The worst mass extinction of all time did far more than nearly denude the planet of life. This vast catastrophe—probably triggered about 252 million years ago by massive eruptions of the Siberian Traps volcanoes—destabilized life on Earth so drastically, according to a new study, that ecological aftershocks continued to hinder the recovery of life on land for millions of years.
Much of what paleontologists understand about the event, known as the end-Permian extinction, and its aftermath they learned from the fossil record of marine organisms. Up to 95% of known species, including the last of the trilobites, disappeared during the extinction. And the plentiful record of fossil fish and invertebrates indicates that ecosystems in the seas required about 5 million to 8 million years during the following Triassic period to regain their previous diversity and complexity. The story on land, however, has been unclear.
Now paleontologist Randall Irmis of the University of Utah and the Utah Museum of Natural History in Salt Lake City and geologist Jessica Whiteside of Brown University propose online today in the Proceedings of the Royal Society B that things were just as tough on dry land as in the seas. Irmis and Whiteside examined fossil vertebrate data sets from southern Africa's Karoo Basin and the Ural region of Russia. They found that the number of different land-dwelling vertebrate genera dropped during the time interval when the extinction struck. In the post-extinction world, a small cadre of "disaster species" made the most of a bad situation. These survivors, including the "shovel lizard" Lystrosaurus (see picture), were hardy species that could make a living under distressed conditions. Such creatures quickly colonized and dominated the environments that had been shaken up by the mass extinction.
But life was not easy. According to previous geological studies, the global cycle through which carbon is recycled through land, air, and water was disrupted again and again during this time. Previous researchers blamed these cycles on continued volcanic activity. But Irmis and Whiteside propose instead that the perturbations point to a "boom-bust" cycle in which relatively minor changes in temperature, for example, caused some of the surviving species to go extinct. These smaller-scale losses then caused ecosystems to collapse. The events that triggered the end-Permian extinction drastically changed the climate, atmosphere, and other aspects of global ecology, hindering the recovery of life on land during the Early Triassic. Even as survivors of the mass extinction began to recover, the ecosystems they lived in were so fragile that the lingering influences of the end-Permian extinction—such as global warming—could cause those habitats to fail.
This ongoing cycle acted as a reset button, temporarily preventing land vertebrates from evolving the diversity their pre-extinction ancestors had enjoyed. "It seems that the rate of recovery in both marine organisms and terrestrial vertebrates was pretty similar," Irmis says. "They didn't really bounce back until the Middle Triassic, some 5 to 6 million years after the extinction."
Paleontologist Peter Ward of the University of Washington, Seattle, agrees that the new study adds to the big picture of how life recovered from the end-Permian extinction. But our understanding of that time period is still changing, he notes. For one thing, what seems like a long recovery might look shorter as geologists revise figures of how long the Early Triassic actually lasted. If new studies find that the Early Triassic rocks were laid down during a shorter time frame than presently known, then the recovery of Early Triassic life would have happened more rapidly than presently thought.
However long it lasted, the Early Triassic world was likely a harsh one. Citing the work of geologist Lee Kump of Pennsylvania State University, University Park, Ward says that in the millennia after the mass extinction there would have been hot snaps, lowered oxygen levels in lakes and oceans, and possibly the persistence of the poisonous "swamp gas" hydrogen sulfide in the atmosphere. The powerful volcanic eruptions that likely triggered the extinction left a devastating imprint on the planet, which continued to shake up life on Earth even as ecological wounds began to heal.