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
An ammonite shell, with complex chamber walls