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PAST LIFE
Lesson 5 - Page 4

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Ammonites were much more varied than nautiluses. For example, nautiluses grow up to 20 centimeters in diameter, while ammonites range in diameter from a few centimeters to more than 2 meters. In addition, the coiling of ammonite shells was variable; some fossil shells are "unrolled" or open coiled.

The walls of the chambers in ammonite and nautilus shells are very different. Nautilus chamber walls are simple smooth shapes. In contrast, ammonite chamber walls are very complex and intricate. They show arrangements almost like the stitching patterns in clothing. Paleontologists do not yet know why ammonite chamber walls were so complex. One idea is that it allowed the shells to be thinner and stronger. This may have allowed ammonites to move faster and swim deeper in the ocean.

The variety of ammonite sizes, shapes, and structures suggests that they lived differently than nautiluses. Some ammonites may have lived near the sea floor, eating bottom-dwelling organisms or decayed organic material. Others, especially the large ammonites, may have been predators.


An ammonite shell, with complex chamber walls

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