Time

In 1650, James Usher, Anglican Archbishop of Armagh, published a treatise in which he calculated the time to creation to be nightfall on the day preceding October 23, 4004 B.C.

If we compress the history of our the Earth (~ 4.6 billion years) into one calendar year, we can gain some perspective.

In the first four months of that year, the earth was very inhospitable, violent and lifeless.

The first cell living cell developed somewhere at the ocean shoreline sometime between may-June. In August, terrestrial plant life evolved.

Dinosaurs appeared on the scene on December 21 and became extinct by noontime on December 26.

New Year’s Even has been a busy day. Human ancestors appeared around 8 pm. Modern man (Homo sapiens) appeared in Africa around 11: 36 pm on December 31. Civilization, which dates back to about 30,00 years, started at 11:56 pm. The first human arrived to North America from Asia, at 11:58 pm. The industrial revolution took place one second before midnight. In the last seven tenths of a second before midnight, human beings have been very busy indeed. They have increased their population to more than 6 billion, at the stroke of midnight.

In reality, it’s a bit older….

FIGURE 2.1 Geologic Time

The gathering of cosmic gases under gravity’s pull created Earth some 4.6 billion years ago. Yet life became neither abundant nor complicated until the Cambrian period, or slightly earlier, when the first vertebrates appeared. Source: After U.S. Geological Survey publication, Geologic Time.

FIGURE 2.2 Stratigraphy Sediment settling out of water collects at the bottom of lakes. As more sediment collects,

the deeper layers are compacted by the ones above until they harden and become rock. Animal remains become embedded in these various layers. Deeper rock forms first and is older than rock near the surface. Logically, fossils in deeper rock are older than those above, and their position within these rock layers gives them a chronological age relative to older (deeper) or younger (surface) fossils.

In general, the oldest rocks in a layer will be at the bottom. As you move upward through the strata, the rocks (and the fossils they contain) become progressively younger.

Stratigraphy Fossil animals occur in sedimentary rocks deposited on oceanic shorelines, one upon the

other. Subsequent cracks in the Earth’s surface, weathering, or erosion by a river open these ancient sedimentary deposits, exposing their cache of fossils.

Index fossils can be used to match rock strata. This helps establish ages of strata across wide geographic areas.

Each exposure of rocks can be of a different age from other exposures. To build up an overall sequence of fossils, various exposures can be matched where they share similar sedimentary layers (layers of the same ages). From five sites in the southwest United States, overlapping time intervals allow paleontologists to build a chronology of fossils greater than that at any single site.

FIGURE 2.4 Index Fossils After careful study at many well-dated sites, paleontologists can confirm that certain fossils

occur only at restricted time horizons (in specific rock layers). These distinctive index fossils are diagnostic fossil species used to date rocks in new exposures. In this example, the absence of index fossils confirms that layer B does not exist at the third location. Perhaps rock-forming processes never reached the area during this time period, or the layer was eroded away before layer C formed. After Longwell and Flint.

Most fossils are formed when an organism is entrapped in soft sediment at the bottom of a lake or ocean.

Deposits harden into sedimentary rocks. Some fossils are more than just impressions, but entire organisms.

Radioactive Isotope(Parent)

Product(Daughter)

Half-Life(Years)

Samarium-147 Neodymium-143 106 billion

Rubidium-87 Strontium-87 48.8 billion

Rhenium-187 Osmium-187 42 billion

Lutetium-176 Hafnium-176 38 billion

Thorium-232 Lead-208 14 billion

Uranium-238 Lead-206 4.5 billion

Potassium-40 Argon-40 1.26 billion

Uranium-235 Lead-207 0.7 billion

Beryllium-10 Boron-10 1.52 million

Chlorine-36 Argon-36 300,000

Carbon-14 Nitrogen-14 5715

Uranium-234 Thorium-230 248,000

Carbon-14 dating While alive, organisms accumulate both ordinary carbon (C12) and its unstable isotope carbon-14

(C14)into their tissues in proportion to their availability in the atmosphere. When the organism dies, stable C12 persists, but unstable C14 decays at a constant rate and is lost slowly from the fossil. The more time that passes, the more C14 is lost from the fossil, thereby changing the proportion of one to the other with the passage of time. Consequently by measuring the proportion of C12 to the remaining C14, scientists are able to calculate the geologic age of the fossil.

Radiocarbon dating is only useful for carbon-containing materials, and has a relatively short “half-life”.

