Marine Realm and The End-Permian Extinction
The Permian marine fauna was similar
to that of the Carboniferous. Corals, stromatolites, sponges, bryozoans,
brachiopods, and foraminiferans formed reef ecosystems in the warm
shallow waters. Ammonoids, nautiloids, echinoderms, and gastropods
were still common predators. The great diversity of fishes included
agnathans (jawless fishes), chondrichthyans (such as sharks), and
many types of bony fishes. This collection of marine species represents
the last of the Paleozoic Fauna, which first rose to dominance in
the Ordovician, some 200 million years previously.
The end of the Permian was marked by the greatest mass
extinction of the last 600 million years of Earth history, during
which perhaps 90% of marine animal species disappeared. Major groups
such as trilobites, fusulinid
and tabulate corals, acanthodian and placoderm fishes,
and blastoid echinoderms vanished entirely, Although they did survive,
brachiopods, bryozoans, ammonoids, sharks, bony
fishes, crinoids, eurypterids, ostracods, and many echinoderms lost
the majority of their species. Finally, insects
suffered their greatest mass extinction in Earth history.
Several factors have been implicated in this massive extinction.
The formation of Pangea reduced the
shelves, decreasing the area available for shallow-water organisms.
Rapid warming and glaciation both occurred during
the Permian as well. These events do not seem to have happened at
the same time as the extinction event, however. Indeed, a first
extinction pulse actually occurred during the Middle Permian and
may have been caused by a dramatic drop in sea level. A more likely
cause for the end-Permian extinction was a series of volcanic
eruptions in Siberia, which produced massive outpourings of
lava called flood basalts. This volcanism covered an area about
two-thirds the size of the United States and erupted very rapidly
just at the time of the extinction. It may have caused significant
atmospheric disturbances, global warming, and anoxic (low-oxygen)
ocean waters. The other possible cause is the impact of a large
extraterrestrial object, as occurred with the extinction at the
end of the Cretaceous. Direct evidence for such an impact is sparse,
but the available data are consistent with such a cause.