Sumber : http://palaeos.com/phanerozoic/phanerozoic.htm

Introduction

The Phanerozoic represents a relatively brief period of half a billion years (brief that is relative to the age of the Earth and the universe) that constitutes the age of multicellular animal life on Earth.  During this time micro- and multicellular organisms left a detailed fossil record, and built up complex and diverse ecosystems, and life has evolved through countless transformations and millions upon millions of species.

The term Phanerozoic - "visible" or "revealed life", or "evident life" - is generally applied to the Paleozoic, Mesozoic and Cenozoic eras; the relatively short period during which the Earth has been inhabited by multicellular organisms that leave fossil traces in the rocks.  This is in contrast to the "Precambrian", which lasted for a very much longer time, but was characterized only by micro-organisms that generally do not leave fossils.  With the discovery of a complex late Precambrian (Vendian/Ediacaran) biotas the term Phanerozoic has lost much of its meaning, but can still be used perhaps to define the period of the development and evolution of higher groups of organisms like arthropods, molluscs, vertebrates etc that are still alive and predominant today. For although primitive algae existed throughout much of the Precambrian, this was not the case with multicellular animals (Metazoa), which only appeared during the very earliest Cambrian. This eon can also be considered (as suggested by Dr James Lovelock in his book Ages of Gaia) as the modern period in the life of Gaia (following the Archean and the Proterozoic), the maturity or third age of Gaia so to speak, and is characterized as much, if not more, by the presence of abundant free oxygen as by the existence of multicellular organisms or fossil-bearing rock strata.

The following table shows the three eras and eleven geological periods that comprise the Phanerozoic.  Like all geological tables this diagram has to be read from the bottom up; the lowest period in the table, the Cambrian, being the earliest.

eon era periodwhen began My ago ICS

duration My ICS

PhanerozoicCenozoic

Neogene 23.0 23.0

Paleogene 65.5 42.5

Mesozoic

Cretaceous 146 80.5

Jurassic 200 54

Triassic 251 51

Paleozoic Permian 299 48

Carboniferous 359 60

Devonian 416 57

Silurian 444 28

Ordovician 488 44

Cambrian 542 54

The Age of Ancient Life

Of the three main eras that make up the Phanerozoic, the Paleozoic is the longest and most diverse, spanning the period from very early multicellular life

that only inhabited the oceans to quite advanced tetrapods* and reptiles and extensive forests on land.

Early Paleozoic: Age of Invertebrates Coelomate radiation (Cambrian explosion) - origin of major groups of organisms; nervous system, behavior patterns and simple consciousness (the nascent Noosphere);

continents drift apart.

Middle Paleozoic: Age of Fish Tropical conditions.  Extinction of many "experimental" animal groups; diversification of surviving invertebrate groups, rise of vertebrates (fish).  Life moves on land (rhyniophytes, lycophytes, uniramous arthropods, and

proto-amphibians).

Late Paleozoic: Age of Tetrapods* and Reptiles Ice age.  Coal forests of giant lycopsids, calamites, pteridophytes and ferns cover the tropical landmasses.  Southern landmass of Gondwanaland buried

under glaciers; continents drift together.  Reptiles

More on the Paleozoic

 

The Age of Middle Life

The Mesozoic has been called the "age of reptiles", but "age of dinosaurs" would be more appropriate.   There is still controversy over whether dinosaurs

really were stupid sluggish ectotherms ("reptiles") or active high-metabolism (endotherm) creatures more like birds.   Even if we define them as "reptiles" the age of reptiles as such begins in the Permian period of the Paleozoic era anyway.

Tropical (Greenhouse) Conditions.  Pangaea continues during the early Triassic; then landmasses begin to drift apart.  Shallow oceans cover much of the continents, breaking the land into large islands.   Mammals remain

small, possibly nocturnal.  Most modern groups of organisms appear. Vertebrate animals

(mammals, birds, theropod dinosaurs) develop larger brains then their earlier reptilian ancestors.

More on the Mesozoic

The Age of Recent Life

Last of all, the Cenozoic - also spelt "Cainozoic" - is the age of mammals.  During this period, following

the extinction of the dinosaurs, mammals evolved from small shrew-like types into all the diverse types around today, as well as many different prehistoric forms.

The modern world.  Land masses take their present shape.  "Intelligence race" as herbivores develop larger brains and carnivores do likewise.  The climate, originally tropical, becomes increasingly more seasonal as ice age conditions develop, possibly triggered by the rise of the Himalayan mountain uplift.

The Paleozoic (also spelt "Palaeozoic") era lasted from about 540 to 250 million years ago, and is divided into six periods The 320-odd million years of the Paleozoic era saw many important events, including the development of most invertebrate groups, life's conquest of land, the evolution of fish, reptiles, insects, and vascular plants, the formation of the supercontinent of Pangea, and no less than two distinct ice ages.  The earth rotated faster than it does today so days were shorter, and the nearer moon meant stronger tides.

MAK

Paleozoic Geography

Since the continental cratons all move with respect to each other, we need to pick an East-West point of reference to keep things straight. Paleozoic  paleocartographers have somehow fallen into the habit of placing this reference longitude slightly east of Greenland.  For most of the Paleozoic, Greenland remained close to the equator and, after Baltica sutured to Laurentia (North America plus Greenland) during the Silurian, this longitude came to correspond quite closely to the longitude of the future Greenwich, England, which defines the present conventional 0° longitude line.  We will adopt this convention, although it is important to understand that it's just a convention.  We have no absolute measures of East-West continental drift, and must be content with noting movements relative to some arbitrary geographical point.  

The early Paleozoic saw many of the continents clustered around the equator, with Gondwana (representing the bulk of old Rodinia) slowly drifting south across the South poles, and Siberia, Laurentia (North America plus Greenland) and Baltica converging in the tropics.  There was a large ocean between Laurentia and Eastern Gondwanaland.

It seems that Gondwanaland underwent a large clockwise rotation around an axis close to Australia during the Early Paleozoic. Laurentia underwent a large eastward movement, as well as a northward drift.

Baltica joined with Laurentia during the Silurian, drifting from a moderate southern hemisphere position in Cambro-Ordovician time to an equatorial position in Silurian-Devonian time.  The combined continent is sometimes referred to as Euramerica, Laurasia, or Laurussia.  Siberia, and possibly the Kazakhstan terranes, drifted across the equator to the northeast.  All the East and Southeast Asian terranes, as well as the microcontinents which later formed Mexico, the east coast of North America, and southern Europe, were still part of the north coast (India-Australia margin) of Gondwana during the Early Palaeozoic.

During the middle and late Paleozoic (Devonian to Permian), about a third of the Gondwanan mass was torn into small pieces and moved rapidly to equatorial regions. Most of these blocks were assembled by a series of plate collisions into the supercontinent of Euramerica by the Devonian, which by addition of further landmasses became Laurasia by the late Carboniferous. Most of western  Gondwana (South America and Africa), then rotated clockwise and moved northward to collide with Laurasia.  By Permian time, Siberia and the Kazakhstan terranes were sutured to Euramerica (Laurussia) and the Chinese blocks started accreting to them.  The result was the supercontinent Pangaea. MAK, revised ATW050810.  

Paleozoic Stratigraphy

eon era periodwhen began myrs ago ICS

duration myrs ICS

Phanerozoic

Mesozoic Triassic 251 51

Paleozoic

Permian 299 48

Carboniferous 359 60

Devonian 416 57

Silurian 444 28

Ordovician 488 44

Cambrian 542 54

Proterozoic Neoproterozoic Ediacaran ("Vendian") 630 88 

MAK

Paleozoic Climate

The Cambrian climate was probably moderate at first, becoming warmer over the course of the period, as the second-greatest sustained sea level rise in the Phanerozoic got under way.  However, as if to offset this trend, Gondwana moved south with

considerable speed, so that, in Ordovician time, Most of West Gondwana (Africa and South America) lay directly over the South Pole.  The Early Paleozoic climate was also strongly zonal, with the result that the "climate", in an abstract sense became warmer, but the living space of most organisms of the time -- the continental shelf marine environment -- became steadily colder.  However, Baltica (Northern Europe and Russia) and Laurentia (eastern North America and Greenland) remained in the tropical zone, while China and Australia lay in waters which were at least temperate.  The Early Paleozoic ended, rather abruptly, with the short, but apparently severe, Late Ordovician Ice Age.  This cold spell caused the second-greatest mass extinction of Phanerozoic time.  

The Middle Paleozoic was a time of considerable stability.  Sea levels had dropped coincident with the Ice Age, but slowly recovered over the course of the Silurian and Devonian.  The slow merger of Baltica and Laurentia, and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow sea floor.  As plants took hold on the continental margins, oxygen levels increased and carbon dioxide dropped, although much less dramatically.  The north-south temperature gradient also seems to have moderated, or metazoan life simply became hardier, or both.  At any event, the far southern continental margins of Antarctica and West Gondwana became increasingly less barren.  The Devonian ended with a series of turnover pulses which killed off much of Middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.

The Late Paleozoic was a time which has left us a good many unanswered questions.  The Mississippian Epoch began with a spike in atmospheric oxygen, while carbon dioxide plummeted to unheard-of lows.  This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous.  These were far more severe than the brief Late Ordovician Ice; but, this time, the effects on world biota were inconsequential.  By the Cisuralian, both oxygen and carbon dioxide had recovered to more normal levels.  On the other hand, the assembly of Pangea created huge arid inland areas subject to temperature extremes.  The Lopingian is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the end-Permian extinction.  

Image: Devonian sea floor scene from the OTS Heavy Oil Science Center.

ATW041218.  Text public domain.  No rights reserved.

Paleozoic Sites

As one might expect from such a vast interval of time, there are a great many Paleozoic sites to choose from.  Rather than attempt the impossible task of describing the scars left by 300 My of geological time, we thought we would briefly summarize the ten Paleozoic sites which, in our judgment, had left the greatest mark on paleontology.  That, at least, is what we thought.  As it turned out, after going through the agonizing job of paring down the list, we found that we could not get much below twelve or fifteen sites.  Rather than make some kind of difficult, rational choice, we have simply hacked off the Permian and about half the Carboniferous, as well as randomly discarding some of the many Devonian sites.  So you will not see anything from Isheevo or the Karoo, the red beds of Texas, Mazon Creek, Bear Gulch or even Canowindra.  Some of these sites are covered in detail elsewhere on this site (which is, of course, one of the ten indispensible sites of Holocene time).  Accordingly, without wasting a single electron or pixel more on vain regrets:

1)  Chengjiang: Terreneuvian of South China.  This site is discussed at Chenjiang.  The English spellings are somewhat variable, "Chenjiang" being another popular variant.  The correct spelling seems to be 澄江 .  It might be better to referred to as the Maotianshan Shale.  This is less accurate (it is more properly the Qiongzhusi Formation), but seems least likely to be misspelled by ignorant foreigners, such as ourselves.  The Chengjiang fossils are dated at 525-520 Mya, or perhaps a bit younger, corresponding most nearly to the Botomian Age in our system [1].  Outcrops of the Qiongzhusi occur in scattered locations south of Kunming in eastern Yunnan Province, Chengjiang County, near the towns of Chengjiang and Ercai.  Additional sites have now been opened further south.  Of all the sites mentioned here, Chengjiang is geologically the oldest and historically the youngest.  The fossil potential of the region was discovered by Dr. Hou Xianguang in 1984.  Many of the fossils have been recovered -- and many lost forever -- in connection with phosphate mines in the area.  The incredible soft-tissue preservation of the fossils here seems to have resulted from rapid burial, complete sediment anoxia, and replacement of organic remains with pyrite or phosphates -- nothing magical, except the absolutely unreasonable number of such sites, of varying ages, in Yunnan Province.   

The faunal list from Chengjiang is a virtually complete census of the major metazoan taxa of the time, and includes our personal favorite of all early chordates, Haikouella.  There seems to be little selectivity.  There are now Chengjinag fossil images all over the web. However, many of the Chengjiang organisms remain undescribed, simply for lack of competent describers, and new specimens are being discovered at an extraordinary rate.   

2)  The Burgess Shale: Middle Cambrian of Canada.  The Burgess Shale is slightly younger than Chengjiang.  The Shale is located near the town of Field, in southeastern British Columbia, high in the Canadian Rockies.  The closest major town is Banff, about 90

km to the east.  The site was discovered by Charles Walcott of the Smithsonian Institution in 1909, and the Walcott Quarry is named after him.  The deposits are deepwater, benthic sediments, but the fauna probably represent a reef community swept off the reef and buried in an anoxic bottom by a mudslide.  The Burgess is actually far less spectacular than Chengjiang, but it attained great fame (ironically, just at the time that Chengjiang was starting to produce large quantities of fossils) due in part to Jay Gould's book, Wonderful Life.  

The Burgess Shale's influence on paleontology has been, in part, due to the fact that Gould chose this book to set out some of his most interesting and controversial ideas about evolution, and in a manner readable by almost everyone.  Gould argued  that the end results of evolution were essentially random because the process was chaotic [2].  Thus even the tiniest change in Proterozoic conditions might have resulted in an entirely different modern fauna.  His proof was the diversity of phyla in the Shale, hinting at an enormous initial diversity in the Cambrian Explosion which was quickly pruned away, largely by happenstance.  As it has turned out, Gould was certainly wrong about the Burgess Shale.  Chengjiang -- and closer examination of the Burgess fauna -- have shown that Walcott was more correct than Gould.  The great majority of Burgess animals can now be assigned with confidence to well-known phyla.  However, his ideas about evolution may well be correct, if the pruning process actually occurred in the lower Early Cambrian or even before metazoans became morphologically recognizable.  

The supercontinent Pangea divides into Laurasia in the north and Gondwana in the south. The climate is hot and tropical worldwide. On land, the dinosaurs reign supreme. In the oceans are various kinds of marine reptiles, as well as ammonite and belemnite molluscs and many other invertebrate groups. Plants include ferns and gymnosperms. Mammals are small and insignificant, but probably numerically common.

