Marine life thrived and diversified
during the Silurian; many forms were similar to those of the earlier
Ordovician Period. Although the later
Period is often called the “Age of Fishes,” the
Silurian also was an important time in the evolution of this group.
In particular, it was marked by the wide and rapid spread of jawless
fishes (agnathans). Only two of their jawless
descendants—lampreys and hagfishes—survive today, but
many different kinds populated the Silurian seas.
Among the invertebrates, brachiopods were especially
diverse and abundant. Many survived from the Ordovician, but other
new forms evolved as well, so that brachiopods represented nearly
80% of all Silurian marine invertebrate species. Although the planktonic
graptolites lost many species in the end-Ordovician
mass extinction, they also remained abundant. The common genus Monograptus
included many species that are useful today as biostratigraphic
fossils. Tabulate and rugose corals, calcareous
bryozoans formed shallow-water tropical reefs.
were among the active predators of the time. Trilobites
began to decline in diversity after having reached their peak in
the Cambrian and Ordovician.