Paleontology

I

Introduction

Paleontology, study of prehistoric animal and plant life through the analysis of fossil remains. The study of these remains enables scientists to trace the evolutionary history of extinct as well as living organisms (see Evolution). Paleontologists also play a major role in unraveling the mysteries of Earth’s rock strata (layers). Using detailed information on how fossils are distributed in these layers of rock, paleontologists help prepare accurate geologic maps, which are essential in the search for oil, water, and minerals. See Dating Methods.

Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500, scholars had engaged in a bitter controversy over the origin of fossils. One group held the modern view that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.

II

Fossils and Stratigraphy

Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how Earth evolved. The study and comparison of different strata is called stratigraphy.

Fossils provide most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one—that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 542 million years of Earth’s history into eras, periods, and epochs. The part of Earth’s history before about 542 million years ago is called Precambrian time, which began with Earth’s birth, about 4.5 billion years ago.

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The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.8 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.

III

The Paleozoic Era

The Paleozoic Era lasted about 290 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic Era are those of invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.

A

Cambrian Period

At the beginning of the Cambrian Period (542 million to 488 million years ago) animal life was entirely confined to the seas. By the end of the period, all the modern phyla of the animal kingdom existed. The characteristic animals of the Cambrian Period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic Era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian Period were brachiopods, bryozoans, and foraminifers (see Foraminifera). Plants of the Cambrian Period consisted of algae in the oceans.

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