Land life spared in Permian extinction, geologists argue

BALTIMORE — The greatest extinction in Earth’s history might not have been so great after all. A suspected colossal die-off of roughly 75 percent of land species didn’t accompany the Permian extinction around 252 million years ago, a team of geologists contend.

That divisive result comes from new work in South Africa that redates the demise of Dicynodon — a mammal relative whose disappearance defines the terrestrial extinction event in the rock record. The new timeline places the creature’s disappearance at more than a million years before the Permian extinction in the ocean, in which 90 percent of marine species vanished.

Furthermore, the researchers argue, the new evidence raises doubts that a mass extinction on land even happened at all.

Although many experts are not convinced, the established understanding of the Permian extinction is “up in the air,” said geologist John Geissman of the University of Texas at Dallas. Geissman contributed to the work, which was presented November 4 at the Geological Society of America’s annual meeting and published in the October Geology. “We need to rethink the Permian crisis,” he said.

While the Dicynodon dating does raise questions, it isn’t infallible and doesn’t invalidate the consensus that the Permian extinction extended across land and sea, said Jennifer Botha-Brink, a paleobiologist at South Africa’s National Museum in Bloemfontein. “People forget that biology is messy,” she said. “You can never draw a line of when the extinction was. It’s an interval; it’s a changeover.”

Voluminous volcanic outpourings in what is now Siberia probably sparked the severe environmental changes that made the planet uninhabitable for many species (SN: 9/19/15, p. 10). While scientists confidently peg the marine extinction to 251.88 million years ago, the terrestrial die-off is trickier to nail down.

In South Africa’s Karoo Basin, fossilized animals and plants from the Permian are embedded inside layers of accumulated rock. The changeover from Dicynodon fossils to those of another mammal relative called Lystrosaurus marks when scientists think the mass extinction on land occurred.

Scouring the basin, geoscientist Robert Gastaldo of Colby College in Waterville, Maine, Geissman and colleagues made a lucky find: seven zircon crystals that can provide accurate ages for the surrounding rock. Zircon forms with small amounts of uranium that decays into lead over time. Comparing the relative abundance of uranium and lead in zircon provides a precise age of the crystal’s formation.

The zircon crystals date to about 253.48 million years ago and were found about 60 meters below the rock layer that marks the Dicynodon–Lystrosaurus transition. Based on how fast rock accumulated in Karoo Basin, the researchers estimate that the 60-meter gap formed over 200,000 to 300,000 years. That means the Dicynodon extinction took place around 253.2 million years ago, so about 1.3 million years before the Permian marine extinction and a million years before the volcanic eruptions in Siberia started.

The researchers also found traces of land plant and animal species thought to have gone extinct alongside Dicynodon in younger rocks. Those findings don’t point to a single widespread extinction event that abruptly wiped out land species within a short time span, Gastaldo said.

The new findings support the idea that the Permian extinction lacked a land component, agrees Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History & Science in Albuquerque. Given the new dating, the rock layers that actually match up with the marine extinction event are higher in the basin than previously thought. Those layers require further study. But at first glance, they don’t appear to record any major fossil changes indicative of an extinction event, Lucas said.

Botha-Brink isn’t convinced. The accumulation of new rock layers isn’t always constant. So the 60-meter gap between the zircons and fossils may have formed over a much longer period of time, and the Dicynodon die-off may have indeed coincided with the marine extinctions, she said.