The Mesozoic Era lasted more than 180 million years.  During this time, many modern forms of plants, invertebrates, and fishes evolved.  On land, dinosaurs were the dominant animals, while the oceans were populated by large marine reptiles, and Pterosaurs ruled the air.  For most of this period, the climate worldwide was warm and tropical, and shallow seas covered low-lying landmasses.  At the beginning of the Mesozoic, all of the world's continents were joined into the supercontinent of Pangea, which rifted into Laurasia in the north and Gondwanaland in the south.  By the end of the era most of continents had separated into their present form.

The Mesozoic Era is divided into three periods, each lasting many millions of years: the Triassic, Jurassic, and Cretaceous.  The Triassic saw the emergence of many modern invertebrate groups, and on land the archosaur reptiles replaced the therapsids.  In the oceans Ichthyosaurs such as Shonisaurus became as large as whales.  The Jurassic was the height of the dinosaur era, with giants such as Brachiosaurus, Stegosaurus, etc, and mammals tiny and shrew-like.  Distinctive plants like ferns, Cycads, Bennettitales, and Cheirolepidiaceae conifers characterized the landscape.  During the Cretaceous period, the first flowering plants appeared, birds and fish diversified, and new types of dinosaurs appeared.  The climate cooled and unique dinosaurs evolved on different continents.

The Mesozoic era came to an end with the great terminal extinction event known as the K-T (Cretaceous-Tertiary) event.

Image: illustration © Doug Henderson, reproduced with permission

The Mesozoic Era: Stratigraphy

 

Period Epoch Age Range (Mya)

Duration (My)

Cretaceous K

Upper/late K2

Maastrichtian k6 70.6 - 65.5 5.1

Campanian k5 83.5 - 70.6 12.9

Santonian k4 85.8 - 83.5 2.3

Coniacian k3 89.3 - 85.8 3.5

Turonian k2 93.5 - 89.3 4.2

Cenomanian k1 99.6 - 93.5 6.1

Lower/early K1

Albian b6 112.0 - 99.6 12.4

Aptian b5 125.0 - 112.0 13.0

Barremian b4 130.0 - 125.0 5.0

Hauterivian b3 136.4 - 130.0 6.4

Valanginian b2 140.2 - 136.4 3.8

Berriasian b1 145.5 - 140.2 5.3

Jurassic J

Upper/late J3

Tithonian j7 150.8 - 145.5 5.3

Kimmeridgian j6 155.7 - 150.8 4.9

Oxfordian j5 161.2 - 155.7 5.5

Middle J2 Callovian j4 164.7 - 161.2 3.5

Bathonian j3 167.7 - 164.7 3.0

Bajocian j2 171.6 - 167.7 3.9

Aalenian j1 175.6 - 4.0

171.6

Lower/early J1

Toarcian l4 183.0 - 175.6 7.4

Pliensbachian l3 189.6 - 183.0 6.6

Sinemurian l2 196.5 - 189.6 6.9

Hettangian l1 199.6 - 196.5 3.1

Triassic T

Upper/late T3

Rhaetian t7 203.6 - 199.6 4.0

Norian t6 216.5 - 203.6 12.9

Carnian t5 228.0 - 216.5 11.5

Middle T2Ladinian t4 237.0 -

228.0 9.0

Anisian t3 245.0 - 237.0 8.0

Lower/early T1

Olenekian t2 249.7 - 245.0 4.7

Induan t1 251.0 - 249.7 1.3

Mesozoic Climate

Some of the main outlines of Mesozoic climate are matters of general agreement, but almost no one is very satisfied with the explanations for what has been observed.  Here's the usual story:

The Triassic, particularly the first half of the Triassic, was dry and highly seasonal, with particularly large annual temperature variations in the vast continental interior of Pangea, the world-spanning continent of the Triassic.  Low sea levels probably exaggerated these temperature extremes.  Water acts as a heat sink -- it takes much more heat to warm a cup of water than it does to warm a cup of rock.  Water also circulates, so that heat doesn't build up in one place.  The net result is that water tends to stabilize temperatures.  Land areas near the ocean are warmed or

cooled by winds which pass over the ocean and by rains from evaporated ocean waters.  It is generally agreed (a) that the low sea levels of the Triassic contributed to temperature extremes in the interior of Pangea and (b) that the interior of Pangea probably included huge areas of desert.    

During the Jurassic, sea levels began to rise, probably due to an increase in sea-floor spreading.  This seems paradoxical, but the mechanism is explained in the image.  This caused flooding of large areas of the continents.  As a result, the deserts began to retreat, and continental temperatures stabilized.  Pangea also began to break up into smaller units, which brought more land area in contact with the ocean.  The presence of nearby oceans also increased humidity, so that climates worldwide became wetter as well

as warmer.

During the first half of the Cretaceous, this process continued.  In addition two climate trends which began in the Jurassic became quite pronounced in the Cretaceous.  The mechanism for these events is not fully understood.  First, the temperature gradient from North to South became almost flat -- much more so than would be predicted from ocean circulation models.   In other words, average temperatures were about the same everywhere on Earth, from the poles to the equator.  Second, average temperatures were much higher than today, probably by about 10C°.  Higher CO2 (carbon dioxide) levels certainly played a part, but the paleoclimate data do not match theoretical predictions. 

The later Cretaceous story is more complex, and more controversial.  Many researchers, but not a real consensus, believe that sea temperatures near the equator may have become a bit too warm by the Aptian-Albian, perhaps actually incompatible with ocean life.   In addition, some data suggest that land areas near the equator were not jungle- or forest-covered, that plant diversity was low, and that these regions were arid despite being close to the sea.   Deep ocean circulation may also have broken down.  That is, water continued to circulate horizontally, but not vertically.  The deep oceans weren't getting oxygen, and "black shales" appeared in the Aptian-Albian and High Cretaceous.  These are large volumes of organic matter in the oceans which never completely decomposed because of lack of deep ocean oxygen.  Still, the north-south temperature gradient remained very flat.  

Things cooled off a little during the End-Cretaceous, but it's unclear how much or how regularly.   The climate at the very end of the Mesozoic is particularly controversial.  

Unfortunately, the data only match this story to a limited degree, and there are internal inconsistencies.  Here are a few of the problem areas.

1)  If temperature extremes in the Triassic were as great as general circulation models predict, one would expect rather hefty ice-build-up in at least some polar regions.  Glaciers leave a rather distinct geological signature, and we simply don't have any evidence of Triassic glaciers or polar caps.

2)  Conversely, there is evidence of the kind of rapid sea level changes associated with polar ice in the Mid-Cretaceous, which is rather hard to accept.  Miller et al. (2003).

3)  CO2 levels are usually invoked to explain Cretaceous warmth and the flat Cretaceous temperature gradient.  This makes sense, since the very active mid-ocean spreading ridges might well have been associated with out-gassing of CO2 from deep within the Earth.  Unfortunately, neither the geology of the period nor the stable carbon isotope records really support the idea as well as they might.  

4)  Even the most sophisticated quantitative models can't reconstruct the flatness of the Cretaceous temperature gradient.  Either our temperature estimates are off, or some important factor is missing from the models.  Since dinosaurs and semi-tropical vegetation are known from within 10° of the Cretaceous poles, the problem is likely to be with the theory.  A recent study of a mid-latitude continental interior (in eastern Russia) -- far from the ocean in even Late Cretaceous times, suggest that temperatures were very even and that these regions were damp and non-seasonal even in the Mid-Cretaceous.  

Links: Mesozoic Dinosaurs - Enchanted Learning Software, Lecture 24-, Global Climate Change Student Guide, Pz-Mzclimate, Global Climate and Phytogeography in the Early Mesozoic, Pangaean climate during the Early Jurassic- GCM simulations and ..., The Vilui Basin and the Late Cretaceous Continental Interior ..., Mesozoic LAND ECOSYSTEMS, Geological Society - Abstracts.

ATW041023.  Text and ocean crust image public domain.  No rights reserved.

Mesozoic Life

Dragonfly Bivalve Ammonite Belemnite PterosauriaAraucariacean conifer

If you're looking at this section, you may be a beginner without much previous knowledge.  Of course, you may simply have been searching the web for an old Nirvana CD and you ran across this page because, as it happens, you're also a moron.  In either case, it is unlikely that you have much background in Mesozoic zoology (or, for that matter, much taste in music).   Accordingly, we'll keep this pretty basic and concentrate on the familiar tetrapods.

The Mesozoic came after the Paleozoic.  The Paleozoic Era ended with the Permian Period, which ended with a sort of general meltdown, sometimes called the "PT" or "End-Permian" extinction.  We still aren't certain exactly what happened, but the fact that much of central Siberia turned into a sort of volcanic bubble bath for a few million years didn't help.  This was, bar none, the worst mass extinction in the last 600 My.  Don't get this one confused with the "KT" extinction at the end of the Mesozoic -- the one which finished off the dinosaurs 200 My later.  That was a sumo match by comparison.  That is, it eliminated some very large and conspicuous folks very quickly, but it was all very fast and civilized.  

The PT extinction dragged on for at least a few hundred thousand years and resulted in the loss of perhaps 98-99% of all species of animals.  The survivors of the End-Permian radiated into a world that was rather empty, and the new life forms that evolved from those survivors were sometimes quite different from those whch had come before.  For example, of all the therapsids (mammal ancestors) in the world at the end of the Permian, only a few cynodonts and dicynodonts were left.  Not surprisingly, they multiplied like rabbits and spread out all over the world.  As they did, they encountered environments and ecological challenges quite different from those in their South African (probably) home base.  So, different populations evolved in different directions.  

In addition, the great legion of their dinocephalian cousins had vanished completely, leaving those large-herbivore and carnivore jobs empty.  Some of those slots were filled by newly modified cynodonts and dicynodonts; but, many of those positions were taken over by archosaurian reptiles, instead.  So, not only were the surviving groups changed in composition, but the balance between them changed as well.  Among the tetrapods, the newly expanded range of the archosaurs created an opening for the evolution of the archosaurian

dinosaurs and pterosaurs, both of which appeared just a few million years after the PT extinction.  These went on to drive the dicynodonts to extinction and reduce the cynodonts to a marginal population of small, furtive night-dwellers -- the mammals.  A similar, but larger, set of vacancies in the marine job market created new opportunities for other major reptile lines,

which evolved several different groups of specialized sea-going forms, including the sauropterygians (plesiosaurs and their kin), ichthyosaurs, and mosasaurs. 

This same story could be told about molluscs and echinoderms and even plants, which suffered much less than animals from the effects of the PT events.  In each case, one or two of the big groups were completely eliminated, the rest were changed, and the old ecological balances of the Paleozoic were very thoroughly unbalanced.  The entire Triassic, and most of the Jurassic, was spent getting all that sorted out.  At the end of this period -- about the Late Jurassic and earliest Cretaceous -- there was another burst of evolutionary creativity associated with rising seas and relatively warm, equable climate throughout the world.  Familiar examples include birds, placental mammals and angiosperm (flowering) plants. Again, even more fundamental changes were going on in the sea: rudist molluscs, new types of sharks, planktonic foraminifera, and several new types of algae, to name but a few.  

Both temperature and sea level reached maxima in Aptian-Albian time, or perhaps a little later.  By this time, things were getting a bit too warm in the seas, and there was some climatic deterioration.  The Late Cretaceous saw a remarkable evolution of smaller animals of all kinds, perhaps at the expense of the giants of earlier Mesozoic ages.  So, for example, we find the first examples of modern lizards and snakes, and mammals which were probably primates.  

Of course, all these vermin might have come to nothing if a small asteroid hadn't happened to land in Mexico, 65.5 Mya.  But it did, and the cycle of disaster, evolution, dispersal, and recovery continued.  Speaking of which, if you're still interested in that Nirvana CD, forget it.  Sure, Cobain could have been the TS Elliot of the Twenty-First Century had he, likewise, taken a different path.  But he didn't either, and its just no use pretending otherwise.  

ATW050205.  Text public domain.  No rights reserved.

Mesozoic Reef Systems

It is easy to type "Mesozoic Reef Systems," just as it is easy to type the words "The History of the Asia."  In both cases, it's a bit harder to say anything meaningful in a few words.  We might try a few verbal pictures instead.  

The Mesozoic began with the universal desolation of the end-Permian world.  Most reef systems were devastated beyond recovery.  The frothy and exhuberant dream castles of Late Permian calcareous sponge were were now in ruins -- crumbling blocks of lifeless rock, around which no fishes swam.  Instead, there sprouted, here and there, the squat and flaccid mushroom shapes of pale stromatolites.  These glowed a ghost-like green against the garish,

toxic shades of fungal blooms which gnawed like ghouls opon the last decaying flesh of Permian life.  The seas were weirdly clear.   The rich planktonic rains of fusilinid forams, diatoms, and softer-bodied forms, uncounted and unknown, were gone.  In deeper seas, the drifting galaxies of crystal radiolarian stars were swept away.  All ocean life was strangled by anoxic waters reaching through unheard-of depths; and nothing lived that did not feed on death.  

Almost two million centuries later it closed with a riot of shape and of form, leaving reefs made of corals and sponges and clams,  leaving mountains of algae and snails, leaving brachiopods by the billion or more, leaving walls built by rudists on carbonate platforms with foraminiferan floors.   For throughout the Triassic, Jurassic, Cretaceous, the oceans continued to rise.  And as long as the waters continued to rise, the corals continued to grow.  Like the rudists and algae and sponges and clams, they grew to the tops of their tropical seas, where the sun made a tropical glow.

While an interesting exercise, the attempt to deliver scientific information in blank verse suffers from certain unavoidable inefficiencies.  We will therefore return to our regular bland diet of tasteless literary grits -- with but with an occasional metrical lapse for particular pieces and bits.  ATW040909.

Image: Jurassic corals from Jurassic Reef Park

Marine Life in General

Mesozoic oceans were populated by a rich and diverse fauna of fish, reptiles, and a variety of cephalopod mollusks including the ammonites and the belemnites (nautiloids were also present but less common). Both these molluscan groups were adapted for speed and mobility. Fish were mostly slow moving heavy scaled ("ganoid") types, which were probably not as agile (teleosts only become predominant towards the later Cretaceous).

