Resource: Fossils: An Ancient Sea in Indiana

Fossils reveal a great deal about earlier life forms. Paleontologists use them to reconstruct body shapes, which provide information about evolutionary processes and trends. Most prominent animal fossils, including those displayed in museums, are mineralized bones. From these skeletons, researchers can often learn about an animal's gender, physical capabilities, growth patterns, life expectancies, and pathologies. These are revealed through clues preserved in the fossils, as well as through comparisons to present-day life forms, which help scientists imagine what an ancient creature may have looked like.

In addition to bones, shells and teeth, also made primarily of inorganic compounds, are frequently preserved as fossils. In rare cases, soft parts such as muscles, skin, and other organs leave some kind of trace. Muscles, for example, leave marks where they attached to bones. Although completing a skeleton with missing components may be more straightforward than interpreting fossilized soft parts, the latter also provide useful information. For example, by studying the muscle attachment points on bone fossils, paleontologists can make inferences about the size and shape of muscles. This information helps them reconstruct the animal.

Scientists generally search for models on which to base reconstructions of complex organisms. The models may be modern living things with bones or other structures similar to the fossil or fossils in question, or they may be previously discovered fossils that bear some resemblance to the fossil. Using these models, scientists make inferences that allow them to manufacture a more or less complete physical interpretation of a plant or animal, even if many or most of the hard and soft parts are missing. For instance, based on their models, paleontologists might make an educated guess that an animal skull was covered in skin and hair or fur, even if evidence for these is lacking in the fossil itself. Or, if a scientist recognizes tell-tale holes in a skull that suggest it is that of a reptile, the scientist would assume that the skin covering the head and body was scaly or bumpy and without fur or feathers.

Of course, sometimes paleontologists get it wrong. For many years, Tyrannosaurus rex reconstructions showed the dinosaur standing erect the way a human does. But recently, scientists concluded that the creature's posture was more horizontal, like that of a bird. Many museums have since changed their displays to reflect this new understanding.

To view images of fossils from different geologic time periods and to learn more about the environmental factors that dictate whether organic remains become fossilized, check out Fossils.

To view an animated sequence of how organic remains may be fossilized and later discovered, check out How a Dinosaur Became a Fossil.

To learn about geologic time and how the dynamic conditions that have characterized Earth's 4.6-billion-year history have influenced every living thing -- including their remains -- check out Deep Time.

• In what type of environment did all of these creatures live?
• How long ago were these animals living on this sea floor?
• Which part of the animal is preserved in the rock? Which parts are not preserved?
• How were these creatures preserved as fossils on this sea floor?
• What animals that exist today do you think are related to these fossilized creatures?