Iguana-sized dinosaur cousin discovered in Antarctica

Antarctica wasn’t always a frozen wasteland. About 250 million years ago, it was covered in forests and rivers, and the temperature rarely dipped below freezing.

It was also home to diverse wildlife, including early relatives of the dinosaurs. Scientists have just discovered the newest member of that family — an iguana-sized reptile whose genus name, Antarctanax, means “Antarctic king.”

“This new animal was an archosaur, an early relative of crocodiles and dinosaurs,” said Brandon Peecook, a Field Museum researcher and lead author of a paper in the Journal of Vertebrate Paleontology describing the new species. “On its own, it just looks a little like a lizard, but evolutionarily, it’s one of the first members of that big group. It tells us how dinosaurs and their closest relatives evolved and spread.”

Collected during a 2010-2011 expedition to Antarctica led by senior author Christian Sidor, professor of biology at the University of Washington and curator of vertebrate paleontology at the UW’s Burke Museum of Natural History & Culture, the fossil specimen consists of portions of the backbone, limbs and skull. The specimen is now part of the permanent collections at the Burke Museum and is one of more than 300 vertebrate fossils from Antarctica in its collection, collected over the course of four expeditions and resulting in one of the largest Antarctic vertebrate fossil collections in the country. During the Burke’s most recent Antarctic expedition in 2017-2018 Sidor led his team back to Graphite Peak, the site where Antarctanax had been found, which was also where the first vertebrate fossils in Antarctica were discovered in 1967.

Although the new specimen is an incomplete skeleton, paleontologists still have a good feel for the animal, named Antarctanax shackletoni — the latter part a nod to polar explorer Ernest Shackleton. Based on its similarities to other fossil animals, the researchers surmise that Antarctanax was a carnivore that hunted bugs, amphibians, and relatives of early mammals.

The most interesting thing about Antarctanax, the authors say, is where and when it lived.

“The more we find out about prehistoric Antarctica, the weirder it is,” says Peecook, who was a doctoral student in the UW Department of Biology at the time the fossil was collected and is now also a research associate at the Burke Museum. “We thought that Antarctic animals would be similar to the ones that were living in southern Africa, since those landmasses were joined back then. But we’re finding that Antarctica’s wildlife is surprisingly unique.”

   

About two million years before Antarctanax lived — the blink of an eye in geologic time — Earth underwent its largest mass extinction. Climate change, caused by volcanic eruptions, killed 90 percent of animal life. The years immediately after that extinction event were an evolutionary free-for-all. With the slate wiped clean by the mass extinction, new groups of animals vied to fill the gaps. The archosaurs, including dinosaurs, were among the groups that experienced enormous growth.

“Before the mass extinction, archosaurs were only found around the equator, but after it, they were everywhere,” said Peecook. “And Antarctica had a combination of these brand-new animals and stragglers of animals that were already extinct in most places — what paleontologists call ‘dead clades walking.’ You’ve got tomorrow’s animals and yesterday’s animals, cohabiting in a cool place,” he added.

“Fossil exploration in Antarctica is really difficult, given all of the logistics involved. But since so little work has been done the potential for making important new discoveries is high — and that’s what Antarctanax represents,” said Sidor. “The same rocks that yielded Antarctanax also yield some of the earliest mammal relatives from after the mass extinction.”

The fact that scientists have found Antarctanax helps bolster the idea that Antarctica was a place of rapid evolution and diversification after the mass extinction.

“The more different kinds of animals we find, the more we learn about the pattern of archosaurs taking over after the mass extinction,” said Peecook. “Antarctica is one of those places on Earth, like the bottom of the sea, where we’re still in the very early stages of exploration. Antarctanax is our little part of discovering the history of Antarctica.”

UNIVERSITY OF WASHINGTON

Header Image – Part of the fossil type specimen of Antarctanax shackletoni, showing well-preserved vertebrae and a humerus. Credit : Brandon Peecook/Field Museum

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