Shark Teeth
(c) 2000 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Shark teeth, like sharks, have been around for more than 400 million years. Their teeth are almost the only fossils they leave behind, because their skeletons are made of cartilage—the same stuff that stiffens your nose and ears—rather than bone. They leave a lot of teeth, because unlike most other animals they grow new ones throughout their lives.
The teeth on the left are modern specimens from the beaches of South Carolina. The teeth on the right are fossils I collected as a child, probably from Maryland, laid down at a time when sea level was higher and much of the eastern seaboard was underwater. Geologically speaking they're very young, perhaps from the Pleistocene or Pliocene. Even in the short time since they were preserved, the mix of species has changed.
Note that the fossil teeth are not petrified. They're unchanged from the time the sharks dropped them. An object doesn't need to be petrified to be considered a fossil, merely preserved. In petrified fossils, the substance from the living thing is replaced, sometimes molecule for molecule, by mineral matter such as calcite, pyrite, silica, or clay. To see a petrified fossil, visit the fossil wood page.
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