Free ebooks ==> http://www.Ebook777.com
with certain rare members adapted to life in freshwater. They have existed
since the Cretaceous about 140 million years ago.
Sponges were common reef builders in the tropical Tethys Sea during
the Mesozoic, particularly in the Cretaceous. Likewise, hexacorals, which
ranged from the Triassic to the present and were the major reef builders of the
Mesozoic and Cenozoic seas, experienced their greatest abundance and diver-
sity in the Tethys. The brachiopods reached their peak in the Jurassic and
declined thereafter. The gastropods, including snails and slugs, were most
abundant in the Cretaceous, when the modern carnivorous types appeared.
Gastropods increased in number and variety throughout the Cenozoic and are
presently second only to insects in diversity.
The rudists were a significant group of reef-dwelling clams restricted to
seas of the upper Jurassic and Cretaceous.The echinoids, including the urchins,
became abundant for the first time in the Jurassic and Cretaceous but have
declined to obscurity since then. The tubes of annelid worms are particularly
common in Cretaceous marine strata. The star-shaped columnals of certain
crinoids (Fig. 160) are occasionally common in Triassic and Jurassic rocks.The
floating-swimming crinoids underwent a short, widespread evolutionary burst
in the late Cretaceous, which makes them useful for dating rocks of the period.
Figure 160Cogwheel-
shaped crinoid columnals
in a limestone bed of the
Drowning Creek
Formation, Flemming
County, Kentucky.
(Photo by R. C. McDowell,
courtesy USGS)217CRETACEOUS CORALS