Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell as pictured above. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs. They belong to a group of predators known as cephalopods, which includes their living relatives the octopus, squid, cuttlefish and nautilus.
Ammonites had hard, coiled shells. The animal added chambers to the shell as it grew, actually living only in the chamber closest to the opening.
Paleontologists use certain ammonites to date rock formations.
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