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Dawn of an age

Early Triassic fossils provide clues to the rise of mammals, dinosaurs

 

It’s the greatest murder mystery in the history of the planet.

About 250 million years ago, life on Earth collapsed. More than 90 percent of the organisms in the ocean disappeared. Some 70 percent of land animals also died off. It all happened in a geological heartbeat, though there is some debate on the actual duration, as well as whether life flatlined all at once or in waves.

Clues as to how the Permian-Triassic (P-T) extinction unfolded can be found in fossils entombed in the hard sedimentary rock of the central Transantarctic Mountains, one of the world’s longest ranges, which bisects Antarctica’s two great ice sheets.

Several teams of paleontologists spent part of the austral summer there, flying and hiking among the peaks and outcrops to collect the rare and unusual bones of vertebrates that once burrowed and bounded across a much different landscape — including at least one never before found on the icy continent.

“What we’re doing is recording what happened on land step by step, almost blow by blow, about what happened to cause the extinction,” explained Roger Smith, a paleontologist from Cape Town’s South African Museum.

“These animals adapt to the environments. By looking at the animal, we can get some indication of what the environment was like,” added Smith, a member of a team collecting everything from mammal-like therapsids from the Early Triassic to dinosaurs that lived in the region during the Jurassic, a stretch of time about 100 million years long.

Smith, co-principal investigator Chris Sidor from the University of Washington, and PhD student Adam Huttenlocker, also from UW, are particularly interested in the deeper time period. Colleagues led by principal investigator William Hammer from Augustana College focused on the Jurassic.

What caused the Great Dying, as the P-T boundary is sometimes called, is the big mystery. At the time, all of the landmasses had pushed and shoved together into a supercontinent called Pangaea, which would have caused changes in ocean and atmospheric circulation. The vast interior may have become extremely arid.

There is evidence that the oceans had turned anoxic, nearly depleted of oxygen, at the end of the Permian. Extreme volcanism may have triggered greenhouse conditions that would have caused planetary temperatures to rise, as well as acidify the ocean. Similar environmental changes are happening today.

Based on specimens the Karoo Basin in Africa, where Smith has worked for nearly three decades, he believes drought was the major cause of death across Pangaea. But the exact cause of that drought — supercontinent assembly, volcanism, shifting circulation patterns, or even an impact event — is not within the scope of this project.

In fact, the scientists are just as fascinated with the flip side of the P-T extinction – recovery. “The way in which nature bounces back is just as intriguing as the extinction event itself,” Smith said.

For example, a type of therapsid known as Lystrosaurus bolted out of the gate from the Late Permian into the Early Triassic, becoming the most common animal of the time. These and other mammal-like therapsids, which combined reptilian characteristics with some mammalian features such as canine teeth, were able to survive whatever catastrophe befell the world at the P-T boundary.

And that’s a good thing for us, according to Huttenlocker. “Eventually, they gave rise to mammals in the Late Triassic,” he said.

It also helped that the gorgonopsians, another type of large carnivorous therapsid that ruled during the Permian, took a big blow during the extinction. “They basically ruled the land. If they hadn’t been given this tremendous knock, there would have been no [ecological] space for the others to come through, especially the ones that led to humans,” Smith said.

It’s also after the P-T extinction that the first of the common ancestors of dinosaurs arose. And one of the few and most complete specimens of these archosauromorphs, called Prolacerta, was discovered this season at a site called Graphite Peak.

As Smith tells the story, the team was excavating a Lystrosaurus, one of the common therapsid herbivores, when they found the remains of a Prolacerta-like animal resting underneath. It’s the most complete and best preserved skeleton of this type of animal in the region, he said.

“This thing is beautiful,” Huttenlocker said.

Another intriguing find came from Graphite Peak, but in the geological formation directly beneath the Lystrosaurus-bearing rocks. A handful of bones here and from Coalsack Bluff, not yet identified, are about as close as scientists have come to finding actual bones from before the P-T boundary in Antarctica. Plants flourished during the Permian — and numerous fossils have been collected over the years — but never anything from a vertebrate.

“Of course, years of collecting [in Antarctica] means a few months,” noted Smith, referring to the short field seasons available to researchers in such remote polar locations.

The handful of earliest Triassic bones includes a clavicle that appears to belong to an amphibian and a partial skeleton of a small reptile.

“We’ll be able to fill in the story better of how vertebrates came down here and when precisely they came down here,” Huttenlocker said of the finds. “It’s filling in a little bit more of that story of what’s going on earlier on. We still don’t have a good Permian record yet, but we’re working our way down.”

A PhD student with Sidor, Huttenlocker’s research involves the growth strategies used by vertebrates as they passed across the extinction boundary. Did they grow smaller (a pattern observed following other extinction events)? Did they shorten the time it took to grow to adulthood? To answer such questions, he looks at the microstructure of the bones.

“I take priceless fossils, relics from antiquity, and cut them up, to look inside their bones so as to look at the growth rings that they record,” he explained.

Huttenlocker can compare growth rings between animals in Antarctica and the Karoo Basin, for example, to see if they grow differently in different environments.

The comparison is particularly apt because scientists like Smith believe the two ecosystems, joined together in Pangaea (and still later as the southern supercontinent Gondwana) were remarkably similar. Even down to the fauna.

“It’s been quite a revelation to find not just the same animals, but the same down to species level, to prove there was genetic flow between South Africa and here,” he said.

One difference between the two places is the ease in which the fossils can be extracted.

Antarctica is not just a tough place to visit. The rocks undergo little weathering, so those that are exposed are in the sort of condition one might expect to find several meters below the surface.

For extraction, jackhammers give way to gas-powered, diamond-bladded rock saws. “Saw down, chisel out, saw down, chisel out,” said Smith, almost like repeating a mantra.

Added Huttenlocker: “It’s no trivial task. This is the hardest rock a paleontologist will ever work on.”

Of course, even to get to the point of sawing into the rock, one must first find the fossils. A trained geologist, Smith seems to possess a particularly keen eye for spotting irregularities in the rock that might indicate a fossil is lurking nearby. It’s also a matter of confidence, he said.

“It’s not a matter of if you are going to find it, but when,” Smith said. “We have definitely done what we came out to do. We came out to find fossils in these places, and we have done it.”

In fact, the team collected about 40 specimens, which should arrive at UW in Seattle this month, according to Sidor.

One specimen that didn’t make it back was an exposed fossil — a rib cage or backbone — that Sidor found buried under about a meter of rock in 2003 but didn’t have the time or tools to retrieve.

“I wasn’t able to relocate the specimen at Fremouw Peak,” Sidor said later via email. “However, that didn’t matter, because we found so much more that it was probably our most productive locality.”

NSF-funded research in this story: Christian Sidor, University of Washington, Award No. 0838762.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs