Phylum Chordata From Sea Otters to Sea...
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Phylum ChordataFrom Sea Otters to Sea Squirts
The Big Four! pharyngeal slits - a series of openings that connect the inside of the
throat to the outside of the "neck". These are often, but not always,used as gills.
dorsal nerve cord - a bundle of nerve fibers which runs down the"back". It connects the brain with the lateral muscles and otherorgans.
notochord - cartilaginous rod running underneath, and supporting,the nerve cord.
post-anal tail - an extension of the body past the anal opening.
Chrodate Body Plan
The basic chordate body plan consists of a tube within a tube. Theinner "tube" is the digestive tract and the outer "tube" is the bodywall. The cavity space between the two tubes is the coelom. Note thatthe coelom is lined with mesoderm. The major chordatecharacteristics, notochord and dorsal nerve cord (shown here as theneural tube), are also part of the body plan.
The evolutionary relationship between the echinoderms andhemichordates (acorn worms) is indicated by the similarity of theirlarvae. Both larval forms are small, motile, and swim by utilizingbands of cilia.
Weirdos from the Burgess ShaleSome Designs Failed
Chordate Characteristics The Big Four!
Bilateral symmetry; segmented body; three germ layers; well-developed coelom.
Notochord (a skeletal rod) present at some stage in life cycle.
Single, dorsal, tubular nerve cord; anterior end of cord usuallyenlarged to form brain.
Pharyngeal gill slits present at some stage in life cycle.
Other Characteristics: Postanal tail, usually projecting beyond the anus at some stage but
may or may not persist.
Segmented muscles in an unsegmented trunk.
Ventral heart, with dorsal and ventral blood vessels; closed bloodsystem.
Complete digestive system.
A cartilaginous or bony endoskeleton present in the majority ofmembers (vertebrates).
The Burgess Shale
- The swimming larvae resemble tadpoles and possess all of the four key chordatecharacteristics.
"Tail Cord." There are approximately 1500 described species of these marineliving chordates. They are commonly called tunicates (also sea squirts) because ofthe tough, nonliving tunic that surrounds the animal. As sessile adults they lose thenotochord and the tail, their dorsal hollow nerve cord becomes reduced, but theyretain the pharyngeal gill slits.
Example: tunicates, sea squirts
Subphylum Cephalochordata -"Head Cord"
There are only 25 species described in this subphylum ofmarine lancelets. They are all small, slender, laterally compressed, translucent
Textbook example of the four key chordate characteristics. Onceconsidered to be the ancestor of the chordates, they are stillbelieved to more closely resemble the earliest prevertebrates thanany other animal known.
Examples: Amphioxus (the lancelets)
A Typical Cephalochordate BodyPlan
The Lowly Chordate on theSeafloor--Who would have
Subphylum Vertebrata -"Backboned"
This is the largest and most diverse subphylum ofchordates whose evolution has been guided by the basicadaptations of: the living endoskeleton
advanced nervous system
This group is divided into those without jaws and those with jawedmouths.
Superclass Agnatha ("Without jaws"): Those vertebrates withouttrue jaws or appendages including the hagfishes and the lampreys.
Superclass Gnathostomata ("Jawed mouth"): Those vertebrates withjaws and usually with paired appendages.
-Class Chondrichthyes - The cartilaginous fishes.-Class Osteichthyes - The bony fishes.-Class Amphibia - The amphibians.-Class Reptilia - The reptiles.-Class Aves - The birds.-Class Mammalia - The mammals.
Superclasses and Classes ofSubphylum Vertebrata
Characteristics: Slender, eel-like body.
Median fins but no paired appendages.
Fibrous and cartilaginous skeleton; notochord throughout life.
Heart with one atrium and one ventricle.
Dorsal nerve cord with differentiated brain.
Digestive systems without stomach.
Sense organs of taste, smell and hearing.
Examples of living Agnaths:
Class Myxini - The hagfishes
Class Cephalaspidomorphi - The lampreys
The first vertebrates to evolve were the agnathans. Their traces are found in rocks of theLate Cambrian period, more than 520 million years old. These first fishes had no jawsin which to seize and manipulate prey. Nor did they have paired fins, to stabilize theirbodies in the water. The bony armor has been preserved in the rocks, and gives thefossil agnathans their collective name of ostracoderms, meaning "shell-skins."Despitetheir lack of jaws, ostracoderms dominated the seas and freshwaters of the northernhemisphere for about 130 million years, from Early Ordovician to Late Devonian times.
Major Evolutionary Step #1
The Evolution of Jaws--Why is this soimportant?
1) ancestral fishes were jawless and somewhat similar to today's agnathans2) anterior gill arches began to be modified into jawlike structures 3) functional jaws were first exhibited in the Placodermi 4) shows the configuration of jaws in most modern fishes
The placoderms, or "flat-plated skins," were a strange assemblage of heavilyarmored jawed fishes. Several large, interlocking plates formed a bony head shield,while another series of plates encased the front part of the body in a trunk shield.The rest of the body was usually naked, with no scaly covering. This group was thefirst to exhibit jaws, which exhibited broad dental plates instead of teeth. They firstappeared in the Early Devonian and had died out by the Early Carboniferous.
