Paleozoic Life History - The Changing Earth: Exploring Geology and Evolution - page 568

excellent guide fossils for the
Devonian through Cretaceous
periods (

Figure 21.10).
Near the end of the Devonian,
another mass extinction occurred
that resulted in a worldwide near-
total collapse of the massive reef
communities. On land, however,
the seedless vascular plants were
seemingly unaffected. Thus, ex-
tinctions at this time were most
extensive among marine life, par-
ticularly in the reef and pelagic
communities.
The demise of the Middle
Paleozoic reef communities high-
lights the geographic aspects of
the Late Devonian mass extinc-
tion. The tropical groups were
most severely affected; in con-
trast, the higher latitude com-
munities were seemingly little
affected. Apparently, an episode
of global cooling was largely re-
sponsible for the extinctions near
the end of the Devonian. During
such a cooling, the disappear-
ance of tropical conditions would
have had a severe effect on reef
and other warm-water organ-
isms. Cool-water species, in con-
trast, could have simply migrated
toward the equator. Although
cooling temperatures certainly
played an important role in the
Late Devonian extinctions, the
closing of the Iapetus Ocean and
the orogenic events of this time
undoubtedly also played a role by
reducing the area of shallow shelf
environments where many ma-
rine invertebrates lived.

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Figure 21.8 Middle Devonian Marine Reef Community Reconstruction of a Middle
Devonian reef from the Great Lakes area of North America. Shown are corals, cephalopods,
trilobites, crinoids, and brachiopods.

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Figure 21.9 Silurian Brackish Water Community Restoration of a Silurian brackish water
scene near Buffalo, New York. Shown are algae, eurypterids (large scorpion-like arthropods),
gastropods, worms, and shrimp.

stromatoporoids ( Figure 21.8). Although the fauna of these

Silurian and Devonian reefs was somewhat different from
that of earlier reefs and reef-like structures, the general com-
position and structure are the same as in present-day reefs.
The Silurian and Devonian periods were also the time
when eurypterids (arthropods with scorpion-like bodies
and impressive pincers) were abundant, and unlike many
other marine invertebrates, eurypterids expanded into
brackish and freshwater habitats (

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Carboniferous and Permian
Marine Communities
The Carboniferous invertebrate marine community re-
sponded to the Late Devonian extinctions in much the
same way that the Silurian invertebrate marine community
responded to the Late Ordovician extinctions—that is, by
renewed adaptive radiation and rediversifi cation. The brachio-
pods and ammonoids quickly recovered and again assumed
important ecologic roles. Other groups, such as the lacy bryo-
zoans and crinoids, reached their greatest diversity during the
Carboniferous. With the decline of the stromatoporoids and
the tabulate and rugose corals, large organic reefs such as

Figure 21.9). Ammo-
noids , a subclass of the cephalopods, evolved from nau-
tiloids during the Early Devonian and rapidly diversifi ed.
With their distinctive suture patterns, short stratigraphic
ranges, and widespread distribution, ammonoids are

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