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Judging from the patterns of their chamber walls, ammonites evolved
quickly. This means that ammonite fossils are very useful for telling
relative geologic time. For example, ammonites from the Paleozoic Era
all have simple chamber wall patterns. In the first part of the Mesozoic
Era, ammonites appear that have more complex folded chamber walls. By
the middle of the Mesozoic Era, the patterns become very elaborate. A
paleontologist can tell the age of a rock from just the ammonite fossils
it contains.
Ammonite fossils are found in many places throughout the world. While
they may seem unusual to us, in their time they were as common as fish
in the oceans. Paleontologists do not know why ammonites became extinct.
They vanished at the same time as the dinosaurs, about 65 million years
ago. Their extinction may have been related to environmental changes due
to the movement of the plates.
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Fossil ammonite
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