Historical Geology Understanding Our Planet\'s Past

(Marvins-Underground-K-12) #1
The cephalopods were the most spectacular, diversified, and successful
marine invertebrates of the Mesozoic seas. The nautiloids grew to lengths of
30 feet or more. Because of their straight, streamlined shells, they were among
the swiftest creatures of the deep. The ammonites, the most significant
cephalopods, had a variety of coiled-shell forms (Fig. 161) identified by their
complex suture patterns.This makes them the most important guide fossils for
dating Mesozoic rocks.
The ammonite shells were subdivided into air chambers.The suture lines
joining the segments presented a variety of patterns used for identifying var-
ious species. The air chambers provided buoyancy to counterbalance the
weight of the growing shell. Most shells were coiled in a vertical plane. Some
forms were spirally coiled. Others were essentially straight, which often made
swimming awkward. Their large variety of coiled-shell forms made the

Figure 161Cretaceous
ammonite fossils on display
at the Museum of Geology,
South Dakota School of
Mines at Rapid City.


Historical Geology

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