From the Jurassic onwards Plankton increased in diversity, with phytoplankton such as coccoliths, diatoms and silicoflagellates together with a zooplankton dominated by foraminifera and radiolarians. Arthropods such as the amphipod, decapod and isopod crustaceans together with nudibranch mollusks and the annelid and polychaete worms were also probably part of the plankton. MAK020428

Invertebrates

Annelida

The fossil record of Mesozoic annelids, like the fossil record of all annelids, is poor.  We can only make a few, general remarks.  

The end-Permian extinction more or less destroyed the entire Paleozoic benthic fauna.  The Mesozoic benthic communities, developed an entirely new style, possibly (i.e., this is complete speculation) based on the very few anoxia-tolerant detritivores who would have flourished in the benthic carnage of the end-Permian.  Whatever their origin, Mesozoic and Cenozoic benthic communities are dominated by infaunal (burrowing) deposit-feeders, rather than epifaunal suspension feeders.  This was surely good for the annelids who are quite handy with low-oxygen, burrowing ways of making a living.  Oligochaetes probably evolved in the Late Jurassic.  However, they were unable to employ the usual annelid skills on land until the Late Cretaceous, when angiosperms began creating large quantities of humus, permitting the evolution of the oligochaete earthworms.  

Brachiopoda

Brachiopods suffered greatly during the end-Permian extinction.  They were able to make a considerable come-back during the Late Triassic, but ultimately declined and were ecologically replaced by bivalves.  Their fate may have been tied to substrate.  The brachiopods of the Late Triassic resurgence were strongly associated with carbonate shelves, the classic reef environment of the Late Paleozoic and Early Mesozoic.  The rise in sea levels during the Jurassic and Early Cretaceous drowned these platforms on a global basis.  That is, the residents of the carbonate platforms gradually found themselves too deep in the water column for sunlight to sustain photosynthesis, and the shelf ecosystems collapsed.  This permitted the bivalves to "mussel" their way in, as they were better adapted to the soft and unstable sand & mud sea bottoms within the new photic zone. In fact, with the evolution of the rudists, the bivalves were able to make their own quick and sloppy reefs on even the softest substrate.

As a consequence, the surviving Mesozoic brachiopods became off-shore specialists, occupying deeper-water and more cryptic environments in crevices and on submarine cliffs below the photic zone.  Some developed poisonous tissues.  The more robust and globose terebratulides such as Terebratella and probably some species of Tichosina were free on the substrate.  A few of these developed semi-infaunal strategies.  

Mesozoic brachiopods, like many other invertebrates, show considerable differentiation between Tethyan (tropical) and Boreal (subtropical and temperate) types in the Late Triassic and Jurassic.  Also like many other invertebrates, these distinctions broke down in the Cretaceous, as rising sea levels and flattened climate zonation homogenized most marine fauna.  

Bryozoa

Early Mesozoic bryozoans were largely cheilostomes and cyclostomes. During the Early Cretaceous, however, the cyclostomes declined while cheilostomes diversified. The reasons for this replacement are unclear. Both suffered massive extinctions in Maastrichtian time, possibly coinciding with the more general KT extinctions.  The cheilostomes rebounded during the Cenozoic.  The cyclostomes generally did not.  McKinney & Taylor (2001).

Cnidaria

Mesozoic cnidarians are mostly known from their greatest success story, the scleractinian corals.  Several groups of scleractinians developed tight symbiotic relationships with photosynthetic zooxanthellae with a resulting huge boost to their productivity.  The scleractinians suffered considerably from the drowning of the carbonate platforms on which their reefs were based during the Late Jurassic and Cretaceous.  However, they recovered quickly after the KT extinctions.  

Echinodermata

The End-Permian extinction at the end of the Paleozoic Era took a heavy toll on the stemmed echinoderms.  The blastoids became extinct at that time and the crinoids suffered heavy losses.  In general, Paleozoic echinoderms were epifaunal suspension and detritus feeders.  Like so many high school students, their strategy was to sit more or less stationary on the sea bottom with their mouths open and wait for food to come to them.  In the Mesozoic and Cenozoic, the echinoderms became more like undergraduates -- still bottom-feeders, but now willing to dig for it (infaunal detritus feeders) or, if sufficiently pressed, to go and hunt for it (armored herbivores and carnivores).  

This use of rather heavy armor runs counter to a general trend among Mesozoic life forms to shed heavy plates and to depend more on speed, or on other behavioral adaptations for survival.  However, behavioral strategies depend on having the neural equipment to select a response and adapt it to local conditions.  Echinoderms are poorly adapted for this sort of thing because they are attractive, but brainless. So as time went on, echinoderms, like other attractive but

brainless organisms, were increasingly forced to rely on heavy make-up, intimidating ornament, and a thick skin.  The surviving crinoids, for example, were articulates, with rounded, closely fitting armor plates, usually bearing elaborate ornamentation.  Some also gave up sessile life, left their stems behind, and became motile.  These swimming crinoids, the Rovecrinidae, are discussed briefly elsewhere. 

However, for the most part, the old crinoid fauna simply died out.  The future of the Echinodermata lay with the Echinoidea and Asteroidea.  Echinoids are rare in Paleozoic faunas, but radiated extensively during the Mesozoic and Paleogene.  Paleozoic, and even

Triassic, urchins have no compound plates, and the interambulacral plates are constructed in many columns [1].  These earliest sea urchins are generally small and lack strong spine development -- characters which developed over the course of the Mesozoic.   

Porifera

Sponges as a whole did well and slowly diversified until the very end of the Mesozoic.  However, this general trend is made up of varying fates of different groups of sponges.  Demosponges and calcisponges recovered from the end-Permian extinction and dominated the reef fauna once more in many locations during the Late Triassic.  However, they were gradually replaced by scleractinian corals.  Hexactinnelids and some stromatoporoids continued as important frame builders for the coral reefs of Jurassic Europe. Demosponges and hyalosponges became more common in the Cretaceous.  As sea levels rose, these sponges were sometimes able to thrive in regions which had become too deep for the corals.   Mesozoic stromatoporoids (demosponges probably not related to the Paleozoic forms) were significant reef-builders in the Cretaceous.  All types of reef-building sponges virtually disappeared at the KT boundary and never recovered.    ATW040905

Vertebrates

Mesozoic Tetrapods

The Mesozoic era was an extremely long period of time, which saw the rise and fall of successive "dynasties" of life. At least half a dozen succesive evolutionary communities or empires of land vertebrates (tetrapods) can be distinguished. Identifying them by characteristic large herbivores, these can be called the lystrosaur (Earliest Triassic [Induan]), kannemeyeriid- traversodontid (primarily Gondwanan, though this may be sampling bias) (Early [ Olenekian] to Late (Carnian) Triassic ], plateosaur-vulcanodontid (Late Triassic [Norian] - Early Jurassic), sauropod- stegosaur (Middle to Late Jurassic), iguanodont- nodosaur (Early to Mid

Cretaceous), and ceratopsian-hadrosaur (Late Cretaceous - Laurasia only, Gondwana was predominantly Titanosaurid, with Abelisaurid carnivores) communities or "empires". In the sea one finds what could be perhaps termed the mixosaur- nothosaur (Mid Triassic), shastasaur (Late Triassic), ichthyosaur- plesiosaurid- rhomaleosaurid (Latest Triassic [Rhaetian] - Early Jurassic), ophthalmosaur- pliosaurid- metriorhynchid (Middle Jurassic-Early Cretaceous), and protostegid- elasmosaurid- mosasaur communities (Mid to Late Cretaceous).

The following diagram is from Fig. 3. of Dr Robert T. Bakker's 1977 paper "Tetrapod Mass Extinctions - A model of the regulation of speciation rates and immigration by cycles of topographic diversity" in A. Hallam, ed. Patterns of Evolution as illustrated by the Fossil Record, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, pp.439-68

Although subsequent research has modified some of the family rankings and stratigraphic correlations, the basic pattern remains.

Diversity of tetrapods, Early Triassic to Cretaceous. Standard marine stages are indicated by initials in boxes at top. Narrow extensions of bars indicate that the family is present but very rare. Families known from only one formation are omitted. Roman numerals at top show the successive "dynasties". Biomass D for large herbivores for Triassic taken from Fig. 2; D calculated for a few large Jurassic and Cretaceous samples from the following formations: 1, Tendaguru (Kimmeridgian); 2, Morrison (Kimmeridgian/Tithonian); 3, Old Man (Campanian); 4, Lower Edmonton A (Campanian/Maastrichtian); 5, Lower Edmonton B (Maastrichtian); 6, Lance-Hell Creek-Frenchman (latest Maastrichtian).

Family abbreviations:

Marine Aquatics: A = henodontids; B = pachypleurosaurids; C = mixosaurids; D= placocheliids; E = nothosaurids; F = shastasaurids; 0 = pliosaurids; H = metriorhynchids; I= rhomaleosaurids; J = ichthyosaurids; K = plesiosaurids and cryptoclidids; L = stenopterygiids; M = teleosaurids; N = rhamphorhynchids; O = protostegids; P = mosasaurids; Q = cheloniids; R = toxocheliids; S = ichthyornids; T = hesperornids; U = elasmosaurids; V = polycotylids; W = ornithocheirids (pteranodontids); X = ornithodesmids.

Large Fresh-Water Aquatics: A =amphisbaenids; B = varanids; C = pelomedusids; D = dermatemyids; E = crocodylids; F = trionychids; G = pholidosaurids; H = goniopholids; I = glyptopids; J = emyids; K = champsosaurids.

The sampling of non-marine tetrapods during the first eight stages of the Jurassic is so poor that the records are not worth plotting on this compilation.

Large Terrestrial Herbivores: A = camarasaurids; B = diplodocids; C = stegosaurids; D = brachiosaurids; E = cetiosaurids; F = camptosaurids; G = hypsilophodontids; H = panoplosaurids; f = ceratopsids; J = iguanodontids; K = hadrosaurids; L = protoceratopsids; M = titanosaurids; N = pachycephalosaurids; 0 = euoplocephalids.

Marine Reptiles

illustration © Walking with Dinosaurs © 1999 ABC, BBC

During the Mesozoic era there were a number of lineages of marine reptiles.  In comparison with the mammals, many more types of reptiles, having attained an existence on land, returned to the seas.  Aquatic adaptation (whether freshwater, estuarine, or marine) is a common phenomenon among reptiles, due to their low metabolic rate, tolerance of anoxia and of low body temperatures, and easy ability to make use of fermentative metabolism for muscle activity.  Moreover it does not require great structural or physiological changes, as indicated by the modern marine iguana.  Reptiles move with a naturally sinuous motion, which is easily adaptable to swimming (as it comes originally from the swimming motion of the fish).  In marine iguanas aquatic locomotion requires only one quarter the metabolic activity of terrestrial locomotion.  When looked at this way, it is not surprising that reptiles have returned to the water, like their tetrapod and fish ancestors, whenever conditions were favourable.

The following is a list of aquatic reptiles, with some basic data.  Note: Many of these pages are under construction or very incomplete.  So here they are: the marvelous Mesozoic marine reptiles:

Picture name time-span habitat location approx size food

Plesiochelyidae

late Jurassic [Kimmeridgian] to early

estuarine, near

Central to East Laurasia

shell c.75 cm???

invertebrates, fish, plant

Cretaceous shore marine material?

Desmatochelyidae + Protostegidae

Cretaceous Desmatochelyidae - Albanian to Maastrichtian Protostegidae - Turonian to Maastrichtian

open ocean

Desmatochelyidae - cosmopolitan Protostegidae - west Laurasia ("inland sea") only?

Desmatochelyidae - av. 1.2 meters (shell) Protostegidae - up to 4 meters long (Archelon)

jellyfish, other invertebrates, fish?

Toxochelyidae

late Cretaceous (Coniacian to Maastrichtian)

open ocean

west Laurasia ("inland sea" to west Atlantic)

shell 60-120 cm

jellyfish, other invertebrates, fish?

Cheloniidae

latest Cretaceous [Maastrichtian] to Recent

open ocean Worldwide

75 cm to over 1 meter long

jellyfish, other invertebrates, fish?

Ichthyosauria

Triassic to Jurassic, a few stragglers into the Cretaceous

open ocean world wide 1 to 23

meters

mostly fish, also cephalopods, smaller reptiles

Placodontia Triassicnear shore marine

Tethys Sea 1 to 2.5 meters

probably shellfish (Bivalve mollusks)

Pachypleurosauridae

Middle Triassic [Early Anisian to Ladinian]

estuarine, near shore marine

world wide 20 cm to 1 meter

fish, crustacea, etc

Nothosauridae

Middle to early late Triassic [Anisian to Carnian]

near shore marine

world-wide 2 to 8 meters mostly fish

Plesiosauria

rare in the Triassic common in the Jurassic and Cretaceous

open ocean ( a few estuarine species)

world wide 2 to 14 meters

fish, cephalopods, other reptiles

Mosasauroidea

Aigialosauridae Late Jurassic to Early late Cretaceous (Tithonian to Turonian) Mosasauridae: late Cretaceous (Turonian to Maastrichtian)

open ocean

Aigialosauridae: Europe Mosasauridae: world wide

Aigialosauridae: av. 1 meter Mosasauridae: 2.5 to 17  meters

fish, cephalopods, other reptiles

Thalattosauria Triassic

near shore marine

Tethys Sea, also west Laurasia

about 1.5 meters

fish, crustacea etc

Nanchangosaurus

earliest Middle Triassic (Early Anisian)

near-shore and estuarine

China no info (~1 meter?)

probably fish, crustacea, etc

Teleosauridae Jurassic

estuarine, near shore marine

world wide 2 to 6 meters mostly fish

Metriorhynchidae

Jurassic to Early Cretaceous

open ocean world wide 2 to 6 meters mostly

fish

During the 65 million years of the Cenozoic Era (also spelled "Cainozoic"), or Age of Mammals, the world took on its modern form.  Invertebrates, fish, reptiles etc were essentially of modern types, but mammals, birds, protozoa and flowering plants still evolved and developed during this period.

