The Largest Mass Extinction of the Phanerozoic
The Great Dying -The Permian-Triassic Event and Hypothetical Causes
Nov 15, 2008
Dubbed “the great dying,” and “the mother of all mass extinctions,” the Permian-Triassic (P/T) extinction was the largest loss of biodiversity in Earth’s history. Not only were approximately 90 percent of all living species wiped from the planet, but the event changed the basic taxonomic composition on land and in the oceans.
“There's been strong agreement ever since Sepkoski and Raup (1982) that it was much worse than the K/T [the Cretaceous-Tertiary extinction of the dinosaurs] or any other extinction,” says John Alroy, a paleontologist with the National Center for Ecological Analysis and Synthesis, a research facility based at the University of California at Santa Barbara.
The Permian World
According to Donald R. Prothero, in his 1998 text Bringing Fossils to Life, published by McGraw Hill, the Permian oceans were characterized by rugose and tabulate coral-dominated reef systems and were home to an abundance of brachiopods, crinoids and ammonoids. At the start of the Triassic period, approximately 245 to 240 million years ago, those families, along with the trilobites, graptolites and blastoids were wiped out, leaving niche space for new families and genera to evolve. For example, the bottom-dwelling brachiopods were replaced by the more hearty bivalves, leaving only the genus Lingula, a brachiopod still in existence today. The loss of the major coral groups ended reef-building processes for several million years to be replaced by the scleractinian corals, which first appear in the Triassic and take over the reef-building responsibilities.
In the terrestrial realm, the extent of the damage to fauna was just as great. Sixty-one percent of tetrapod families were eradicated, giving way to the Triassic-dominant Lystrosaurs, used by Alfred Wegener in his efforts to prove the existence of Pangea. Only six of the 44 Permian reptilian groups are present during the Triassic, leaving niche space to later be filled by the dinosaurs.
Timing of the P/T Extinction
Using layers of ash deposited above and below the P/T boundary, the timing of the event has been constrained at approximately 250 million years before present by Samuel Bowring of the Massachusetts Institute of Technology and his collaegues in his 1998 Science paper, "U/Pb Zircon Geochronology and Tempo of the End-Permian Mass Extinction." Using key fossils, such as the conodont Hindeodus parvus and the ammonite Otoceras woodwardi, P/T strata have been correlated worldwide in places as far apart as China, Antarctica, Sicily, Canada, Japan, Italy and South Africa.
Hypothetical Causes for the P/T Extinction
Although paleontologists are in agreement that the P/T extinction was the largest in magnitude, it is a matter of contention concerning the hypothetical causes of this major event.
Impact theorists proposed the idea that an extraterrestrial object collided with Earth at that time, leading to global devastation. In her 2001 Science paper, "Impact Event at the Permian-Triassic Boundary: Evidence from Extraterrestrial Noble Gases in Fullerenes," Luanne Becker of the University of Washington cited the presence of helium and argon gases found in complex carbon molecules called fullerenes. These gases, she explains, are evidence of an impact comparable to the one that would exterminate the dinosaurs almost 200 million years later.
However, Doug Erwin, a paleontologist with the Smithsonian Institution National Museum of Natural History, disagrees with the impact scenario. “There simply isn't any evidence for impact,” he says. “Hammond and Zare in Geochimica earlier this year [2008] showed that the fullerene data may be the result of an artifact of analysis. In any event, no one has produced any replication of the evidence which has been provided in favor of impact.”
Rather, Erwin says, “the most likely cause of the extinction is the effects of the eruption of the Siberian Traps, based on the magnitude of the eruptions and the (increasingly) close coincidence in time.” The Siberian Traps are a large area of volcanic activity in Russia, created by the presence of a mantle plume. The massive eruptions that produced an approximated one to four million cubic kilometers of lava, would have led to extreme changes in climate as well as possible sea level fluctuations. But, Erwin says, “the causal connection to extinction I think remains obscure.”
