during the Devonian. They are among the most common fauna and impor-
tant for long-range rock correlations of this period. The conodonts were of
lesser importance in the Permian and Triassic, when the shallow seas that they
inhabited became highly restrictive.
The mollusks were well represented. The first appearance of freshwater
clams suggests that aquatic invertebrates had successfully conquered the land
by the Devonian. Arthropods also invaded the land. They included advanced
insects, spiders, and centipedes with poisonous jaws, which first appeared in
the Devonian. Ostracods, tiny bivalved crustaceans also known as mussel
shrimp, increased in abundance but declined in the late Permian when the seas
contracted due to continental collisions. The flowerlike crinoids were com-
mon in the Devonian seas, reaching a peak in the late Paleozoic. However, the
crinoids and their blastoid cousins failed to survive past the Permian.
The nautiloids and ammonoids (Fig. 97) appeared in the early Devonian
about 395 million years ago. They had external shells subdivided into air
chambers.The suture lines joining the segments presented a variety of patterns
used for identifying various species. The air chambers provided buoyancy to
counterbalance the weight of the growing shell. Most shells were coiled in a
plane, some forms were spirally coiled, and others were essentially straight.
The now-extinct nautiloids grew upward of 30 or more feet long.With
their straight, streamlined shells, they were among the swiftest and most spec-
tacular creatures of the Devonian seas.Their neutral buoyancy, maintained by
air chambers, and jet propulsion for high-speed travel, by expelling water
under pressure through a funnel-like appendage, contributed to the nautiloid’s
gr eat success.
The belemnoids, which probably originated from more primitive nau-
tiloids, were abundant during the Jurassic and Cretaceous but became extinctFigure 96The conodonts
show their greatest
diversity in the Devonian.
Historical Geology