Describe and discuss the marine communities of the Ordovician, Silurian, and Devonian. How do they compare with
present-day marine communities? What will be an ideal response?
The Middle Ordovician was noted for a major marine transgression causing widespread
epeiric seas over much of the North American craton. This shallow marine environment was
home to a variety of brachiopods, bryozoans, and corals. Acritarchs also were abundant. A
major mass extinction occurred at the end of the Ordovician and was followed by a
rediversification of marine organisms in the Silurian and Devonian marine communities.
Eurypterids and ammonoids made up a large part of the populations during these periods.
Brachiopods, bryozoans, gastropods, bivalves, corals, crinoids, and graptolites were some of
the groups that rediversified during the Silurian. The Silurian and Devonian periods were also
dominated by reefs and reef-like structures. Another mass extinction occurred at the end of
the Devonian.
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