Fossil

H

Tracks and Trails

When animals walk through soft sediment such as mud, their feet, tails, and other body parts leave impressions that may harden and become preserved. When such an impression is filled with a different sediment, the impression forms a mold and the sediment that fills the mold forms a cast. Molds and casts of dinosaur tracks are relatively common and help paleontologists understand how these creatures moved.

I

False Fossils

Minerals can sometimes grow within rocks into shapes that resemble fossils. Dendrite crystals are often mistaken for fernlike fossils. Flint nodules in chalk can look like a variety of different life forms. Mineral concretions in sediments are sometimes mistaken for fossilized eggs. It is only with close study that the true nature of false fossils can be discovered.

Modern animals and plants sometimes become mummified or coated in travertine (calcium carbonate salts from springwater), or they may die while trapped in cracks in older rock strata. These remains are not true fossils, but trapped animals and plants may eventually fossilize with time.

III

Where Fossils Form

Fossils are found in all parts of the world, from Greenland to Antarctica. They can be found in cores drilled in and retrieved from the ocean floor, and on top of the highest mountains. Their wide geographical distribution is a result of the way the earth’s surface has changed throughout its history.

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The earth’s crust is made up of several large tectonic plates that float on the earth’s liquid mantle (see Plate Tectonics). These tectonic plates have moved throughout geological time, forming large land areas and mountain ranges, and forming and closing off seas. Some land that is now in the polar regions was once closer to the equator, and many modern mountain ranges were once under water.

The global climate has also changed over geological time, alternating between periods of warmth and ice ages. These climatic conditions affected the distribution of life on the earth and are reflected in the fossil record. Fossils are abundant in rocks that were formed in tropical and equatorial regions for the same reason that life is most abundant at these latitudes today—a warm, tropical climate supports a wider variety of life forms than does a cold climate.

The types of fossils found in a particular region depend on the age of the rocks that are currently eroding at the surface. Some areas have become famous for the types of fossils found there, such as China and the badlands (rugged, rocky areas with little vegetation) of the United States and Canada, where an abundance of dinosaur fossils from the Cretaceous Period (138 million to 65 million years before present) have been found. Some fossils are restricted to small areas and some are distributed globally. The most widespread fossils are the remains of organisms that lived in oceans and could move with the currents, such as foraminifera, and those that lived on land and were spread by wind, such as spores. Fossils of graptolites (marine invertebrates that lived in colonies) in rocks of marine origin and of ferns on land are now found on all continents. Certain species of shallow-water trilobites, and dinosaurs that were restricted to land, are found only at particular localities.

Different types of fossils are found in different geological formations, depending on the prehistoric environment represented and the age of the rock. Older rocks are found on low, eroded continents near the edges of large oceans. Younger rocks are found more commonly where there is active mountain building and volcanic activity. Old fossils are most commonly found where an old mountain range has eroded, such as in eastern North America and northern Europe, or where two old continents have collided, such as in Russia. Younger fossils are found at the ocean side of young mountains where an ocean plate is colliding with a continental plate, such as in western North and South America and in New Zealand.

IV

Learning From Fossils

Paleontologists use fossils to reconstruct how prehistoric organisms might have looked. Fossils that are found grouped together can suggest how an organism interacted as part of a community. Sometimes the microscopic structure of an organism is preserved, as well as different growth stages from embryo to adult. Such remains allow paleontologists to determine how closely related fossil organisms are to one another and to living organisms. When studying extinct organisms with no obvious living relatives, such as graptolites, paleontologists look at the microscopic structure and chemical composition of the remains to determine if there is a living relative.

Paleontologists must sometimes compare the fossils of extinct organisms with living organisms to draw conclusions about the nature, behavior, or habits of prehistoric life forms. For example, the inner chambers of the extinct ammonites (squidlike mollusks with a spiral shell) can be compared with the inner chambers of the living nautilus. The sharp, serrated teeth of Tyrannosaurus are similar to those of living carnivores, indicating that this dinosaur was also a meat eater. Similarly, the flattened teeth of Hadrosaurus, which resemble the teeth of living herbivores, suggest that this duck-billed dinosaur was a plant eater.

Some fossils reveal information about how a species grew. Paleontologists have found fossils of the empty shells of trilobites, for example, that reveal that the animals shed their shell-like skeletons as they grew into adult forms, much as shrimps and crabs do today. Vertebrates have internal skeletons that cannot be shed at different growth stages. In order for paleontologists to gather information about the growth stages of vertebrates, therefore, they must study the fossilized bones of animals that died during certain stages. For example, paleontologists have discovered a dinosaur nesting site in Montana that contains skeletal fossils of the duck-billed Maiasaura that represent various stages from embryo to adult.

Prehistoric organisms interacted with one another in much the same way as living organisms do today. Paleontologists have identified predators and their victims using evidence such as the teeth marks of mosasaurs (large, carnivorous marine lizards) on ammonites. Evidence of fighting between rivals can be seen in the fossils of some crocodiles, in which the jaws or ribs have been broken and have healed. Prehistoric animals also suffered from disease and deformities, as evidenced by such fossils as arthritic hip joints of plesiosaurs or split segments of trilobites. Fossil plants show evidence of parasitism and disease, as well as evidence of having been fed on by insects and larger animals.

A

Evolution

The fossil record contains evidence of how life has changed and evolved throughout the earth’s history. The earliest fossils are more than 3.5 billion years old. They are simple, microscopic, single-celled bacteria called blue-green algae. There is little evidence of change in the life forms on earth over the next 3 billion years, except that cyanobacteria (formerly known as blue-green algae) began growing in layered colonies called stromatolites. The first complex life—jellyfish and worms—appears in the fossil record about 680 million years ago. The first vertebrates evolved about 570 million years before present, at the border between Precambrian time and the Paleozoic Era. At this point, the seas also became abundant with a variety of life forms.

About 400 million years before present, some living organisms migrated onto land, and pioneering plants and arthropods became common. Vertebrates soon took advantage of this new habitat, and reptiles appeared about 330 million years before present. Early mammals appeared about 100 million years later, during the Mesozoic Era, when dinosaurs roamed the earth. After the extinction of the dinosaurs 65 million years before present, mammals moved into habitats left vacant by the dinosaurs and developed, with other survivors, into the creatures that exist today. Flowering plants appeared about 120 million years before present, becoming abundant after the extinction of the dinosaurs.

The fossil record also reveals how individual species evolved over time. It is possible to study such changes by comparing older fossils found lower in a sedimentary formation with the younger fossils found higher in the formation. The study of the succession of geological time represented by these sediments and fossils is called stratigraphy.

The fossil record suggests that evolution may have progressed at different rates—sometimes gradually, and at other times in short bursts. This is difficult to prove, however, because sedimentation is rarely continuous over long periods of time. Paleontologists theorize that rapid evolutionary events commonly occur after a major extinction, such as that of the dinosaurs. This may be so because populations of different species move into the newly unoccupied position, or niche, within a community left vacant by the extinct species.

Convergent evolution occurs when an animal with a shape that was well suited to its function becomes extinct, and a new animal that replaces the extinct one evolves a similar shape to perform a similar function. This type of evolution has occurred in dolphins and porpoises that moved into the environmental niche left vacant by the extinction of ichthyosaurs. Although there are substantial differences between the extinct ichthyosaurs and today’s dolphins and porpoises, and although their ancestry is very different, the basic form that dolphins and porpoises adapted for living in the ocean is similar to that of ichthyosaurs.

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