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.

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.