FIGURE 2.5 Radiometric Dating (a) Sand flows regularly from one state (upper portion) to another (lower portion) in an hourglass. The more sand in the bottom, the more time has

passed. By comparing the amount of sand in the bottom with that remaining in the top and by knowing the rate of flow, we can calculate the amount of time that has elapsed since the flow in an hourglass was initiated. Similarly, knowing the rate of transformation and the ratios of product to original isotope, we can calculate the time that has passed for the radioactive material in rock to be transformed into its more stable product. (b) Half-life. It is convenient to visualize the rate of radioactive decay in terms of half-life, the amount of time it takes an unstable isotope to lose half its original material. Shown in this graph are successive half-lives. The amount remaining in each interval is half the amount present during the preceding interval. (c) A radioactive material undergoes decay, or loss of mass, at a regular rate that is unaffected by most external influences, such as heat and pressure. When new rock is formed, traces of radioactive materials are captured within the new rock and held along with the product into which it is transformed over the subsequent course of time. By measuring the ratio of product to remaining isotope, paleontologists can date the rock and thus date the fossils it contains.

How do fossils form?

Pleisiosaur fossil This Mesozoic reptile, although not formally part of the dinosaurs, was their contemporary.

It was adapted to aquatic life.

Plant fossils A favorable splitting of this rock yields a view of the pressed plant fossil held within

(bottom) and its impression on the other face of the rock (above).

Dinosaur footprint At the time, this footprint of a dinosaur pressed into soft mud and became preserved in the

now hardened rock.

There are many other types of trace fossils. The study of such fossils is called “ichnology”.

Insect in amber This mosquito was caught, then imprisoned in stick tree sap that subsequently hardened

into this amber, preserving the insect within.

Martian microbes? Meteorites from Mars survived passage through Earth’s atmosphere and fell in areas of the

poles. Microscopic squiggles resemble microfossils from early Earth history, suggesting a similar early history for life on Mars. However, such microfossils could also be produced by simple chemical reactions rather than represent the remains of small, living organisms.

FIGURE 2.6 Geological Time Intervals The Earth’s history, from its beginnings 4.6 billion years ago, is divided into four major eons

of unequal length—Hadean, Archean, Proterozoic, and Phanerozoic. Each eon is divided into periods, and periods into epochs. Only epochs of the Cenozoic are listed in this figure.

FIGURE 2.7 Fossil Eggs This clutch of dinosaur eggs from about 70 million years ago is thought to be from a

Segnosaur, an enigmatic carnivorous (or omnivorous) species about which we know little. These eggs, found in China, were laid together in pits or holes in the ground that may have been lined with plant material, which did not fossilize. Each egg is about 6 cm in diameter. Photograph kindly supplied by Lowell Carhart, Carhart Chinese Antiques.

FIGURE 2.8 Fossil Ichthyosaur Small skeletons are seen within the adult’s body and next to it. This may be a fossilized

birth, with one young already born (outside), one in the birth canal, and several more still in the uterus. Such special preservations suggest the reproductive pattern and livebirth process in this species.

FIGURE 2.9 Archaeopteryx The original feathers have long since disintegrated, but their impressions left in the

surrounding rock confirm that the associated bones are those of a bird.

FIGURE 2.10 Fossil Dig in Wyoming (a) Partiallyexposed dinosaur bones. The work crew prepares the site and notes the

location of each excavated part. (b) This Triceratops femur is wrapped in a plastic jacket to prevent disintegration or damage during transport back to the museum. Photos courtesy of Dr. David Taylor, Executive Director, NW Museum of Natural History, Portland, Oregon.

FIGURE 2.11 Restoration of a Fossil (a) This skeleton of the extinct short-faced bear, Arctodus simus, is positioned in a likely

posture in life. (b) Scars on the bones from muscular attachments and knowledge of general muscle anatomy from living bears allow paleontologists to restore muscles and create the basic body shape. (c) Hair added to the surface completes the picture and gives us an idea of what this bear might have looked like in its Alaskan habitat 20,000 years ago.

FIGURE 2.12 Making Fossils The remains of extinct animals that persist have escaped the appetites of scavengers,

decomposers, and later tectonic shifting of the Earth’s crustal plates in which they reside. Most surviving fossils are of dead animals that quickly became covered by water and escaped the notice of marauding scavengers. As more and more silt is deposited over time, the fossil becomes even more deeply buried in soil compacted into hardened rock. For the fossil held in the rock to be exposed, the Earth must open either by fracture or by the knifing action of a river.