The Paleozoic Era
The largest mass extinction in the history of life on Earth—the Permian Mass Extinction—marks the end of the Paleozoic Era. Following this event, also called the Permo-Triassic Extinction Event
approximately half of the marine invertebrate families and nearly 95 percent of the marine invertebrate species were wiped out. Earth had seen the last of the trilobites, the reef-building colonial tabulate
corals, the solitary and colonial rugose corals, a group of echinoderms called blastoids, and a group of large single-celled organisms called fusulinids. Although brachiopods and bryozoans survived the
extinction, their numbers were greatly diminished.
The Permian Mass Extinction
In contrast to the mass extinctions of the Middle Paleozoic, the Permian Mass Extinction affected both marine invertebrate animals and many of the terrestrial organisms that thrived in the lowland
swamps during the late Paleozoic. More than 65 percent of the amphibians and reptiles did not survive, nor did almost one third of all insects. Life was certainly not the same after the Permian. But what
could have caused such widespread catastrophe?
Geologists and paleontologists have tried to unravel the mystery for years. No single theory has met with agreement in the scientific community. However, two possible causes, working in concert, may
have led to the Permian Mass Extinction.
Marine Regression
Most geologists hypothesize that a major marine regression, or a lowering of sea level, may have triggered the mass extinction of ocean life during the Permian. Regressions, even large ones, are not
uncommon throughout geologic history. A large regression during the time of a single supercontinent, however, was unique. Now, as then, most marine animals lived in the relatively shallow water of the
continental shelf, which surrounds a continent. The shelf drops off steeply to the deep ocean at a place called the continental slope. When sea level is high, the entire continental shelf is flooded and
marine animals have a large area in which to live. During a regression, however, sea level may lower to the point that the continental slope is nearly exposed, and there is very little inhabitable space
on the continental shelf for marine animals. Because there was only one continent during the Permian—Pangaea—the impact of a major regression and the subsequent loss of marine habitats was particularly
devastating.
LINK UP: Find out more information about mass extinctions.
Climatic Change
The second major cause of the Permian Mass Extinction may have been climatic change. Scientific studies provide evidence of global warming at the end of the Permian. Recall that global warming is
an increase in temperature worldwide. During the Permian, the warming was likely caused by relatively high levels of atmospheric carbon dioxide. As was explained earlier, the continental shelf was exposed
during the Permian marine regression, and the organic material that was trapped in the sediments were then exposed to the atmosphere and oxidized. This process, in turn, lowered levels of atmospheric
oxygen. At the same time, large amounts of carbonate rocks that had been deposited earlier in the Paleozoic were exposed and chemically eroded. One product of the chemical erosion of carbonate (CaCO3)
rock is carbon dioxide (CO2). Carbon dioxide in the atmosphere traps heat that radiates from Earth when the surface is warmed by the Sun. The trapped heat then warms the atmosphere and keeps temperatures
on Earth at an optimal level for life—a phenomenon known as the greenhouse effect. However, increases in atmospheric carbon dioxide can accelerate the greenhouse effect and lead to global warming, as
happened during the Permian.
The combined impact of marine regression and global warming made this period in Earth’s history a stressful time for life. Marine habitats shrank as a result of the regression that occurred around Pangaea,
and the resulting global warming affected organisms on land as well as in the sea. Additional stresses also may have been present at that time—scientists simply do not know. What is known, however, is
that life on Earth was irrevocably altered at the end of the Paleozoic.
Activity
Design an experiment to show how an increase in temperature affects an organism, such as a green plant. Be sure to identify your variables and control. With your teacher’s approval, carry out the
experiment.
