Geology Reference
In-Depth Information
ter and saltwater), and some have leg-like appendages that could have helped them
move across short distances on land — similar to crabs today.
Figure 19-2: A
eurypterid.
Ammonoids and nautiloids
Ammonoids are the spiral-shelled distant relatives of modern squid and flourished dur-
ing the late Paleozoic. They went extinct along with the dinosaurs at the end of the
Mesozoic (see Chapter 20). Ammonoid evolution in the Paleozoic can be traced through
the increasing complexity of their shells.
Ammonoid shells were spiral shaped with individual chambers attached along sutures.
The suture patterns of ammonoids became more complex through time, as illustrated in
Figure 19-3. This feature allows geologists to accurately date rock layers when they find
ammonoid fossils. (In Chapter 16, I explain relative dating using fossils.)
Nautiloids appear similar to ammonoids because some of them also have curved shells.
They were both part of the larger group of mollusks. Early nautiloids had straight shells,
while the modern living nautilus has a curved shell. None of the nautiloids developed
the elaborate shell sutures seen in ammonoid fossils. A straight-shelled nautiloid is illus-
trated in Figure 19-4.
Figure 19-3:
Changes in am-
monoid shell su-
tures through time.
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