Traditionally, the Cenozoic Era was divided into two very unequal periods, the Tertiary (which made up the bulk of the Cenozoic), and the Quaternary, which is only the last one and a half million years or so.  The Tertiary is in turn divided into Paleogene and Neogene.  We

do not adopt this use of the "Tertiary" as a formal stratigraphic division for the following reasons:

More than 95% of the Cenozoic era belongs to the Tertiary period, an unreasonable division which reflects the arbitrary manner in which the geological epochs were first named. From 1760 to 1770, Giovanni Arduino, inspector of mines in Tuscany and later professor of mineralogy at Padua, set forth the first classification of geological time, dividing the sequence of the Earth's rocks into Primitive, Secondary, and Tertiary. During the 18th century the names Primary, Secondary, and Tertiary were given to successive rock strata, the Primary being the oldest, the Tertiary the more recent. In 1829 a fourth division, the Quaternary, was added by P. G. Desnoyers. These terms were later abandoned, the Primitive or Primary becoming the Paleozoic Era, and the Secondary the Mesozoic. But Tertiary and Quaternary were retained for the two main stages of the Cenozoic. Admittedly, attempts to replace the obsolete "Tertiary" with a more reasonable division of Palaeogene (early Tertiary) and Neogene (later Tertiary and Quaternary) have not been completely successful, but most of the newer geological timelines have rejected the Tertiary.

Revised ATW040925

Geology of the Cenozoic

During the Cenozoic, the fragmentation of continental landmasses continued as the Earth's surface took on its present form.   The major geologic events of the Cenozoic can be thought of as two basic processes.  First, four different large fragments of the Gondwanan supercontinent moved north and became, to varying degrees, attached to the Laurasian landmass.  This resulted in a number of spectacular mountain-building events which climaxed about the Early Miocene.  Second, the north-south Atlantic spreading zone continued to widen the Atlantic, contributing to geologic strains in East Africa and the western parts of the Americas, as these continents were pushed into contiguous plates by the growing Atlantic Ocean.  

The defecting Gondwanan fragments were South America, Africa, India, and

Australia.  South America has not pushed far enough north to cause a the geological equivalent of a high speed collision with North America.  Instead, the impact was cushioned by a sort of air bag of small plates in what is now the Caribbean Sea.   In particular, the

approach of the American continents pinched off part of the Pacific crust, a region containing the sea bottom south and west of Cuba.  The cushioning effect of these intervening plates delayed the formation of a land bridge between the Americas until the Middle Pliocene (Piacenzian), and has confined the effects of continental collision to relatively mild and sporadic vulcanism around the Caribbean and its southern and western margins.  See image from Kerr et al. (1999).  Various other representations can be found at Prof. Manuel Iturralde's wonderful site on the Caribbean plate: Comparison of different opinions...

A similar, but less pneumatic, effect has softened the impact of Africa on Europe.  The numerous microplates of the Mediterranean have been repeatedly rearranged and compressed as Africa approached from the south.  Nevertheless, Africa's attempt to subduct under the European Plate has been a little like a hippopotamus trying to hide under a bed sheet -- there have been some inevitable little lumps and wrinkles.  Some of these, like today's Alps, are difficult to overlook.  Other, older ranges which run generally east to west across Europe are also products of this process.  In addition, the approach of Africa squeezed shut the old Tethys Seaway, which played such a large part in Early Mesozoic tetrapod history, leaving only a few puddles, such as the Mediterranean Sea and the Black Sea.  

The impact of India doesn't seem to have been mitigated at all.  A land bridge between India and the Asian mainland was not established until the Eocene.  However, the continental shelves of Asia and India had been in contact for some time before this, and elevation of the Himalayas has been ongoing throughout the Cenozoic.  Initially, most of the impact was in the East, as India attempted to subduct under Asia to become the basement level of Tibet.  In the Miocene, the force of the collision was distributed further west, forming the high plateaus of Afghanistan and Iran, with collateral consequences as far west as Eastern Europe.   Perhaps the same fate awaits Australia, the last of the Gondwanan refugees.  However, Australia has been dawdling along in the Pacific and has only recently begun to interact with the outlying portions of the Indonesian plates.  

While the northern and southern continents have been getting progressively cozier, the Mid-Atlantic spreading ridge has been busy separating east from west.  In the north, after splitting Greenland from North America, the rift abruptly changed course in the Paleogene and began to separate Northern Europe from Greenland.  As a result, the last land bridge between North America and Europe was broken in the Eocene.  The westward pressure on the Americas may well have been responsible for the Laramide Orogeny in the Western United States during the Paleogene, and the seamless merger of the subduction zones of North and South America later on.  It is less clear that it has had any role in the more recent events which raised the current complex set of north-south mountain ranges in North America. 

 On the other side, in East Africa, the eastward pressure of the Mid-Atlantic ridge, combined with the opposite forces generated by the impact of India, created enormous stresses.  As a result, the Arabian peninsula was rotated and torn off the East coast of Africa, and a series of deep faults have begun to fracture the African plate.  Late in the Cenozoic, the main rift

valley running through Ethiopia, Kenya, and points south, became the home of several species of large, noisy, and nearly hairless apes.  

Image: The satellite image is from NASA.  It shows the southern part of the rift system in East Africa with a few of the great lakes which have developed in the rift valleys.  Part of Lake Tanganyika is in the upper left corner.  The lake in the upper central portion is Lake Malawi.

ATW040924.  All text public domain.  No rights reserved. 

Timescale

The following table gives the component periods and epochs that make up the Cenozoic.  In addition to the Neogene and Paleogene Periods below, the helpful folks at the ICS have added a sub-era (or possibly period, subperiod, or supra-age), the Quaternary, which begins at the base of the Gelasian Age and extends through the present day.  Obviously, this makes no sense at all, since the same body abolished the "Tertiary" just a year or two ago -- not to mention the fact that the rules on chronostratigraphic units don't seem to allow for a unit which begins in the middle of one epoch and ends in another.  It would make much more sense to move the Gelasian Age (a unit which was only created in 1996) to the Pleistocene.  Then the Pleistocene would become the age of continental ice sheets, which is precisely the reason it was thought necessary to recognize the "Quaternary."  This sensible proposal was actually considered, but it was far too late.  The ICS had already established a Subcommission on Quaternary Stratigraphy.  Kings may die or abdicate.  Nations may be conquered and governments dissolved.  Even whole continents may fractured and dispersed by cataclysmic rifting.  But committees do not vote themselves out of existence.  

Period Epoch AgeBase (duration)

Geomagnetic Polarity Zone (base)

Approximate Central Paratethys Stage

European Neogene Mammal Zones (base)

South American Land Mammal Ages ("SALMA")

North American LandMammal Ages ("NALMA") Paleobiology Database (2006)

Neogene Holoce

ne

0.0118 (0.0118)

Pleistocene Late

0.126 (0.1142)

Middle 0.781 (0.655)

Lujanian (0.3)

Early 1.81 (1.029)

C1 (1.8) Ensenadan (1.5)

Irvingtonian (1.8), Rancholabrean (1.02)

Pliocene

Gelasian 2.59 (0.78)

MN 17 (2.5), MmQ1 (2.0)

Uquian (2.5)

Piacenzian

3.60 (1.01)

MN 16 (3.2)

Chapadmalalan (3.0)

Zanclian 5.33 (1.73) C2 (4.2) Dacian

MN 15 (4.2), MN 14 (4.9)

Montehermosan ( 5.4) Blancan (4.9)

Miocene

Messinian

7.25 (1.92) Pontian MN 13

(7.1)

Tortonian

11.6 (4.35)

C3 (7.4), C4 (9.7)

Pannonian

MN 12 (7.7), MN 11 (8.7), MN 10 (9.7), MN 9 (11.3)

Huayquerian (9.0), Chasicoan (10)

Hemphillian (10.3)

Serravallian

13.7 (2.1)

Sarmatian, later Badenian

MN 7/8 (12.7), MN 6 (13.8)

Mayoian (12), Laventenian (13.8)

Clarendonian (13.6)

Langhian

16.0 (2.3)

earlier Badenian

MN 5 (16.0)

Colloncurian (15.5), Friasian (16.3)

Barstovian (16.3)

Burdigalian

20.4 (4.4) C5 (19.1)

Karpatian, Ottnangian, Eggenburgian

MN 4 (16.8)

Santacrucian (17.5)

Hemingfordian (20.6)

Aquitanian

23.0 (2.6) Egerian Colhuehua

pian (21)

Paleogene

Oligocene

Chattian 28.4 (5.4)

C9 (28.2), C8 (26.5), C7 (24.9), C6 (24.1)

MN 1 (23.9)

Harrisonian (24.8)

Rupelia 33.9 C11 Deseadan Geringean

n (5.5)(30.6), C10 (29.3)

(29)

(30.8), Whitneyan (33.3), Orellan (33.9)

Eocene

Priabonian

37.2 (3.3)

C17/16 (36.3), C12 (33.0)

Tinguirican (36)

Bartonian

40.4 (3.2)

C19 (41.3), C18 (40.4), C17 (37.6)

Chadronian (38)

Lutetian 48.6 (8.2)

C21 (48.6), C20 (45.1)

Divisaderan (42), Mustersan (48)

Duchesnean (42), Uintan (46.2)

Ypresian

55.8 (7.2)

C23 (52.6), C22 (50.6)

Casamayoran (54.0-51.0)*

 Bridgerian (50.3),  Wasatchian (55.4)

Paleocene

Thanetian

58.7 (2.9)

C24 (56.6)

RÃochican (57.0-55.5)

Clarkforkian (56.8)

Selandian

61.7 (3.0)

C25 (58.4)

Itaboran (59.0-57.5)

Tiffanian (60.2)

Danian 65.5 (3.8)

C29 (65.5), C28 (64.6), C27 (63.4), C26 (61.7)

Peligrosan (62.5-61.0), Tiumpampan (64.5-63.0)

Torrejonian (63.3), Puercan (65), Lancian (69.7)

Climate

During the Paleogene the climate worldwide was warm and tropical, much as it had been for most of the preceding Mesozoic.  The Neogene saw a drastic cooling in the world's climate, possibly caused by the Himalayan uplift (Tibetan plateau) that was generated by the Indian subcontinent ramming into the rest of Asia (and is still going on now).  During the Pleistocene, the continuing cooling climate resulted in an ice age, or rather a series of ice ages with interspersed warm periods

Life

With the end Cretaceous extinction event and the extinction of the ammonites and most of the belemnites, teleost fishes dominated neritic (near shore) and pelagic faunas. Plankton recovered and basically belonged to modern groups. Coleoidea, Crustaceans, nudibranch mollusks and polychaete worms make up a large part of the larger zooplankton. The large marine reptiles of the Mesozoic were replaced by cetacean mammals (dolphins, whales and their kin) that first appeared during the Eocene. And while the protostegids (which included giants like Archelon) disappeared with the end-Cretaceous extinction, modern sea turtles survived quite happily.

The Paleogene saw the diversification of many mammalian and bird groups, flourishing in the tropical conditions.  During the early Paleogene the continents were isolated by shallow seas, and different lineages of Mammals evolved on each one.  Mammals included many giant yet small-brained rhinoceros-like types - the Asiamerican uintatheres, and brontotheres and the African arsinoitheres.  There were huge flightless carnivorous birds - the Laurasian diatrymids (left) and the

South American phorusrhacids - 2 meters tall with cruel curved beaks, that mimicked the great theropod dinosaurs of the Mesozoic.  All these animals lived in tropical forests.  The champsosaurs, crocodile-like "eosuchian" reptiles - living fossils of their time - survived the dinosaurs and the K-T extinction but died out later in the Paleogene.  In the seas the first archaic toothed whales appeared.  Giant marine protozoa, (foraminifers) the size of lentils evolved during the Eocene.  Bivalve and Gastropod molluscs were basically the same type as today.  The nautilids experienced their last mild evolutionary radiation.  Transitional forms ancestral to modern coleoid cephalopods evolved.  Echinoderms, corals, bryozoa and sponges were basically of modern type.  On land insects were generally of modern type.   Ants were even more numerous then they are today.

During the Neogene modern mammals and flowering plants evolve, as well as many strange mammals that are no longer around.  The most astonishing thing to happen during the early Neogene was the evolution of grass.  This led to the evolution of long-legged running animals adapted to life on the savanna and prairie.  The horse family - Equidae - was an especial success story during the Neogene.  Horses and other grazing mammals evolved high-

crowned teeth to cope with a diet of abrasive grass.  There were still many forest animals however.  The Mastodons lived on every continent except Australia.  Many strange mammals - litopterns, notoungulates, ground sloths, borhyaenids, etc - continued to evolve in isolation in Hominids appeared in the Africa savannas, the Australopithecines.    The oceans were inhabited by whales basically like modern forms, which had replaced the archaic toothed whales.  They were the most intelligent animals of their time, but they never developed the use of tools or a memetic noosphere.  In the north Pacific were the Desmostylids - a sort of cross between an elephant and a seal.  Also in the seas were the largest carnivorous sharks ever to live - the Carcharodon megalodon, a predecessor of the modern White Pointer but much larger and heavier.

The Pleistocene period saw essentially modern flora and invertebrate species.  However many mammalian types were of species and genera now extinct, and generally of large size - the various species of mammoth, the Irish "elk" (left), a large diversity of rhinos, the giant ground sloths, the diprotodonts of Australia, and many more.   Man evolved as an ice-age mammal in Europe.  A

combination of human hunting ("stone age overkill") and climatic change served to kill off most worlds megafauna.

