species was restricted to narrow areas around the tropics. Species trapped in
confined waterways that were unable to move to warmer seas were particu-
larly hard hit. Furthermore, the accumulation of glacial ice in the polar regions
lowered sea levels, thereby reducing shallow water shelf areas.This limited the
amount of habitat and consequently the number of species.
Ocean temperature is by far the most important factor limiting the geo-
graphic distribution of marine species. Climatic cooling is the primary culprit
behind most extinctions in the sea. Species unable to migrate to warmer
regions or adapt to colder conditions are usually the most adversely affected.
This is especially true for tropical faunas that can tolerate only a narrow range
of temperatures and have nowhere to migrate. Since lowered temperatures
also slow the rate of chemical reactions, biological activity during a major
glacial event should function at a lower energy state, which could affect the
rate of evolution and species diversity.
The greatest extinction event took place when the Permian ended 250
million years ago. The extinction was particularly devastating to Permian
marine fauna (Fig. 128). Half the families of marine organisms, including
more than 95 percent of all known species, abruptly disappeared. On land,
more than 70 percent of the vertebrates died out. So many plants suc-
cumbed to extinction that fungi briefly ruled the continents. Two distinct
die outs occurred in the space of 1 to 2 million years. About 70 percent of
the species became extinct during the first event, and 80 percent of the
remaining species died out in the second episode.The most pronounced loss
of species took place between 252 and 251 million years ago. The final
extinction pulse might have lasted less than 1 million years. In effect, the
extinction left the world almost as devoid of species at the end of Paleozoic
as at the beginning.
Groups attached to the seafloor and filtered organic material from sea-
water for nutrients suffered the greatest extinction. Corals, which require
warm, shallow water for survival, were hardest hit by the extinction. They
were followed by brachiopods, bryozoans, echinoderms, ammonoids,
foraminifera,and the last remaining trilobites.Another major group of animals
that disappeared were the fusulinids, which populated the shallow seas for
about 80 million years, during which their shells accumulated into vast
deposits of limestone.They were large, complex protozoans resembling grains
of wheat.They ranged from microscopic size up to 3 inches in length. Plank-
tonic plants also died out,devastating the base of the marine food web, upon
which other species depended for their survival.
The more mobile creatures, such as bivalves, gastropods, and crabs,
escaped the extinction relatively unharmed.Those organisms, especially the
shelled types, because they could buffer their internal organs from changes in
ocean chemistry, were less likely to be wiped out. They were better able toHistorical Geology