Geological time spans billions of years, and includes the history of all life on this planet. By studying fossils, we have learned more about the history of the Earth and the organisms that have inhabited it. For scientists of the natural and cultural world, clues to the past come in the form of not just fossils, but bones, and artifacts as well. Whether they are created naturally or by humans, these "clues" provide a glimpse into what life was like long ago. Even what seem to be simple objects point to a wealth of information and scientific concepts such as; geologic time, plate tectonics, and rocks and minerals.
There are certain general ideas that can be drawn from different fossils accordingly. Different fossils, depending upon how they were preserved, tell us different things. For example, fossils that are preserved in amber give us an extraordinary amount of information about the anatomy of that organism; since the organisms that are preserved in amber, mostly insects, are usually preserved intact without any disintegration of organs, muscles, and coloring. Even bones may tell a great deal about the soft anatomy. For instance, the area where the muscle attaches to the bone leaves marks that indicate size, shape, and functions of these varied organs. Also, the cavities and the channels in skulls give us an idea of their intelligence, behavior, and their principle features. Certain parts of certain fossils can also tell us about growth, injury, disease, form, function, activities, and instincts. Fossils also record the successive evolutionary diversification of living things, the successive colonization of habitats, and the development of increasingly complex organic communities. Fossils can tell a great deal about their surroundings and the conditions under which they lived. Finally, study of fossils contributes greatly to the study of evolution. They are the only direct record of what has occurred in sequences of reproducing populations and in the course of the time on an evolutionary scale.
UC Berkeley by Mani K., December 1996. (http://www.ucmp.berkeley.edu/paleo/fossils)


In the distant past, when most fossils formed, the world was different from today. Plants and animals that have long since vanished inhabited the waters and land. A region now covered with high mountains may have been the floor of an ancient sea. Where a lush tropical forest thrived millions of years ago, there may now be a cool, dry plain. Even the continents have drifted far from the positions they occupied hundreds of millions of years ago. No human beings were present to record these changes. But paleontologists, or scientists who study prehistoric life, have pieced together much of the story of Earth's past by examining its fossil record.

Fossil Formation

Most fossils are found in sedimentary rocks. These fossils formed from plant or animal remains that were quickly buried in sediments—the mud or sand that collects at the bottom of rivers, lakes, swamps, and oceans. After thousands of years, the weight of upper layers of sediment pressing down on the lower layers turned them into rock. A few fossils are whole plants or animals that have been preserved in ice, tar, or hardened sap.

Recording changes on Earth.
Paleontologists use fossils to determine how Earth's climate and landscape have changed over millions of years. For instance, they have found fossils of tropical palm trees in Wyoming, an area that has a cool climate today. These fossils indicate that the climate in that area has cooled. Paleontologists have found fossil oysters in Kansas and other areas that are far inland today. Such fossils reveal that a shallow sea once spread over these areas.
Fossils also provide evidence supporting the theory of continental drift—the idea that the positions of the continents have changed over hundreds of millions of years. Paleontologists have found similar kinds of fossil dinosaurs on all of the modern continents. It is unlikely that similar species could have evolved on separate continents. As a result, most earth scientists believe that when the dinosaurs first appeared—about 230 million years ago—nearly all of Earth's land mass was united as a single supercontinent. In contrast, fossils of mammals show complex differences from continent to continent. This indicates that after about 200 million years ago, when mammals were beginning to develop and spread, the supercontinent was breaking apart. The continents were drifting slowly to the positions they occupy today.