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What Is a fossil?

Fossils are naturally-preserved physical traces of long-dead organisms. Usually, these traces consist of an organism's hard parts, such as bones, teeth, shells, or wood. Occasionally, when conditions are optimal, soft parts of organisms can also fossilize, such as impressions of skin, body outlines, and, more commonly, leaves. Other traces of objects made by organisms, such as footprints, burrows, and nests, also qualify to be called fossils. Most definitions of fossils require that the organism's body part or other physical trace be more than 10,000 years old in order to truly be called a fossil. Created by American Museum of Natural History.

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Video transcript

People have different definitions of what fossils are, but basically the key point is that fossils are some trace of an ancient extinct organism. Most of the fossils, especially when it comes to dinosaurs, are bones like these big vertebra or teeth from a dinosaur, hard parts. And, if you're looking for other kinds of fossils, seashell fossils of various kinds, there are fossil leaves in wood from plants, all of those qualify as fossils because they're traces of ancient organisms. Sometimes, other types of body parts like skin and fossil footprints that were left by animals are also found, and these too qualify for fossils. So basically, a fossil is any trace of an ancient organism, either a hard part, or a soft part, or some product that they left behind and usually this trace has to be over ten thousand years old in order to be considered a fossil.