Huge Jawed Predators
These fish are common in Silurian Rocks in Ohio
Members have a cartilaginous skeleton, pectoral fins and pelvic fins,probably evolved in the sea from ancestors of Placoderms during theDevonian or earlier.
These possess a lateral line system.By lacking the armor of their ancestors, this group exhibits agreater degree of mobility.Also, the appendages' bone types are featured in the tetrapodvertebrates. Extant representatives are sharks and rays.
Major Characteristics of ClassChondrichthyes:
Characteristics: A heterocercal caudal fin.
Skin with placoid scales.
Endoskeleton entirely cartilaginous.
Respiration by means of 5 to 7 pairs of gills with separate andexposed gill slits, no operculum.
No swim bladder or lung.
Internal fertilization and separate sexes.
Sharks, Skates and Rays
Chimaera or ratfish
Cartilaginous Body Plans
Agnatha- jawless, armored protovertebrates without paired appendagesPlacodermi- jawed armored extinct fishes with paired appendagesChondrichthyes- cartilaginous fishesOsteichthyes- bony fishes with an operculum and swim bladder. Thisgroup diversified into lung- ray- and lobe-finned fishes, the latter ofwhich gave rise to the amphibians.
The "bony fishes" evolved in freshwater, apparently diverging from anancestor common to placoderms and chondrichthyes about 425 or 450MYA. In members of this group, calcium phosphate reinforces theskeleton.Other characteristics of bony fishes include: mucus production a very well developed lateral line system, a swim bladder (whichevolved from lung-like sacs derived from the pharynx and used forrespiration in many early bony fishes) and an operculum.
Both bony and cartilaginous fishes diversified extensively in theDevonian and Carboniferous, leading to ray-finned fishes, lobe-finnedfishes, and lungfishes. Lobe-finned fishes (similar to the coelocanth)gave rise to amphibians.
Interestingly, although the ray-finned fishes include the modern fishesand are the most diverse group of vertebrates, they are a "dead-end" interms of evolution, since there are no "more advanced" groups which
evolved from them.
Although among the fishes the lobe-finned fishes were not as successfulas the ray finned fishes, they proved to be in the mainstream of evolutionbecause they gave rise to the other groups of vertebrates.
One of the members of the lobefinned group provided the ancestor of thefirst land animals, the amphibians. This transition was accomplishedrelatively quickly in their evolution; the first lobe-finned fishes appearedin the Early Devonian, and by the end of that period, some 20 millionyears later, the amphibians had set foot on dry land.
A Living Colecanth
Paleontologists agree that the amphibians must have evolved from one ofthe 3 groups of lobe-finned fishes (lungfish, coelacanths, or extinctrhipidistians). However, there is disagreement on which group is themost likely ancestor. Most paleontologists consider that amphibiansevolved from the rhipidistian fishes, based on the remarkable similarityin the pattern of bones in their skulls and fins/limbs. Otherpaleontologists, however, believe that the lungfish were ancestral to theamphibians, since the development of the lungs, nostrils and limbs ofliving lungfish is strikingly similar to those of living amphibians.
Major Evolutionary Step #2
The Move to Land
Why Make that First Step????
Age of Amphibians
This figure illustrates the similarities in bone structure between the lobe-finnedfishes (such as coelacanths) and amphibians. The muscles and bony axis of thepaired fleshy fins of these fishes provide a structure that could easily haveevolved into the limbs of an early amphibian. Similarly, there seems littledoubt that these fishes possessed lungs like those of amphibians. Livinglungfish, for example, have them, and a similar structure (though single) ispresent in the living coelacanths.
That First Step!
What Makes an Amphibian an Amphibian ?
Ichthyostega-That First Step!!
Fossils of the earliest amphibian, Ichthyostega, were found in Greenlandand were dated about 370 MYA when Greenland was part of aEuramerican continent that lay near the equator, and stretched fromtoday's western North America to eastern Europe. Members of this genusare most often considered to represent the type of animal which firstexhibited a terrestrial existence, unlike its ancestors, the lobe finnedfishes.
Pangaea RevisitedA remarkable feature of the distribution of these early amphibians,and of their relatives, the reptiles, is that until the middle of thePermian period (about 100 million years later), nearly every one ofthem has been found only on this former Euramerican continent.This strongly suggests that this continent was the homebase inwhich they first evolved and diversified. Only after the middle ofthe Permian--when Asia and the southernlandmass of Gondwanahad become attached to Euramerica to form the supercontinent ofPangaea--did the amphibians and reptiles spread throughout theworld.
Implications for Biodiversity
Major Step#3-The Amniotic Egg
The evolution of this type of egg was a major step in the conquest ofthe terrestrial environment. Probably, it was first exhibited in stemreptiles.
The most important modifications which enabled subsequent vertebratesto become independent of the aquatic environment were 1) modificationsof the outer covering into a multilayered skin which is fairly impermeableto water and 2) evolution of an amniote egg.
What Makes a Reptile aReptile??
