Antarctanax shackletoni: Iguana-Sized Reptile Roamed Antarctica 250 Million Years Ago

Saturday, February 2, 2019

Along the banks of a river, three archosaur inhabitants of the dense Voltzia conifer forest cross paths: Antarctanax shackletoni sneaks up on an early titanopetran insect, Prolacerta broomi lazes on a log, and an enigmatic large archosaur pursues two unsuspecting dicynodonts, Lystrosaurus maccaigi. Image credit: Adrienne Stroup, Field Museum.

Some 250 million years ago (early Triassic period), an early relative of dinosaurs and crocodiles lived in what is now a frozen continent, Antarctica, according to an international team of paleontologists from the United States and South Africa.

The ancient creature, named Antarctanax shackletoni, was a carnivore that hunted bugs, early mammal relatives, and amphibians.

Its small, incomplete skeleton was collected from Graphite Peak in the Transantarctic Mountains during a 2010-2011 expedition.

The fossil specimen consists of portions of the backbone, limbs and skull.

Antarctanax shackletoni was an archosaur, an early relative of crocodiles and dinosaurs,” said lead author Dr. Brandon Peecook, a paleontologist at Field Museum.

“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.”

The most interesting thing about Antarctanax shackletoni, though, is where it lived, and when.

“The more we find out about prehistoric Antarctica, the weirder it is. 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,” Dr. Peecook said.

About two million years before Antarctanax shackletoni lived Earth underwent its biggest-ever mass extinction event — the end-Permian mass extinction. Climate change, caused by volcanic eruptions, killed 90% of all 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 one of the groups that experienced enormous growth.

“Before the mass extinction, archosaurs were only found around the equator, but after it, they were everywhere,” Dr. Peecook said.

“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.”

The discovery of Antarctanax shackletoni bolsters the idea that Antarctica was a place of rapid evolution and diversification after the end-Permian 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,” Dr. Peecook said.

“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 shackletoni is our little part of discovering the history of Antarctica.”

“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 shackletonirepresents,” said senior author Professor Christian Sidor, a researcher at the University of Washington and curator of vertebrate paleontology at the Burke Museum of Natural History & Culture.

“The same rocks that yielded Antarctanax shackletoni also yield some of the earliest mammal relatives from after the mass extinction.”

The study was published in the Journal of Vertebrate Paleontology.


Brandon R. Peecook et al. A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction. Journal of Vertebrate Paleontology, published online January 31, 2019