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Phanerozoic Eon, the span of geologic time extending about 542 million years from the end of the Proterozoic Eon (which began about 2.5 billion years ago) to the present. The Phanerozoic, the eon of   visible   life,   is   divided   into   three  major   spans   of   time   largely   on   the  basis   of   characteristic assemblages of life-forms: the Paleozoic (542 million to 251 million years ago), Mesozoic (251 million to 65.5 million years ago), and Cenozoic (65.5 million years ago to the present) eras. Although life clearly   originated   at   some   time,   probably   quite   early,   in   the   Proterozoic   Eon,   not   until   the Phanerozoic did a rapid expansion and evolution of forms occur and fill the various ecological niches available. The key to this great Phanerozoic expansion appears to lie in the development of plants able to carry out the photosynthetic process and thus release free oxygen into the atmosphere. Before this time, the Earth’s atmosphere contained negligible amounts of free oxygen, and animals, in which energy transfers involving the process of respiration are critical, were unable to develop. During the Phanerozoic, the Earth gradually assumed its present configuration and physical features through such processes as continental  drift,  mountain building,  and continental  glaciation.  Thus, although the Phanerozoic Eon represents only about the last one-eighth of time since the Earth’s crust formed, its importance far exceeds its relatively short duration.

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Phanerozoic Eon

The phanerozoic is the most recent eon in geological terms. It started about 542 million years ago, and continues to this day. The name "phanerozoic" (meaning visible or apparent life) reflects that this was the period when large organisms (visible to the naked eye) appeared in

great profusion. In the early days of geology, this was the designation given to strata where metazoan fossils first started to appear. Today, this definition remains largely true, though we now know that in some places at least, metazoans appeared earlier than the start of the phanerozoic (in the Ediacaran or Vendian period of the preceding proterozoic era). At any rate, this was the period when metazoans started to diversify and multiply greatly. This rapid expansion of metazoan life forms is sometimes referred to as the Cambrian Explosion. The phanerozoic is a brief period in the Earth's history, about half a billion years, a bit less than 12% of the time that the Earth has existed, but almost all metazoan life is confined to this period.

The Phanerozoic Eon

Eon Era PeriodMillion Years Ago

Phanerozoic

Paleozoic

Cambrian 542 490

Ordovician 490 444

Silurian 444 416

Devonian 416 360

Carboniferous 360 299

Permian 299 251

Mesozoic

Triassic 251 200

Jurassic 200 145

Cretaceous 145 65

Cenozoic

Paleocene 65 55.8

Eocene 55.8 33.9

Oligocene 33.9 23.0

Miocene 23.0 5.33

Pliocene 5.33 1.80

Pleistocene 1.80 0.0117

Holocene 0.0117 present

The causes of this (relatively) sudden explosion of multicellular life are not well understood. Life on Earth appeared at least 3.5 billion years ago, in the paleoarchean, as evidenced by stromatolites found in rocks of that age in Australia. For much of Earth's history, life was unicellular and prokaryotic. But eukaryotes appeared about 1.4 billion years ago, during the

mesoproterozoic, and the first evidence of multicellular life is almost as ancient - about 1.2 billion years old, during the Ectasian period of the mesoproterozoic. Why then, did it take another half billion years for more complex forms to appear? Assuming, of course, that the fossil record is reliable in this regard, which is by no means certain. Small soft-bodied animals do not fossilize well, and rocks of such great age are relatively uncommon. So it may well be that complex life goes back earlier than we currently know. As mentioned above, the discovery of Ediacaran fossils also produced a change in our thinking, pushing back the appearance of complex multicellular life by another 20+ million years.

Such ideas also have implications about how we think of evolution. The sudden appearance of a diversity of organisms during the Cambrian was one of the reasons why evolutionary biologists such as Stephen Jay Gould proposed the theory of punctuated equilibrium - that evolution follows a pattern of relative stasis interrupted by bursts of rapid change. However, with the discovery of earlier fossils, such as those from the Ediacaran, it is becoming clearer that these "bursts" of change were not as abrupt as we had thought, that they were part of a more gradual process of evolution. Also, modern re-interpretations of early Ediacaran and Cambrian fossils indicate that they might not be as "diverse" as we previously thought. Some of the forms of these fossils (such as critters with 5 eyes or 3 legs) were so different from modern life that people assumed that numerous new phyla suddenly appeared, and competition between them caused some to become extinct, leaving behind only the more successful ones. But modern interpretations show that it is not necessary to assume such a plurality of phyla, that the great diversity seen in the fossil record may reflect modifications and variations of much fewer fundamental body plans.

Given these things, it is still remarkable how much life changed at the start of the phanerozoic. This was truly the beginning of large, macroscopic, complex life - the period in which such life became established, evolved, and led to most of the forms we see today.

Landmasses during the PhanerozoicPlate tectonics drives the movement and formation of continents, and over the history of the Earth, continents have appeared, changed shape, moved around, and sometimes disappeared. Here is a short blurb I wrote on the presumed history of different continents. Prior to the phanerozoic, the supercontinent Rodinia existed between about 1.1 billion and 750 million years ago. Rodinia was a barren, desolate landmass, since life had not yet colonized land. Around 750 million years ago, Rodinia started to rift apart, and broke into 3 major pieces: Proto-Laurasia, Proto-Gondwana, and the Congo craton. Proto-Laurasia drifted southwards towards the South Pole. Proto-Gondwana rotated northwards, and for a time, the Congo craton lay between these two landmasses, forming another supercontinent - Pannotia - around 600 million years ago, just before the start of the phanerozoic. Since the two major landmasses lay towards the poles, joined by a much smaller landmass at the equator (the Congo craton), the climate is presumed to have been very cold, with extensive glaciation.

Temperature record of the Earth during its 4.6 billion year history (from Barry Saltzman, Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, Academic Press, New York, 2002, fig. 1-3).

The radiation of complex life starts with this period, around 563 million years ago, in the Ediacaran. This was a period of major changes. Pannotia existed for a very short time - around 60 million years - because the collisions that formed it were glancing collisions between landmasses that were already rifting and breaking up. Around 540 million years ago, the breakup of Pannotia was complete - into 4 major landmasses: Laurentia, Gondwana, Baltica and Siberia.

The last supercontinent was Pangaea, which formed during the early Permian, about 300 million years ago. This landmass existed more or less intact for the next 100-150 million years, until it started to break up in the early-mid Jurassic. The break up was in 3 phases, beginning with the early/mid Jurassic up to the end of the Cretaceous and the beginning of the Cenozoic, around 50-60 million years ago.

These events are important in understanding the distribution of plants and animals, for example, in explaining why North American dinosaurs in the Cretaceous did not spread across the rest of the world (north America had already broken away from the Pangaean landmass during the mid-Jurassic). Also, the breakup and rifting of this huge landmass created many shallow seas, which were important habitats for many animals that existed during the time.

Climate during the PhanerozoicIt is difficult to estimate the climate or average temperature of the Earth in past geological ages. Various indirect means are used to deduce these things, and the degree of error is very high the farther back in time we go. More accurate readings are available for relatively recent times, based on ice cores taken from the Antarctica or the Greenland ice cap. The Vostok and EPICA cores from the Antarctica go back at least 450,000 years. For earlier periods, the records are much less reliable.

The accompanying climate graph shows the estimated temperature of the Earth over its 4.6 billion year history. We assume that the Earth cooled down rapidly from the hot, semi-molten state in which it was formed, and within the first 100 million year or so, the surface was cool

enough to support liquid water. Of course, surface temperatures were still much higher than they are today. By about 4 billion years ago, the Earth was on a cooling trend, but average global temperatures were still much higher than today (about 25-28 °C compared to about 15 °C today).

Atmospheric Oxygen Levels, two different records, based on (1) Berner, R, et al., 2003, Phanerozoic atmospheric oxygen, Ann. Rev. Earth Planet. Sci., V, 31, p. 105-134, and (2) Falkowski, P, et al., 2005, The rise of oxygen over the past 205 million years and the evolution of large placental mammals, Science, V. 309, p. 2202-2204 (Sept. 2005)

Note that the time scale (y-axis) on the climate graph is not to scale. The pre-cambrian is shown on a much smaller scale, followed by the paleozoic and mesozoic on the same medium scale, and finally the quaternary on a much larger scale. In the early (pre-cambrian) record, we see high average temperatures, with one large dip in the temperature graph, corresponding to the Huronian glaciation, about 2.4 - 2.1 billion years ago during the Siderian period. Life has existed on Earth for at least 3.5 billion years (perhaps even 4 billion years), but around 2.8 billion years ago, with the evolution of photosynthetic organisms, oxygen started being produced in large quantities. For many hundreds of millions of years, this oxygen production produced no discernable rise in atmospheric oxygen, due to the presence of various oxygen sinks or buffers. However, around 2.5 billion years ago, enough oxygen had accumulated to produce an Oxygen Catastrophe. Life during that period was anaerobic, and oxygen was toxic to it. During the oxygen catastrophe some trigger either released vast amounts of accumulated oxygen, or else the oxygen buffers reached their capacity, and free oxygen started rising, causing a mass extinction event. This was possibly the trigger for the Huronian glaciation, though the exact mechanism remains unknown.

The next major dip in the graph occurred around 800 - 630 million years ago, during the Cryogenian. This was the most extensive cooling the Earth has ever seen, and resulted in the "snowball Earth" scenario, when glaciers possibly covered the entire Earth. This cooling may have been associated with the formation and breakup of the supercontinent Rodinia, which interrupted oceanic currents that disperse heat from the equator.

The phanerozoic begins with the warming of the Earth after the Cryogenian. The initial radiation of metazoa precedes the phanerozoic by a few million years, in the late Ediacaran period. This is pretty much the period when the Earth had warmed to temperatures similar to the present, after the cryogenian glaciations.

During the first part of the phanerozoic - the paleozoic era, the Earth was somewhat warmer than today. Glaciations were rare, and the climate was much more stable than today. The two notable glaciations during this period occurred during the late Ordovician and late Carboniferous, and both are associated with extinction events. For much of the paleozoic, the Earth was free of glaciers and sea levels were high. This turned many of the lower altitude parts of the continents into shallow seas, where marine life first took hold. During the early paleozoic, most metazoan life was still in the sea - it was not until the middle paleozoic that life on land first took hold.

The middle part of the phanerozoic - the mesozoic, was accompanied by rising temperatures and a stable climate, with no glaciations. In the early mesozoic, the climate was hot and dry. Pangaea was a huge landmass, and the interior parts must have been vast deserts, being far away from the oceans. Despite the high temperatures (probably about 10 °C warmer on average than today), sea levels remained low, because the landmass was clustered into one large continent.

In the Jurassic, Pangaea started to break apart. Although temperatures continued to rise, the spreading of the sea floor increased sea levels, as new crust was formed on the sea floors. This caused flooding of the low lying coastal areas, and together with the increase in coastlines due to the breakup of Pangaea, the climate became much more humid. The vast deserts of the Triassic retreated, and for the most part, were confined to the interiors of continents.

Oxygen levels were lower in the Jurassic than they are today. Good estimates are lacking, so it's hard to say how much lower. The accompanying graph shows the oxygen levels according to two published papers. The more recent record shows the Jurassic starting with oxygen levels below 10% (though they rapidly rise to 15-17% for the rest of the mesozoic). These figures have been disputed. Many people believe that oxygen levels were at least high enough to support natural combustion, which requires atmospheric oxygen levels of at least 12% (at least 15% according to some newer publications).

The climate of the late mesozoic, or Cretaceous, is less certain. Some people believe that the much higher levels of carbon dioxide which existed at the time produced a flat temperature gradient across the whole Earth, with very little dependence on latitude. If this is true, equatorial regions must have been hot enough to be largely deserts, despite the presence of nearby oceans. No glaciers could exist even at the poles with such a flat temperature gradient. This is also disputed, since some computer models indicate that glaciers should exist at least at the poles. However, there is no geological record of any glaciations during the entire mesozoic.

High temperatures would have also warmed the oceans, and some models show that the oceans may have reached temperatures of up to 20 °C, even in the deep ocean. This would have been too warm for sea life, and probably vast volumes of the ocean lacked enough oxygen as well to sustain life. These conclusions remain uncertain, though, and more research is needed to better understand the climate during the cretaceous.

The late phanerozoic, or Cenozoic period shows continuing warm temperatures at the beginning, but a prolonged cooling trend starting towards the end of the Eocene. This was due to the separation of South America from the Antarctica (the opening of the Drake Passage), which allowed the formation of the Antarctica Circumpolar Current, which brings cool water from the depths of the Antarctica to the surface.

The cooling trend continued through the Miocene, with small fluctuations between warmer and colder periods. Towards the end of the Miocene, South America became attached to North America, forming a continuous land barrier between the Atlantic and Pacific oceans. This caused the strengthening of the Humboldt Current and the Gulf Stream, which rapidly cooled down the Arctica. This cooling of the northern hemisphere produced a steeper cooling trend, specially since the start of the Pleistocene, which continue to this day. This has led to cycles of intense glaciations, called ice ages, during which glaciers advance from the north and form mile-thick sheets of ice over the northern landmasses. We are currently in an interglacial warm period.

Paleozoic Era

This was the first and longest era of the phanerozoic eon, covering the first half of of the phanerozoic, from about 542 - 251 million years ago. It started with the breakup of Pannotia, a short lived supercontinent that existed after the parts that made up Rodinia broke apart and briefly combined again, and ends with the formation of Pangaea, the last great supercontinent.

This was a very eventful period for life on Earth, beginning with the appearance of most modern phyla, major radiations of various groups of organisms. There were major radiations

of several groups of organisms. The first land plants appeared, and eventually became vast forests. There was great diversity and experimentation. And then it all ended abruptly, with the largest mass extinction of life that the Earth has ever known.

The paleozoic is divided into the following periods.

CambrianThis is the first period of the first era (paleozoic) of the phanerozoic eon. The beginning of the Cambrian is somewhat controversial, and different geologists place it at different points in time. The International Commission on Stratigraphy decided to date it to 542 ± 0.3 million years ago, based on a carbon excursion that can be precisely dated to that time. It ends about 490 million years ago.

