LATE PALEOZOIC ERA
CARBONIFEROUS PERIOD: 359 to 299 million years ago
PERMIAN PERIOD: 299 to 251 million years ago
In the Carboniferous, the climate began
to cool and Gondwana (which included the South Pole) began a major ice
age. The seas underwent several episodes of expansion and retraction from
the continental interiors. A number of continental fusions were finally
completed: the joining of Ancestral North America to Gondwana (and the
end of the formation of the Appalachian Mountains), the joining of Ancestral
Europe to Siberia (and the end of the formation of the Urals), and the
joining of Ancestral China to Ancestral Siberia (ending the formation
of mountains of central Asia). By the end of the Permian the seas had
largely retreated from the continental interiors. During the Permian,
extensive ice sheets covered Antarctica (which included the South Pole),
Australia, India, Southern Africa, and Southern South America. The climate
warmed from the Early through Late Permian (Seyfert, 1979).
Amphibians diversified into many lineages. Some adapted to aquatic life, some to terrestrial life, and some could reach a length of 4 meters.
Some evolved into reptiles.
From the early, primitive reptiles, a diversity of reptiles evolved including the first terrestrial herbivores, and the ancestors of later groups, such as dinosaurs and mammals.
Synapsid reptiles had become very similar to mammals by the end of the Permian.
End Permian Extinctions
The mass extinction that occurred at the end of the Permian was the worst mass extinction in history. An estimated 96% of marine species became extinct. The dominant groups of the Paleozoic seafloor (crinoids, bryozoans, and brachiopods) were replaced by groups which continue to dominate today (bivalves, gastropods, and echinoderms). Bivalves and gastropods were less affected and lost perhaps 30% of their diversity. Some groups had been reduced by earlier mass extinctions and now were completely wiped out (trilobites, blastoids, tabulate corals, rugose corals, orthid brachiopods, and many mollusks). Fusulinid foraminiferans and productid brachiopods were thriving in the Permian but went extinct at its end. Only one group of crinoids and two groups of ammonoids survived the end of the Permian.
What could have caused this extinction which lasted 8 million years?
A variety of factors have been considered and multiple factors could have contributed. One of the largest eruptions of flood basalts (the Siberian traps) and a large volcanic eruption might have changed the composition of the atmosphere and blocked sunlight (Erwin, 1994).
One of the most significant events of the Permian Period was the joining of the continents. The various landmasses on earth came together in the middle Permian to form a single supercontinent known as Pangea. As the ocean which separated what would someday become the Americas from Europe and Africa disappeared, many shallow marine habitats were lost. The change in bodies of water would have affected climate. Terrestrial ecosystems seem to have collapsed after the events which devastated marine ecosystems, and caused both animal and plant extinctions. Climatic fluctuations may have been a major cause (Looy, 2001, Erwim 1996). Some have argued that the halogens which would have been released by the Siberian Traps at the end of the Permian may have destroyed enough ozone to contribute to an ecological crisis (Visscher, 2004).
The formation of a supercontinent would certainly affect local climates
since rainfall patterns would have been altered. In addition, all of the
organic material from former marine continental shelves which were exposed
to air would have decomposed. This would have used tremendous amounts
of oxygen, perhaps reducing the atmospheric oxygen content by half. It
appears that the worlds single ocean (named Panthalassa) suffered
a major anoxic event which lasted millions of years at the end of the
Permian. As oxygen levels dropped, carbon dioxide levels in the ocean
seem to have increased substantially. Not only would this have been toxic
for marine life, it appears that the carbon dioxide entered the atmosphere
causing a brief greenhouse effect which melted the polar ice caps which
had formed earlier in the Permian (for the first time in tens of millions
of years. Whatever initiated the end Permian extinction (probably either
mass volcanic eruptions and/or a meteorite impact), the result seems to
have been a greenhouse effect which raised the earths temperature
by an estimated 6 degrees Celsius. Both terrestrial and marine ecosystems
were decimated and required tens of millions of years to recover their
previous biodiversity (Benton, 2003; Isozaki, 1997, Erwin, 1996; Erwin
1994; Waters, 1997).