Class Reptilia includes 7,000 species of lizards, snakes, turtles andcrocodilians. They exhibit important characteristics for terrestrial life(keratinized scales, lungs, amniote egg, et al.). Ectothermy allows themto survive using only 10% of the energy expended by mammals ofsimilar size. Reptiles first arose during the Carboniferous, about 300MYA.
From among their members came 2 major
evolutionary lines--one led to the mammals, and the
other to the archosaurs, "ruling reptiles".
Plesiosaurs were large marine reptiles of the Mesozoic. They exhibitedlong narrow flippers which aided locomotion of their sturdy, deepbodies and short tails. There were two major groups (or superfamilies)of plesiosaurians, which differed in the lengths of their necks and intheir feeding habits. The plesiosaurs had long necks and short heads,and fed on smaller sea creatures. The pliosaurs had short necks andlarge heads to enable them to bite and swallow larger prey.
The Ichthyosaurs, fish lizards flourished throughout the Jurassic Period andinto the Cretaceous. They had a streamlined, torpedo-shaped body with astabilizing dorsal fin on the back; short, paired paddles for steering; and astrong, fishlike tail with 2 equal lobes for swimming. Ball-and-socket jointsbetween the tail vertebrae allowed for powerful strokes from side to side. Thetail, together with the great flexibility of the backbone, propelled the animalrapidly through the water- the swimming method used by modern, fast-movingfishes.
Therapsids (reptilian ancestors of the mammals) and the mammalsthemselves all exhibit a skull like that of the synapsids. It has an openingbehind each eye socket which allowed the development of longer jawmuscles. This resulted in stronger jaws that could be opened wide andclosed forcefully, to deal with large prey.
Therapsids were descended from the stem reptiles and are presumed tobe the ancestors of the mammals. "Non-reptilian" characteristics ofmembers of this group were thoracic and pelvic skeletal features whichpermitted the legs to be positioned directly underneath the body.
The Evolution of Birds
Archaeopteryx is considered by many to be a "missing link" because itexhibits both reptilian and avian characteristics. It dates from theJurassic, about 150 MYA. It had teeth and a long pointed tail (likereptiles but unlike birds), but also had feathers and skeletal elements(like birds but unlike reptiles).
South American Phororhachids-35 MYA
Flightless, fast-running predatory birds, some nearly three meters tall,armed with enormous beaks and claws. These birds were the dominantSouth American land carnivores for much of the Cenozoic; recently,phororhachid bones have been found in Florida, suggesting thatphororhachids were not confined to South America.
Mammals evolved from reptilian stock (therapsids) before birds. Fossils220 MY old have been found which are believed to be those of earlymammals. Mammal descendents coexisted with dinosaurs during theMesozoic. Early mammals were small shrew-like organisms which arepresumed to have been insectivorous nocturnal forms. Most agree thatthe extinction of dinosaurs opened adaptive zones which couldsubsequently be taken over by mammals. The Cenozoic, 63 MYA topresent is sometimes called the Age of Mammals.
What Makes a Mammal a Mammal?
Mammalian Characteristics Hair composed of keratin;
4 chambered heart;
mammary glands produce a nutritionally balancedproduct, milk, to nourish neonates;
most are born rather than hatched (eggs arefertilized internally and in placentals, embryosdevelop in utero
many have large brains and are capable learners;
long duration of parental care;
Extinctions Led the Way!
Explosive Adaptive Radiation
South American Mammals
The Great Faunal Exchange!
New World and Old World Monkeys
Two distinct modern groups of monkeys exist, the "new worldmonkeys" and "old world monkeys". Ancestors of these groups existedaround 40 million years ago in Africa and/or Asia.
How did new world monkeys got to South America?New world monkeys (including spider monkeys, capuchins, and
squirrel monkeys) are all:Arborealhave prehensile tailsHave nostrils which open to the side.
Old World MonkeysOld world monkeys include monkeys (mandrills, baboons, macaques andrhesus monkeys) and apes (chimpanzees, gorillas, orangutans, andgibbons).
Both are arboreal and ground dwelling forms among old world monkeysNone have prehensile tails.Their nostrils open downward.
Humans & Ape Distinctions
In searching for fossil relatives of humans, we need to be aware of thedifferences between men and apes. These fall into three functionalcategories that have evolved separately from one another:
Cranial differences Dental differences Locomotion differences
By comparison to apes, humans have: Larger brains (1200 cc vs. 500 cc)
The larger brain creates a number ofdifferences in other cranial features: flatter face
smaller brow ridges
no post-orbital constriction
no sagittal crest
By comparison to apes, humans have: smaller canines
no gap between upper canine and premolar
no sectorial (modified) first lower premolar
relatively small incisors and large molars
very thick molar enamel
a parabolic rather than U-shaped dental arch
Hominid Radiation & Climate
Hominid Phylogeny 1-5 MYA
The Cradle of Humanity
Hominid Adaptive RadiationWHY?
By comparison to apes, humans have: a foramen magnum that points down
a curved lumbar spine
a short, flared (versus long and thin) ilium (theupper most section of the hip bone or pelvis)
a strong, robust talus (ankle bone)
a strong, non-opposable big toe
a complex two-way arch system in the foot
Hominid Foot Prints
Vertebrate Diversity Over Time