As mentioned earlier, multicellular life had started to radiate prior to the Cambrian, in the Ediacaran period of the neoproterozoic. However, this life appears to be somewhat different from the forms found in the early Cambrian. Partly for this reason, it is believed that there was an extinction event at the beginning of the Cambrian. Evidence for this extinction event is also found in the carbon excursion that is used to date the beginning of the Cambrian. It appears that many of the earlier forms of life that appeared in the Ediacaran suddenly disappear at the beginning of the Cambrian, soon to be replaced by other (equally complex) life forms. Even some very ancient life forms (such as stromatolites - colonies of cyanobacteria) which had existed for billions of years, in great profusion, almost vanished in this extinction event, the cause of which is not known.

Trilobites were very common in the Cambrian

The first hard shelled animals appeared during the Cambrian, around 530 million years ago. These are trilobites, a now-extinct class of arthropods, as well as the first crustaceans and mollusks. Note that arthropods may have appeared even earlier. There is some speculation that Parvancorina and Spriggina, elements of the Ediacaran biota about 550 million years old, were also arthropods. Hard shelled organisms fossilize well, hence the profusion of trilobites in the fossil record. The first coral reefs probably appeared in the Cambrian, as well as the first vertebrates, such as Myllokunmingia.

However, there are places where soft-bodied Cambrian fauna have been well-preserved, such as the Burgess Shale, which is about 505 million years old, around the middle Cambrian. Some of the early Burgess Shale finds provoked much controversy, since they seemed to represent animals completely unlike any that are found today, and with very awkward designs. There were examples such as Opabinia, with 5 eyes, a backward facing mouth underneath its head, and a proboscis extending forward from the head to end in a spined claw. Another example was Hallucigenia, which was supposed to have rows of rigid spines on which it walked, with tentacles waving at the top. It was bizarre reconstructions like these which led authors like Stephen Jay Gould to think that the Cambrian was a time of great experimentation due to unusual evolutionary pressure, and that most of these "experiments" failed and did not lead to any modern phyla.

More recent studies have cast doubt, both on the reconstructions, and on the theory. Hallucigenia is not so strange if it's reconstructed upside down - the "tentacles" then become legs for walking, and the rigid spines possibly a defensive mechanism, similar to many later organisms. Opabinia has also been found to be closely related to arthropods, as have many other seemingly bizarre animals from the Cambrian.

So while it remains true that there was a high degree of diversity during the Cambrian, it is less certain if this was anything exceptional, when we find similar degrees of high diversity during the Ediacaran immediately preceding it, as well as during the Ordovician radiation that followed.

Most of Cambrian life evolved and lived solely in the shallow seas that were formed as the supercontinent Pannotia split apart. There is no generally accepted evidence of any life on land at the time. It is possible that some regions of land did have some sort of microbial "scum", consisting of bacteria, algae, or lichens. Such microbial land cover may have evolved even before the Cambrian, though there is little direct evidence of it. Without plants, soils cannot exist. Land would have been either barren rock, with weathered patches of sand. Sand is not capable of holding water. However, films of cyanobacteria have been found even in modern deserts, so it seems that something similar could have existed long ago, before there were any plants. Cyanobacteria, algae and lichens (symbiotes, consisting of a fungus with a photosynthetic partner such as cyanobacteria) may have existed on land in very ancient times.

OrdovicianThe end of the Cambrian was marked by another extinction event, which can be dated to approximately 488.3 ± 1.7 million years ago. This marks the beginning of the Ordovician period, which lasted about 44.6 million years, up to about 443.7 ± 1.5 million years ago. The end of the Ordovician is also associated with a major extinction event, which wiped out about 60% of the existing genera at the time.

Most of the Earth's landmass was still in the southern hemisphere at the time, and the climate was warmer than today. There were numerous shallow seas, and a great deal of sedimentary rock dating to this period still exists today. Towards the end of the Ordovician there were some glaciations, so sea levels rose and fell accordingly, but were generally higher than today.

Artist's Impression of Life in Ordovician

Ordovician fauna was very diverse. There was a significant radiation of life during the Ordovician, representing about 12% of all phanerozoic fauna. There were 4x as many marine organisms as during the Cambrian.

Trilobites diversified rapidly during the Ordovician, reflecting the increasing evolutionary pressure of co-evolving organisms. Some trilobites developed ridges and spines as a defensive measure, others started swimming for the first time, instead of just crawling along the sea floor. Cephalopods (from which the modern Octopus is descended) and crinoids developed first during the Ordovician. Other forms of marine life from this period include primitive nautiloids and sharks, the first mosses (bryozoa), and the first jawed fish.

Two important developments of this period include the great profusion of shell secreting organisms (which sequester carbonate in their shells) and the first appearance of land plants. Plant spores dating from the late Ordovician have been found. It is uncertain what the nature of these first plants was. It seems likely that they were some sort of avascular plants, such as moss (bryozoa), though it is possible that marine fungi first colonized the land, in the form of lichens, which are a symbiotic combination of a fungus and some photosynthetic algae.

Towards the end of the Ordovician there was a series of glaciations, which probably led to the end-Ordovician extinctions. These were fairly severe, causing the loss of about 50-60% of all existing genera. It is thought that a series of glaciations raised and lowered sea levels repeatedly, severely affecting the many shallow seas where life flourished. These glaciations are linked with a lowering of atmospheric carbon dioxide. It is uncertain if life had any role in this, though the diversification of carbonate-shell secreting organisms during the Ordovician seems suggestive.

SilurianThe Silurian began after the end-Ordovician extinction event, about 443.7 ± 1.5 million years ago, and ended about 416 ± 2.8 million years ago. This was a period of recovery after the extinction, during which the glaciers largely retreated, the climate was warm, and there was a minor greenhouse effect going. The climate was relatively stable, unlike previous ages which had been marked by large fluctuations. The warm climate resulted in the melting of glaciers, and therefore sea levels were relatively high.

The split up of Rodinia was far advanced in the Silurian. Land consisted of the supercontinent of Gondwana in the southern hemisphere, surrounded by about 6 smaller continents. The northern hemisphere was mostly a single large ocean. There was no major volcanic activity during the Silurian, although the Caledonian orogeny (which had begun earlier, at the start of the Ordovician), was in full swing.

Silurian biota include the first coral reefs, the first bony fishes, and the first fishes with movable jaws. Arthropods grew to huge sizes, specially the Eurypterids (sea scorpions), which grew to sizes of 6-7 feet, and must have been formidable predators. The earliest common evidence of life on land (arachnids and centipedes) dates from this period, though some arthropods may have colonized land much earlier (in the Cambrian). The earliest recognizable shark scales are from the Silurian. The first leeches appeared also at this time.

On land, the first record of vascular plants dates to this period. There were probably extensive "forests" of mosses, lichens, and the early vascular plants (which show the first signs of xylem and phloem), such as Cooksonia, Baragwanathia, and Psilophyton.

The Silurian ended with a series of minor extinction events, probably due to climate change or impact events.

DevonianThe Devonian period lasted from about 416 ± 2.8 million to 359.2 ± 2.5 million years ago, a duration of almost 57 million years. This was a relatively warm period, glaciation was minor or non-existent, with consequently high sea levels. A lot of land was submerged beneath the water, forming shallow seas. This was a period of high tectonic activity, as the continents were moving closer together in a process that would eventually lead to the formation of Pangaea. Although the climate was warm, carbon dioxide levels fell through the Devonian, as growing forests of plants on land locked away carbon and became buried (there are oil and gas deposits found today in some Devonian rocks).

Artist's Impression of Devonian Forests

There were significant developments in the Earth's biota during the Devonian. The first vascular plants, which had appeared earlier during the Silurian, now spread across much of the land. In the early Devonian, these plants were quite small, probably no more than a meter in height. In the late Devonian, plants such as lycophytes, sphenophytes, ferns and progymnosperms appeared, which were much larger. Many of these plants had true roots and leaves, which were not a feature of earlier plants. The landscape was probably dominated by

huge ferns, and contained many other strange plant-forms, such as the giant fungus Prototaxites (the tall tree-like things in the accompanying artists impression, with tree-like trunks and branches), which formed trunks as wide as a meter and grew to heights of nearly 30 feet.

Towards the end of the Devonian, the seed-bearing plants (spermatophytes) and the first real trees appeared, such as the progymnosperm Archaeopteris, which was probably among the first plants with true wood, thus being a tree. At the end of the Devonian, huge forests existed throughout the land masses, unmolested by land herbivores, which had not yet developed. This enormous diversity of plant life that appeared in the late Devonian is sometimes known as the "Devonian Explosion".

A recent discovery of footprints in what is now southeastern Poland indicates that the colonization of land by animals began fairly early in the Devonian. These footprints have been dated to about 397 million years old, and include a group of animals called tetrapods. Tetrapods developed in the shallow seas long before they walked on land. It is believed that the coasts of such seas, as well as freshwater swamps, formed rich ecosystems, as plants colonized the land. Tetrapods probably evolved in such habitats close to the water's edge, where the profusion of plants made limbs useful in moving around in the underwater clutter of roots, stems, and decaying plant matter. The first tetrapods were probably completely aquatic, and only later did they develop the ability to survive outside the water. These were probably shallow, tidal marine environments, where water surged and retreated with the tides, and it was beneficial to be able to both swim and walk. Some of the tetrapods represented by the footprints from Poland show that they grew up to 10 feet in size.

Devonian fishes, including sharks, ray-finned fishes, placoderms.

It's important to remember that although tetrapods were the first vertebrates to colonize land, arthropods had already done so much earlier. The earliest evidence of life on land goes as far

back as the Cambrian. Track ways are found in what must have been wet coastal sand, of a burrowing organism known as Climactichnites, which was possibly an arthropod. During the early Devonian, the early land vegetation (mostly tiny shrubs and plants, many without root systems and leaves, some without vascular systems), probably provided ecosystems for various types of arthropods and the first true insects (which appeared at the beginning of the Devonian, about 426 million years ago) - such as mites, wingless insects, etc. These early land insects are not well known, but they appeared in the early Devonian, whereas the first land vertebrates did not appear until the end of the Devonian. The development of trees with true roots during the late Devonian led to the creation of the first soils, which were probably colonized by burrowing and crawling insects and arthropods. These plants, soils, and insects formed rich ecosystems, which were probably necessary for the beginning of the movement of the first vertebrates to the land, at the end of the Devonian.

A recently discovered fish fossil, showing a fish embryo attached to its mother through an umbilical cord, shows the first evidence of animals giving live birth to their young. This has been dated to about 375 - 380 million years old, the late Devonian. This fish was a placoderm, a kind of armored fish that was very common in the middle paleozoic. This particular species was about 10 inches long, though other placoderms grew up to 20 feet in size. Placoderms became extinct at the end of the Devonian.

The Devonian ended with two extinction events. The earlier event is dated to about 364 million years ago, when most of the fossil agnathan fishes disappeared. A second wave of extinctions followed soon after. These extinctions primarily involved the marine environment, and primarily the warm and shallow seas where life was profuse. Land plants/animals and deep/cool water fishes were much less affected. These extinction events marked the end for many genera, being more severe than the end-Cretaceous extinction that wiped out the dinosaurs. The cause of the extinction remains unknown, though various theories have been proposed (such as asteroid impact, loss of atmospheric carbon dioxide as it was locked up by forests). A cooler climate at the end of the Devonian, with extensive glaciation, is taken to be the probable cause of the extinction.

CarboniferousThis period extends from about 359.2 ± 2.5 to about 299 ± 0.8 million years ago, a length of about 60 million years. The period is named after "carbon" or coal, since huge deposits of coal dating back to this period have been found all over the world. This period is typically divided into two epochs, the earlier Mississippian, and the later Pennsylvanian.

The dip in temperatures at the end of the Devonian quickly reversed at the start of the Carboniferous, and for at least the first half of the Carboniferous, the climate was quite warm. However, about 320 million years ago, there was a sudden precipitous drop in temperature, the onset of the Permo-Carboniferous glaciation. This point, which is marked by the division of the Mississippian and Pennsylvanian epochs, is seen in the geological record as a minor extinction event, which hit the crinoids and ammonites specially hard. For the rest of the Carboniferous, the climate was much cooler, and shows a record of repeated glaciations. However, the tropics continued to remain warm and tropical coal-generating forest continued to flourish.

Carboniferous Coal Forests

The extensive formation of coal from this period is somewhat puzzling. Coal forms when hydrocarbon material (dead plants and trees) are buried and not decomposed. Over time, pressure and heat turns them into coal. In modern times, this is not so common, because various organisms quickly decompose dead organic matter.

There were probably many reasons for coal formation during this period. One is the development of trees with bark. Bark is composed of lignin, a chemical compound found in cell walls, which probably first evolved in the Carboniferous. Barked trees were very common in the Carboniferous, and they contained a lot of bark. Bark to wood ratios as high as 8:1 were common in trees of this period, and ratios as high as 20:1 have been found for some trees. This contrasts with ratios of about 1:4 for modern trees, so we can see that trees in this period had up to 80 times more bark per volume of wood than trees today.

Lignin is toxic, and inhibits the decay of organic material. It is likely that microbes that can decompose lignin had not evolved at the time. Even today, few organisms other than Basidiomycitic fungi can digest lignin. It probably evolved to protect the trees from insect herbivores, which were the dominant herbivores at the time. This was also a time before effective insectivore animals had developed, so the populations of such insect herbivores much have been very high.

Another factor may have been the late Carboniferous glaciations, and the consequent fall in sea levels. This produced huge lowland swamps in Europe and North America, where it is especially easy for dead vegetation to get buried quickly.

The burial of vast forests and swamplands produced a surplus of oxygen in the atmosphere, which may have peaked as high as 35% (compared to 21%) today. This led to insect and amphibian gigantism, since insects specially are size limited by atmospheric oxygen concentrations, as their respiratory system does not allow for the diffusion of gases very efficiently at large mass to surface area ratios.

As the continents continued to move closer together, in process of forming Pangaea, the shallow seas that had separated continents began to shrink. Much of the marine environment of previous periods had consisted of such shallow seas and extensive shorelines. This had the result of increasing the amount of dry land available as habitat, at the expense of shrinking marine environments. The trend was further aggravated during the later Cambrian, as glaciation locked way water and decreased sea levels. This may have been the reason for the evolution of land vertebrates, which first became fully terrestrial during the Cambrian.

