On land, some 70 percent of the amphibian families and about 80 percent
of the reptilian families also disappeared. Even the insects did not escape the car-
nage. Nearly one-third of their orders living in the Permian did not survive,
marking the only mass extinction insects have ever undergone.The loss of plant
life might have contributed to the disappearance of insects that fed on the veg-
etation. Following the extinction, insects shifted from a variety of dragonflylike
groups, whose wings were fixed in the flight position, to forms that could fold
their wings over their bodies, possibly as a means of protection.
The extinction followed on the heels of a late Permian glaciation, when
thick sheets of ice blanketed much of the planet, significantly lowering ocean
temperatures. As a further blow, one of the largest volcanic outpourings
known on Earth covered northern Siberia in thick layers of basalt, causing
considerable changes in the environment during a 1-million-year period.The
eruptions removed much of Earth’s oxygen and replaced it with choking sul-
furous and carbon dioxide gases. Fossil evidence suggests that the Permian
extinctions began gradually, culminating with a more rapid pulse at the end
possibly due to environmental chaos.
The interior seas retreated from the continents as an abundance of ter-
restrial redbeds and large deposits of gypsum and salt were laid down. Exten-
sive mountain building raised massive chunks of crust. Land at higher
elevations accumulated snow and ice, which accumulated into thick glaciers.
The glaciers, in turn, reflected sunlight, causing surface temperatures to drop.
The most important factor limiting the geographic distribution of marine
animal species is water temperature. Corals, which require warm, shallow
water, were affected the worst as evidenced by the lack of coral reefs in the
early Triassic.
Whatever were the agents of biological stress—climatic changes, shifts in
ocean currents, shallow seas, or disruptions in food chains—the ability of the
biosphere to resist them varied in different parts of the world. However, one
pattern of mass extinctions occurred very consistently.Although each event
typically affected different suites of organisms, tropical biotas, which contain
the highest number of species, were almost always the hardest hit.
When all continents had converged into Pangaea by the end of the Per-
mian around 250 million years ago, the change in geography spurred a great
diversity of plant and animal life on land and in the sea.The formation of Pan-
gaea marked a major turning point in the evolution of life, during which the
reptiles emerged as the dominant species of the world.
The Pangaean climate appears to have been equable and warm through-
out most of the year. However, much of the interior of Pangaea was desert,
whose temperatures fluctuated wildly from season to season. It had scorching
hot summers and freezing cold winters. These climate conditions might have
contributed to the widespread extinction of land-based species during the lateHistorical Geology