Ancient burrows

Triassic-era sediments reveal that a four-legged animal — a reptile of the type from which modern-day mammals evolved — once burrowed in Antarctic soils. The findings, reported in the June Journal of Vertebrate Paleontology, are the first to show that such a creature lived so close to the poles during that time period.

Researchers uncovered a set of burrows in the Transantarctic mountain range, which juts through the ice and cuts from north to south across the continent. The underground passages, a set of tubular shafts with a W-shaped bottom, were likely made by a mammal-like reptile, says Christian Sidor, a paleontologist at the University of Washington in Seattle who led the study.

They also discovered another set of burrows likely made by a smaller reptile.

Earlier excavations in the Karoo Basin of South Africa had uncovered a similar underground den with fossilized remains of a cat-sized reptile “curled up like a little puppy” inside, says Molly Miller, a paleontologist at VanderbiltUniversity in Nashville who was also involved in the study. The creature, called Thrinaxodon, had scales and laid eggs like a reptile, but was also whiskered, warm-blooded and possibly furry.

The Antarctic burrows did not contain any fossilized remains, but researchers can infer how many legs the digging animal had based on nail scratch marks found on the side and roof of the burrows, says Gideon Groenewald, a geologist at the Peace Parks Foundation in Lesotho, South Africa, who found the South African Thrinaxodon remains. The animal’s legs created two grooves on either side of a burrow, leaving a hump in the middle, he added. With these details, scientists can deduce the arm and body length of burrowers and narrow down the possible sources of the burrows considerably.

Ancient burrows are important because they can reveal information about animal behavior and the ancient environment that ordinary fossils can’t, Sidor says.

“Burrows represent the best possible information on the behavior of the animals and give a very good indication of the paleoenvironment in which they lived,” agrees Groenewald.

The resemblance between the Antarctic burrows, called trace fossils, and those found in the Karoo basin suggest that a Thrinaxodon may have dug both, Miller says. “The Antarctic fossil was created at least by a creature of similar size, but we don’t know for sure that it was made by exactly the same animal.”

During the Triassic period, between 280 and 235 million years ago, Antarctica was ice-free and more temperate than it is now, Sidor says. Antarctica was still dark for six months of the year though, which may have spurred land animals to dig protective shelters to keep toasty in winter and comfortable in summer.