tified by the complex structure of their skeletons, which aids in delineating
specific geologic periods. Bryozoans have been very abundant, ranging from
the Ordovician to the present.Their fossils are highly useful for making rock
correlations. Because of their small size, bryozoans are ideal microfossils for
dating oil well cuttings.
Bryozoan fossils are generally found in sedimentary rocks, particularly
when covering bedding surfaces.They resemble modern descendants, with some
larger groups possibly contributing to Paleozoic reef building, resulting in exten-
sive limestone formations.The fossils are most abundant in limestone and less
plentiful in shales and sandstones. Often, a delicate outline of bryozoans can be
seen encrusting fossil shells of aquatic animals, stones, and other hard bodies.
Of particular importance to geologists are the ostracods, or mussel
shrimp,whose fossils are useful for correlating rocks from the Ordovician
onward. Starfish were also common and left fossils in the Ordovician rocks of
the central and eastern United States. Their skeletons comprised tiny silicate
or calcite plates that were not rigidly joined and therefore usually disinte-
grated when the animal died, making whole starfish fossils rare. The sea
cucumbers have large tube feet modified into tentacles and skeletons com-
prising isolated plates that are sometimes found as fossils.
As the Ordovician drew to a close, a mass extinction eliminated some
100 families of marine animals 440 million years ago.The climate grew cold,
and glaciation reached its peak. Ice sheets radiated outward from a center in
North Africa, which hovered directly over the South Pole. Most of the vic-
tims were tropical species sensitive to fluctuations in the environment. AmongFigure 64The extinct
bryozoans were major
Paleozoic reef builders.
Historical Geology