A fossil might be the mineralized bone of a dinosaur.
Or the tooth of a saber-tooth tiger.
It might be the hard shell of a clam, or it might be (if you're really, really lucky) the impression of a soft organism such as a jellyfish.
Any trace of ancient organisms is considered a fossil, and people have found billions and billions of them, representing hundreds of thousands of different species. All these fossils, together, make a kind of textbook showing the way in which many different creatures have changed over the millennia. This change over time is what we call evolution.
Check out some fossils today. You can enjoy looking at the real deal at most natural history museums, most rock shops and some jewelry stores, and even some parks and recreation areas such as Dinosaur National Monument or Fossil Butte National Monument. Check out your local treasures (I know we Southern Californians have a fossil dig for kids at Jurupa Mountains Discovery Center, in Riverside, and also at Ralph B. Clark Regional Park, in Buena Park!).
- the official National Fossil Day website, and
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