A large variety of corals are well represented in the fossil record and
resemble many of their modern counterparts. The tabulate corals, which
became extinct at the end of the Paleozoic, consisted of closely packed polyg-
onal or rounded thecae, some with pores covering the walls of the thecae.The
rugose, or horn corals, named for their hornlike shape, were particularly abun-
dant in the Silurian. They became the major reef builders of the late Paleo-
zoic, finally becoming extinct in the early Triassic.The hexacorals, with thecae
separated by six sepia, or walls, ranged from the Triassic to today and were the
major reef builders of the Mesozoic and Cenozoic eras. The tetracorals (Fig.
82), whose sepia were arranged in groups of four, were another extinct group
of reef-building corals.The rudists (Fig. 83) were mollusks that displaced corals
as the dominant reef builders in the Cretaceous and became extinct at the end
of the period.
In the geologic past, massive coral structures turned into some of the
greatest limestone deposits on Earth. Corals constructed barrier reefs and
atolls.They also played a major role in changing the face of the planet. Coral
reefs contain abundant organic material. Many ancient reefs are composed
largely of a carbonate mud with the skeletal remains of a variety of other
species literally “floating” in the sediment, producing some of the finest fossil
specimens.
Tropical plant and animal communities thrived on the reefs due to the
coral’s ability to build massive wave-resistant structures. Unfortunately, these
were the same species that suffered several episodes of extinction due to their
narrow range of living conditions.The extinctions hit hardest those organisms
anchored to the ocean floor or unable to migrate out of the region because
of physical and biological barriers. Despite the excellent climate and the
extreme success of the trilobites in the lower Paleozoic, they began to declineFigure 82 The extinct
tetracorals were major reef
builders.
Historical Geology