Land vertebrates such as amphibians were very common in the Carboniferous, and much more diverse than they are today. Some grew to large sizes, as long as 20 feet, though most were smaller. The amphibians were partly aquatic, but many fully terrestrial animals also developed in this period, often with scaly skins to prevent dehydration and protect them. The development that made terrestrialism possible was the evolution of the amniote egg, with its several protective membranes, that allowed the eggs to survive and hatch on land. Amphibians are not amniotes - their eggs require water, so the amphibian lifestyle requires close association with bodies of water. The amniote egg may have developed as early as 340 million years ago, near the beginning of the Carboniferous, as evidenced by fossils of Casineria, a small lizard like creature, about 6 inches long. Casineria may have been the first amniote, and therefore the first known vertebrate to adopt a fully terrestrial lifestyle. It is

sometimes placed in the stem group protosauria (or "first lizards"), which includes some amphibians as well as early reptiles.

Early in their history, the amniotes split into two branches: the synapsids (proto mammals) and the sauropsids (proto birds/reptiles). This split happened about 320 million years ago, probably near the beginning of the Permo-Carboniferous glaciations, and the beginning of the Pennsylvanian epoch. Hylonomus, the earliest confirmed reptile, dates back to about 315 million years ago. It was about 6-8 inches in size, and probably ate insects and centipedes. The earliest undisputed synapsid was Archaeothyris, a somewhat larger lizard like creature (about half a meter in length), dated to about 306 million years ago.

By the end of the Carboniferous, both synapsids and sauropsids had diversified into a number of groups and spread across the land, possibly in response to the drier climate of the late Carboniferous.

Insects continued to proliferate during the Carboniferous. The high concentrations of atmospheric oxygen (35%) led to gigantism, as seen in Meganeura, a giant dragonfly-like insect with a wingspan of over 2.5 feet. Other groups to emerge included the ancestors of mayflies, and the ancestors of cockroaches.

The marine environment also greatly diversified during the Cambrian. The foraminifera first became prominent in the marine fauna. Echinoderms, radiolaria, sponges, brachiopods, annelids, gastropods and cephalopods were all numerous among marine invertebrates. Trilobites also existed, but were less common than in earlier periods. Aquatic vertebrates also diversified, in both freshwater and marine environments. Some were very large, such as the rhizodonts, which grew up to 20 - 25 feet, making them the largest freshwater fish ever known.

Sharks had a major evolutionary radiation in the Carboniferous, probably due to the extinction of the placoderms at the end of the Devonian. Sharks probably took over the different niches that had been previously occupied by placoderms.

Among land plants, many earlier Devonian lineages continued, but new plants also appeared. Horsetails and cycads first appeared. Seed ferns and other early gymnosperms continued to flourish, as did various lycophytes. Towards the end of the Carboniferous, the first conifers appeared, usually situated away from the water, in higher, drier ground.

PermianThis was the last period of the Paleozoic, and dates from about 299 ± 0.8 to 251 ± 0.5 million years ago, a duration of about 48 million years. It ends with the Permian-Triassic extinction event, probably the worst extinction event in the history of life on Earth.

The Permian saw the completion of the supercontinent Pangaea, which reached its maximum extent about 225 million years ago, though it lasted in some attenuated form for another 100 million years. During the Permian, most of the Earth's land was assembled in a giant C-shaped continent straddling the equator. This assembly of landmasses into one large group drastically reduced the coast lines, decreasing the available shallow marine habitat.

The middle of the "C" shape was the Tethys Sea, and the remaining globe was covered by a single large ocean, known as the Panthalassic Ocean. The large continental landmass meant that the interiors of continents were very arid, and probably large deserts existed. Temperature fluctuations in the interiors of continents must also have been extreme. Monsoon conditions (with highly seasonal rainfall) probably prevailed over much of the land.

The Permian started with the ice age at the end of the Carboniferous. The ice age ended rapidly, and for much of the rest of the Permian, the climate was warm and dry, with alternating warming and cooling periods. Towards the end, the Permian was probably about 60% hotter than today, due to volcanic activity producing greenhouse gases.

The equatorial regions, specially near the Tethys Sea still contained large amounts of swamp land. This area is now the southern region of China, where large Permian deposits have been found. However, further away, conditions were more extreme, and favored the evolution of conifers. This happened somewhere around the middle of the Permian, and conifers quickly spread over much of the inland areas. Many modern trees, such as gingkoes and cycads originated in this period.

Pangaea, about 225 million years ago.

Roaches prospered greatly in the Permian. They were well adapted to the conditions, with an omnivorous digestive system, a gizzard, and sophisticated mouth parts. About 90% of Permian insects were roach-like. Flying insects were dominated by huge predatory dragonflies. Gigantism continued, with species of dragonflies achieving wing spans of over 2 feet. Other important new insect groups that evolved during this period were the beetles and flies.

Land vertebrates included both synapsids and sauropsids. The early Permian was dominated by amphibians and pelycosaurs, which were a class of synapsids. The pelycosaurs varied in size from a few inches up to ten feet and more, eventually giving rise to the therapsids in the

middle Permian. Towards the end of the Permian, a branch of the therapsids known as the cynodonts evolved, which would later give rise to mammals in the Triassic. Initially, the sauropsids were not as successful as the synapsids, but towards the end of the Permian, they gave rise to diapsids, such as archosaurs. The coming end-Permian extinction was to change matters drastically, with the synapsids losing their dominance, and the descendents of the archosaurs (dinosaurs, crocodiles, other reptilians) becoming the dominant vertebrates for the next hundred million years, and more.

The Permian ended with the most catastrophic extinction that life on Earth has ever seen - the Permian-Triassic extinction event, which happened about 251 million years ago. About 90-95% of all marine species and about 70% of all land species became extinct. It's thought that about 99.5% of all living organisms died during this event, leaving behind only about 0.5% to populate the earth in the Triassic. Many ancient lineages, which had survived for hundreds of millions of years, such as trilobites, disappeared after this extinction. Indeed, so many species died out that instead of enumerating what perished, it will be easier to list what remained, in the next section on the Triassic.

The cause of the Permian-Triassic extinction is not known. Some of the more popular hypotheses are:

Volcanism in Siberia

Massive flood basalt eruptions in Siberia which continued for nearly a million years, and formed what is today known as the Siberian Traps, may have been the cause. The eruptions may have caused a nuclear winter scenario which lasted for several years, coinciding with major eruptions. Some of these eruptions produced up to 2000 cubic kilometers of lava, and there were many of them, over the course of hundreds of thousands of years. By contrast, the largest eruption in recorded history, the eruption of Mount Tambora in 1812, only produced about 160 cubic kilometers of ejecta. The eruption of Vesuvius in 79 AD, which wiped out Pompeii and Herculaneum only produced 4 cubic kilometers of ejecta. The eruption of Thera in the second millennium BC (which ended the Minoan Civilization) produced about 60 cubic kilometers of ejecta. The aftermath of the Siberian volcanism may have elevated global temperatures by as much as 5 °C due to greenhouse gases, which is an extreme amount if it happens over a short period.

Deep Sea Methane

This is actually an extension of the Siberian volcanism theory. The rise in temperature caused by the volcanism (about 5 °C) might not be enough to cause such a severe extinction as this, but it might be enough to warm the oceans sufficiently to melt the methane hydrate reservoirs at the bottom of the oceans. Methane is one of the most powerful greenhouse gases, and a massive release of methane would lead to a severe greenhouse effect. This theory is supported by the finding of increased carbon-12 levels in the middle layers of deposits from the end-Permian event, and the particular sequence of extinctions (land based extinctions, followed by marine extinctions, followed by more land based extinctions).

Ocean Venting of Hydrogen Sulfide

The deep sea is a relatively anoxic zone, and periodically, it loses almost all of its oxygen. At such times, bacteria produce large amounts of hydrogen sulfide, which accumulates in the

depths. This hydrogen sulfide can be released suddenly into the atmosphere. This can cause extinctions in several ways. Hydrogen sulfide itself is toxic to aerobic organisms. Once in the atmosphere, it is rapidly oxidized, depleting oxygen in the process, leading to lowered levels of oxygen in the atmosphere. Finally, it can destroy the ozone layers, exposing the Earth to the Sun's ultraviolet radiation.

Impact Event

The Wilkes Land crater in the Antarctica has been dated to roughly 100 - 500 million years old. Since this is roughly (very roughly!) the period of the end-Permian extinction, some people have wondered if there might be a relationship. This theory seems somewhat weak, because the amount of iridium and fractured quartz at the boundary is significantly smaller than the amount found at the extinction event 65 million years ago (when the dinosaurs died). Since the end-Permian extinction was by far the larger extinction event, one would expect more, not less signs of the impact event compared to the later extinction event. However, there can be doubts, since the end-Permian is much older, and the signs of impact are probably more obliterated by time. The crater is certainly larger (about 500 km) than the one associated with the more recent extinction (Chicxulub crater, 180 km), which is expected. But there are other problems with the theory as well, such as fossils in Greenland, which show that the extinction event lasted nearly 80,000 years, far too long to be caused by a single catastrophic event such as an asteroid.

Most scientists believe that the extinction was caused by a combination of some of the theories listed above, together with ongoing effects such as the shrinking of coastlines due to the formation of Pangaea, the change in climate, etc. There are other, more speculative theories as well, such as a nearby supernova in the Milky Way.

Sumber : http://geology.answers.com/fossils/life-in-the-phanerozoic-eon

Geology is the study of the Earth. It examines how the Earth's structures formed, changed, and made it possible to support life. Sometimes deep in the earth, geologists find fossils, petrified remains of creature that once existed long ago. Geologists believe that the Earth is 4.5 billion-years-old, and because of great length of time, scientist use a time scale that divides the Earth's history into four great eons: Hadean, Archean, Proterozoic, and Phanerozoic. Each eon is then further divided into eras, and each era is divided into periods.

Phanerozoic EonThe Phanerozoic Eon consists of three eras: Paleozoic, Mesozoic, and Cenozoic. This eon began 542 million years ago and continues to the present day. It was during this time that multi-cellular life began to flourish on the Earth. All of the planet's creatures, from humans to wooly mammoths to Tyrannosaurus Rex, lived during the Phanerozoic Eon. All of the preceding four billion years of history led up to the explosion of life seen in this eon.

Phanerozoic and Proterozoic BorderThe Proterozoic Eon ran from 2.5 billion years ago to 542 million years ago. During this time, the Earth's atmosphere became able to support oxygen. Single-cell life started during the Phanerozoic, and towards the end of the age soft-shell multi-cellular organisms like those

found in the Francevillian Group Fossils formed. The traditional boundary between the two eons is when trilobites and reef-building animals start to appear. After this point, life on Earth would become increasingly complex.

Paleozoic EraThe first part of the Phanerozoic Eon consists of the Paleozoic Era. It ran from 542 million years ago to 252 million years ago. This era is further subdivided into six different periods. Fossils remain mostly aquatic until the middle of the Paleozoic, revealing different types of fish and shellfish. Insects, amphibians, and reptiles all began to evolve during the latter part of the Paleozoic. This era ended with the greatest mass extinction event ever, which killed up to 96 percent of all aquatic life.

Mesozoic EraThe Mesozoic Era is the second part of the Phanerozoic Eon. It ran from 252 million years ago until 66 million years ago. This was the time of the dinosaurs, which seemed to flourish after the extinction event that ended the Paleozoic Era wiped out most of the life on Earth. The Mesozoic Era consists of three time periods: Triassic, Jurassic, and Cretaceous. The fossil record reveals increasingly complex life forms up until the end of this era due to another mass extinction.

Cenozoic EraThe Cenozoic Era is the final part of the Phanerozoic Eon. It runs from 66 million years ago up to the current day. The Cenozoic consists of three major time periods separated into millions of years. The fossil record reveals that this is the time when mammals began to evolve in new ways after the disappearance of the dinosaurs. Mammals were also able to evolve when the climate rapidly changed, as the fossil record proves during the Ice Ages.

The Phanerozoic Eon is the current eon on Earth. It began at the time when life began to flourish on the Earth. It continued through the great extinctions that brought about the end of the Paleozoic and Mesozoic Eras. The fossil record reveals the great variation of all the life that once lived upon the Earth.

Sumber : http://essayweb.net/geology/quicknotes/carboncycle.shtml

Carbon Cycle

Carbon is the 4th most abundant element in the universe. Is it essential to all life as we know it. All living organisms contain carbon. Additionally, carbon is present in rocks, dissolved in rivers, lakes and oceans, and in the atmosphere as carbon dioxide.

The movement of carbon across these reservoirs is the carbon cycle. Life has a significant role in the carbon cycle. Green plants and bacteria use atmospheric carbon dioxide (plus some dissolved carbonates in the case of aquatic organisms) to create the molecules of life -

carbohydrates, proteins and fats. Organisms higher up in the food chain eat these plants or other animals.

Plants and animals respire, that is, burn fuel in order to live. The products of respiration include carbon dioxide, which is released into the atmosphere. When organisms die, their remains decompose, also releasing carbon back into the atmosphere and the soil.

In addition to the biological turnover of carbon, the Earth itself has a carbon cycle, with carbon being continually released from carbon sources and removed by carbon sinks. The geological carbon cycle works over millions of years, whereas the biological carbon cycle works over periods from days to a few thousands of years.

The image above shows the carbon cycle. The green numbers next to each label represent the carbon reservoirs, in units of billions of tons (gigatons). For example, the atmosphere contains about 750 gigatons of carbon, mostly in the form of carbon dioxide, but also trace amounts of other gases, such as methane. The soil contains about 1580 gigatons, in the form of organic matter, bacteria, etc. Fossil fuel reservoirs hold about 4000 gigatons.

As can be seen, the bulk of the carbon is in the deep ocean, around 38,100 gigatons. The numbers in red show the carbon fluxes (per year) between different carbon pools. The numbers are also in gigatons.

