Ecosystems and the Paleozoic Fauna
in the Ordovician was quite different from that in the Cambrian. Although
many animal groups survived from the Cambrian, some forms disappeared
and other forms appeared for the first time. These changes marked
the Ordovician as the first example of the Paleozoic Fauna.
During this time, bryozoans, mollusks,
(particularly the fan-like crinoids),
and trilobites diversified; these groups would dominate
marine faunas throughout the Paleozoic. Many of the unusual animal
forms seen in the Burgess Shale had gone extinct, but graptolites,
conodonts, nautiloids, and jawless
fishes had their first known appearances.
These new animals radiated alongside the rugose and
corals that replaced the Cambrian archaeocyaths.
In fact, entirely new types of ecosystems evolved in the Ordovician,
associated with the development of many types of carbonate
(limestone) marine shelves (or platforms).
These warm, shallow-water formations were flooded with sunlight, allowing
the establishment of stable reef ecosystems. These
included the first coral/algal reefs, shallow-water
assemblages of mollusks, brachiopods, echinoderms and trilobites,
and more-diverse underwater slope and deeper-water faunas. Many of
these faunas would become even more important in later Paleozoic periods.
Small but significant, lichens and bryophyte
plants also appeared in the Ordovician and began the first of
many colonizations of dry land. However, the end of the Ordovician
saw the second-largest mass extinction of the Paleozoic,
resulting in the loss of many different animal species. One-third
of all brachiopod and bryozoan families disappeared, as well as many
types of trilobites and reef-building animals. Even some of the newly
evolved graptolites and