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Introduction: Name a kind of dinosaur. Give it a try! Do you know anything about what that type of dinosaur ate? How about what it looked like? You have probably watched shows, read books, or studied dinosaurs in school. But how do we know so much about these extinct creatures? Or that they even roamed the Earth? We can thank fossils, and the paleontologists that study them, for what we know about the dinosaurs!

Fossils are any evidence of organisms from the past. Any type of preserved remains, impression, or trace evidence of past life is a fossil.

Fossils are sparse in the rock record. It may seem like we should find fossils everywhere. After all, think about the vast number of plants and animals that have lived throughout Earth’s history! But, most organisms do not preserve well. Animals may eat remains before they can fossilize. Some organisms have only soft body parts that decay quickly. Fossils form in sedimentary rocks as remains are buried and preserved. However, rocks are constantly changing through the rock cycle. Weathering and erosion may break apart the organism and scatter it. Heat and pressure can destroy sedimentary rocks and the fossils they contain. Yet, even with all of these forces working against fossil formation and preservation, the oldest known fossils are over three billion years old!

Not all fossils form the same way. How a fossil forms will depend on the organism and environmental conditions where it died. Fossils can be divided into two main types: body fossils and trace fossils. Body fossils are actual remains of organisms. Shells and bones are examples of body fossils. Trace fossils give evidence that the organism was once alive. Footprints, eggs, and coprolite (fossilized dung!) are all examples of trace fossils. 

The table summarizes some of the main ways fossils form.
Moldimprint left from a fossilshell impression
Casta mold filled in with sediment and mineralsshell mold filled in with hardened sediment
Permineralizationwater deposits minerals that crystallize in cellsmost dinosaur bone fossils
Petrificationwater deposits minerals that replaces organic materialpetrified wood
Carbonizationcompressed remains decay chemically and leave a carbon film impressionleaves or soft body parts can fossilize this way
Ambertree resin traps and preserves organisminsects preserved in amber


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