Geological Carbon CycleThis occurs over millions of years. Rain washes atmospheric carbon dioxide down to the soil and the sea. Carbon dioxide in the soil exists as carbonic acid, which combines with minerals in the soil to form carbonates - a process known as weathering. Over time, these carbonates are eroded and transported by wind and water back to the sea. Carbonates in the oceans eventually sink to the bottom; therefore the oceans are a net carbon dioxide sink.

Plate tectonics drives the sea floor deep underground at the subduction zones. As the sea floor gets buried deeper, it heats up and eventually releases the carbon dioxide, which makes its way back to the surface through volcanoes, hotsprings, or gradual seeps.

Plate tectonics also affects the land. Deeply buried carbonate rocks can be pushed upwards, exposing them on the surface. This is happening in the Himalayas, which contain sedimentary carbonate rich rocks which were formed at the bottom of some ancient ocean. Once at the surface, the rocks are once again exposed to weathering and erosion.

In the end, the fate of all carbon leaving the atmosphere is to enter the sea, and become incorporated in the sea floor. This sea floor then releases its carbon back to the atmosphere when it is subducted deep enough beneath the crust. But during the course of this cycle, the various reservoirs can hold carbon for hundreds of millions of years.

Biological Carbon CycleBiology provides a fast turnover cycle superimposed on the geological carbon cycle. The two biolgical processes - photosynthesis and respiration, together are responsible for carbon turnover at a 1000 times faster rate than the entire geological cycle. These processes happen fast enough that seasonal variations in atmospheric carbon dioxide can easily be detected. For example, in the higher latitudes, sunlight is quite seasonal, with shorter days during the winter. Photosynthesis is therefore greatly reduced during winters, while respiration continues pretty much unhindered. So there is a marked seasonal cycle with carbon dioxide levels being higher in winters than they are in the summers.

In marine environments, there is an additional factor to consider. Many marine organisms (phytoplankton) use carbon to make shells. These shells sink to floor when the organisms die, and the sediment on the ocean floor can be compacted over time to form limestone. Other organic matter can also be buried on the ocean floor, and under certain conditions turn into hydrocarbons such as coal and gas. The oceans can therefore serve as carbon sinks over geological time scales. Eventually, of course, all this carbon will also make its way to the

surface due to plate tectonics. But relatively stable reservoirs can last for hundreds of millions of years.

Human ActivityFor most of human history, we had no net impact on atmospheric carbon. Like any other form of life, our contribution to the carbon reservoir was our bodies, and our carbon production consisted of whatever carbon dioxide we breathed out. Whatever we burned as fuel through cooking fires and heating our homes was too small to make any significant difference.

Things started changing with the beginning of industrialization around 1850. Two things happened: the population of humans increased dramatically, and the per capita production of carbon increased as well due to the fuel requirements of industrialization. This dramatically increased the production of carbon dioxide. Coal and oil have been carbon sinks for hundreds of millions of years. The carbon in fossil fuels was sequestered (that is, not in the atmosphere and not part of the carbon cycle) during this period. This carbon is now being released into the atmosphere as carbon dioxide.

Sumber : http://essayweb.net/geology/quicknotes/carbonexcursion.shtml

Carbon Excursions

Carbon occurs in different isotopes, including 12C, 13C and 14C. The first two are non-radioactive, while 14C is radioactive and decays into 14N, with a half life of 5730 years. Because of the short half life, any 14C present at the beginning of the Earth has long since disappeared, and it would not even exist today, if there were not a regenerating mechanism that constantly produces a small amount due to the cosmic ray bombardment of the upper atmosphere. Even so, the amounts of 14C are so small that we can ignore it for most purposes, unless we are looking specifically for it (e.g., radiocarbon dating).

Of the other two isotopes, about 99% of the carbon in the universe (and Earth) is 12C, while the remaining 1% is 13C. Although isotopes are considered to be identical so far as their chemistry goes, it so happens that the enzymes involved in photosynthesis have a slightly greater affinity for 12C than 13C. For this reason, any biogenic carbon (carbon that is part of a living organism or its remains) is slightly lighter than carbon that is not biogenic. The remaining, non-biogenic carbon, by contrast is isotopically heavy, because more of the lighter 12C has been extracted by living organisms from it than the heavier 13C.

Remember that as biological material does not stay put, it is constantly recycled through the carbon cycle. So when an organism dies, unless its remains are sequestered in some way (as in a fossil) and prevented from mixing with its surroundings, biogenic carbon will disperse into the environment and gradually assume the same isotope ratios as inorganic carbon.

Mass spectrometry can easily determine very small variations in the 12C : 13C ratio, and these ratios are commonly used on inorganic carbon. A positive 13C excursion happens when a lot of 12C is locked up in biological material, and as a result the rest of the carbon (such as carbon dioxide in the atmosphere) becomes heavy with 13C. There is often great debate regarding the

reasons for such excursions, but the fact that the excursion happened can be easily measured with a high degree of repeatability.

Figure showing 13C excursions during the Proterozoic and Phanerozoic. The arrows indicate excursions that occurred during the two major glaciations of the Cryogenian period of the Neoproterozoic - the Sturtian and Marinoan glaciations. Adapted from Bartley and Kah, 2004.

The figure shows the carbon ratios during the Proterozoic and Phanerozoic eons. The arrows indicate excursions that took place during the Cryogenian period, and are associated with two major glaciations that have been implicated in the "snowball Earth" scenario.

The cause for these excursions is a matter of intense debate. The carbon cycle is complex even today, and how it worked in the distant past is a matter of conjecture. Many attempts have been made to relate it to geological events such as the snowball Earth (when in many ways, transfer of material between different parts of the Earth, such as between continents and oceans) was greatly impeded, or volcanism accompanying the end of the glaciations, when large amounts of carbon dioxide was released into the atmosphere. However, the data doesn't provide a perfect fit to any of these explanations, and investigations continue.

Note that the end of the Neoproterozoic and the early Phanerozoic are a period of great instability in this history, with carbon ratios constantly rising and falling. This record is difficult to interpret.

Sumber :

http://www.briangwilliams.us/carbon-cycle-2/modeling-the-phanerozoic-carbon-cycle.html

Modeling the Phanerozoic Carbon CycleTue, 13 Sep 2011 00:03:18 | The Carbon Cycle

Together the carbonate-silicate and organic long-term subcycles play the dominant role in controlling the levels of atmospheric CO2 and O2 over millions to billions of years. In this book I show how these subcycles have operated only over the past 550 million years, the Phanerozoic eon. The Phanerozoic is chosen because of the abundance of critical data such as

abundant multicellular body fossils, relatively noncontroversial pa-leogeographic reconstructions, and relatively agreed-upon tectonic and climatic histories. Such a situation is not available for the Precambrian. The plethora of Phanerozoic geological, biological, and climatic data are extremely useful in trying to recreate the history of the carbon cycle. This will be done in the present book. The reader is referred to the books by Holland (1978, 1984) for discussion of the carbon cycle before the Phanerozoic.

All Phanerozoic carbon cycle models to date use analogous formulations for the mass balance of carbon added to and from the Phanerozic rock record (e.g., Budyko and Ronov, 1979; Walker et al., 1981; Berner et al, 1983; Garrels and Lerman, 1984; Berner, 1991, 1994; Kump and Arthur, 1997; Francois and Godderis, 1998; Tajika, 1998, Berner and Kothavala, 2001; Wallmann, 2001; Kashiwagi and Shikazono, 2003; Bergman et al., 2003; Mackenzie et al., 2003). The simplest approach to carbon mass balance modeling is to introduce the concept of the "surficial system" (Berner, 1994, 1999) consisting of the oceans + atmosphere + biosphere + soils (the reservoirs of the short-term cycle). A generalized mass balance expression for the surficial system is:

where

Mc = mass of carbon in the surficial system

Fwc = carbon flux from weathering of Ca and Mg carbonates

Fwg = carbon flux from weathering of sedimentary organic matter Fmc = degassing flux from volcanism, metamorphism, and diagenesis of carbonates Fmg = degassing flux from volcanism, metamorphism and diagenesis of organic matter Fbc = burial flux of carbonate-C in sediments Fbg = burial flux of organic-C in sediments.

An additional mass balance expression for 13C involving the stable isotopes of carbon has been found to be of great help in doing long-term carbon cycle modeling:

+ 8mgFmg 8bcFbc 8bgFbg where 8 = [(13C/12C) / (13C/12C)stnd - 1] 1000. and stnd represents a reference standard. Equations (1.10) and (1.11), when combined with assumptions about weathering, burial and degassing, can be used to calculate the various carbon fluxes as a function of time. More complicated expressions have been used for carbon mass balance in some models where the surficial system is broken up into its parts and separate mass balance expression are used for carbon in the atmosphere, biosphere, and ocean. However, the simpler approach of equations (1.10) and (1.11) will be emphasized in the present book. By lumping the atmosphere, oceans, life and soils together, processes involved in the short-term carbon cycle are avoided in the modeling, and the use of steady-state becomes possible. A diagrammatic presentation of this approach is shown in figure 1.3.

The weathering and degassing fluxes of carbon integrated over millions of years are much larger than the amount of carbon that can be stored in the surficial system (table 1.1). Adding excessive dissolved calcium and bicarbonate to the oceans eventually would result in the global inorganic precipitation of CaCO3. (Adding too little calcium and bicarbonate would result eventually in an acid ocean and the inability to ever form limestones.) The area of land can hold just so much biomass and soil carbon. Too much CO2 in the atmosphere leads to excessive warming due to the atmospheric greenhouse effect. Because of the inability to store much carbon in the surficial system, over millions of years one can assume that the carbon

loss fluxes, due to organic carbon burial and Ca and Mg silicate weathering followed by Ca and Mg carbonate burial, are essentially balanced by degassing fluxes from thermal carbonate decomposition and organic matter oxidation (Berner, 1991, 1994; Tajika, 1998). In other words, there is a quasi steady state such that:

Figure 1.3. Modeling diagram for the long-term carbon cycle. Fwc = carbon flux from weathering of Ca and Mg carbonates; Fwg = carbon flux from weathering of sedimentary organic matter; Fmc = degassing flux from volcanism, metamorphism, and diagenesis of carbonates; Fmg = degassing flux from volcanism, metamorphism, and diagenesis of organic matter; Fbc = burial flux of carbonate-C in sediments; Fbg = burial flux or organic-C in sediments.

Figure 1.3. Modeling diagram for the long-term carbon cycle. Fwc = carbon flux from weathering of Ca and Mg carbonates; Fwg = carbon flux from weathering of sedimentary organic matter; Fmc = degassing flux from volcanism, metamorphism, and diagenesis of carbonates; Fmg = degassing flux from volcanism, metamorphism, and diagenesis of organic matter; Fbc = burial flux of carbonate-C in sediments; Fbg = burial flux or organic-C in sediments.

This greatly simplifies theoretical modeling of the long-term carbon cycle. It means that the sum of input fluxes to the surficial system are essentially equal to the sum of all output fluxes. For each million-year time step, although input and output fluxes of carbon to the surficial system may change, they quickly readjust during the time step to a new steady state, This is known as the quasistatic approximation. Non-steady-state modeling (Sundquist, 1991) has shown that perturbations from surficial system steady state, for the long-term carbon cycle, cannot persist for more than about 500,000 years.

At steady state, the CO2 uptake flux to form HCO3- accompanying the weathering of Ca and Mg silicates Fwsi is determined from the mass balance expression for bicarbonate (reactions 1.1, 1.2, and 1.6):

Fbc - Fwc represents the carbonate that is formed only from the weathering of Ca and Mg silicates, as opposed to that formed from both Ca and Mg silicate and carbonate weathering (Fbc). Equation (1.13) illustrates the necessity of knowing the rate of carbonate weathering (Fwc) in calculating the rate of silicate weathering.

In GEOCARB (Berner, 1991, 1994; Berner and Kothavala, 2001) and similar modeling (e.g., Kump and Arthur, 1997; Tajika, 1998; Wallmann, 2001) the weathering and degassing fluxes, Fwc, Fwg, Fmc, Fmg are expanded in terms of nondimensional parameters representing how a variety of processes affect rates of weathering and degassing. The parameters are multiplied by present fluxes to obtain ancient fluxes. These non-dimensional parameters are discussed in the next three chapters and provide a window into the inner workings of the long-term Phanerozoic carbon cycle. The last two chapters show how calculations based on long-term carbon cycle modeling can be used to estimate the Phan-erozoic evolution of atmospheric CO2 and O2. The modeling results are then compared to independent estimates of paleo-CO2 and O2 to give some idea of the accuracy and deficiencies of the modeling.

Sumber : http://paleontology.wikia.com/wiki/Phanerozoic

The Phanerozoic (occasionally Phanaerozoic) Eon is the period of geologic time during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. The Phanerozoic eon is still ongoing. Its name derives from the Greek meaning visible life, referring to the large size of organisms since the Cambrian explosion. The time previous to the start of the Phanerozoic is called Precambrian (now divided into the Hadean, Archaean and Proterozoic eons). The exact time of the boundary between the Phanerozoic and the Precambrian is slightly uncertain. In the 19th Century, the boundary was set at the first abundant metazoan fossils. But several hundred taxa of Precambrian metazoa have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Precambrian-Phanerozoic boundary either at the classic point where the first trilobites and archaeocyatha appear; at the first appearance of a complex feeding burrow called Trichophycus pedum; or at the first appearance of a group of small, generally disarticulated, armored forms termed 'the small shelly fauna'. The three different dividing points are within a few million years of each other.

The Phanerozoic is divided into three eras — Paleozoic, Mesozoic, and Cenozoic. In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists. The term seems to be falling into disuse in more modern literature.

The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the emergence of terrestrial plants; the development of complex plants; the evolution of fish; the emergence of terrestrial animals; and the development of modern faunas. During the period covered, continents drifted about, eventually collected into a single landmass known as Pangea and then split up into the current continental landmasses.