At the boundary between the Permian and Triassic periods, 252 million years ago, multi-celled life on planet Earth was nearly terminated. This PT mass extinction represents the greatest dying in the fossil record, with more than 90 percent of species lost. New results from South Africa provide the best-ever picture of the PT extinction on land, suggesting that it was a much more complex process than would be expected for a comet or asteroid impact.
Ever since the KT mass extinction (65 million years ago) was traced to the impact of a comet or asteroid, some scientists have looked for evidence of an impact at the PT boundary 252 million years ago. We have reported here recent research by Luann Becker
and others that indicate such an impact took place. A location for the impact crater has even been suggested, in an undersea structure near Australia. However, many other scientists remain skeptical of the impact explanation for the PT event.
This week, an article in the journal Science reports that new fossil evidence found in the Karoo Basin of South Africa indicates a gradual decline for the Permian land creatures, combined with a sharp extinction event that is coincident with the previously studied PT extinction of marine organisms studied in China.
The article, authored by NAI Investigator Peter D. Ward of the University of Washington at Seattle and his collaborators, presents the first detailed study of vertebrate extinction patterns from the late Permian and early Triassic periods. They found that various species of therapsids, the "mammal-like reptiles," became extinct at different times, consistent with extended climate or habitat change. While the estimated time interval for these changes is geologically brief (several million years in duration), it is much longer than the time-scale for the end-Cretaceous (KT) extinction, which was caused by an impact.
The contending Permian extinction theories that induce such environmental change include increase in temperature (perhaps due to an enhanced atmospheric greenhouse effect) and lowered levels of oxygen. Changes of this sort might have been the result of large-scale volcanic eruptions from what is now Siberia. It is perhaps no coincidence that these huge Siberian eruptions took place at the same time as the great dying at the end off the Permian period.
Whatever the cause, the Permian extinction signaled the end for most species, including the top land predator of the time, the dimetrodon, an 11 foot long, 250 pound reptile with a fin-like sail lining its back, and the diverse varieties of ubiquitous placoderms, heavily armored fish with bright red backs and silver bellies, flattened like stingrays or spiny like blowfish.
A Permian atlas of the world would contain one continent, Pangea. On the swampy land lived insects, amphibians, and reptiles among ferns and coniferous trees. The warm, shallow oceans hosted reefs of clam-like and snail-like shelled creatures, bony fish, sharks, coiled predators related to the octopus, and trilobites. The dawn of the dinosaurs was still 25 million years in the future.
This new work is described in a news story from the University of Washington. For those who want more information, including personal stories of a decade of field work in South Africa, Peter Ward has also recently published a trade book called Gorgon: Paleontology, Obsession, and the Greatest Catastrophe in Earth's History.