Ancient Mammal Relative Dug Burrows in Antarctica?

Sara Goudarzi
for National Geographic News
June 9, 2008
Fossilized burrows found in Antarctica could be the first evidence that four-limbed land animals, or tetrapods, lived in the region as far back as 245 million years ago, paleontologists have announced.

Although no animal remains were discovered, the burrows act as "trace fossils" that reinforce the theory that Antarctica was once ice free and supported a very different array of wildlife than it does today.

The burrows were preserved when sand from a nearby river overflowed into them and hardened, creating casts of the open spaces.

The largest den is only about 6 inches (15.2 centimeters) wide and 14 inches (35.5 centimeters) long.

Based on their size, some of the burrows might have belonged to members of a genus of cat-size mammal relatives known as Thrinaxodon, the scientists say.

"[Thrinaxodon] bones have been found in the Antarctic already," noted lead study author Christian Sidor, a biologist at the University of Washington and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture.

"So we think this is the animal that was making the larger of the two burrows that we describe in the paper."

Sidor's study is set to appear in the June issue of the Journal of Vertebrate Paleontology.

No Bones

The fossils were collected in 2003 and 2006 toward the outer edges of Antarctica from the Fremouw Formation at Wahl Glacier and from the Lashly Formation at Allan Hills.

The newfound burrows date back to the early part of the Triassic period, 251 to 199 million years ago—the start of the Mesozoic era. (Explore an interactive prehistoric time line.)

During this time, all of Earth's landmasses were joined into a supercontinent called Pangaea, although the regions where the burrows were unearthed were within the chilly Antarctic Circle.

Researchers believe the burrows provided much needed shelter for the mammal-like Thrinaxodon.

"When it's very cold or even when it's very hot outside, burrows seem to maintain a relatively even temperature," Sidor said.

But finding Antarctic burrows that contain ancient animal remains is very difficult, in part because fossilized bones are most often found inside rocks.

Antarctica today contains more glaciers and ice than exposed rock.

"Without rocks, there's no way you're going to find bones," Sidor said.

"Rock outcrops—the places for us to search—are very remote and very limited, which makes anything found in Antarctica a tremendous discovery."

Paleontologists in South Africa have found similar burrows from the same time period containing tetrapod remains.

"The same types of animals lived in South Africa as in Antarctica," Sidor said, "so we expect the burrows to be similar."

Preparation and Luck

Anthony Martin is a paleontologist at Emory University in Atlanta who was not involved with the study.

"Burrows give a major survival advantage to whoever is living in them," Martin said.

(Related: "Digging Dinosaur Discovered Inside Fossil Den" [March 21, 2007].)

"So there were good reasons for why Mesozoic animals were burrowing to avoid freezing temperatures outside the burrow, all while getting away from predators or safely raising their young."

According to Martin, fossil burrows are much more common than most geologists and paleontologists realize.

But pinpointing them requires knowing what they look like and precisely where to look.

"Fortunately these paleontologists had the ideal combination of preparation and luck on their side, which worked out very well for them," Martin said.

"After all, you can't go to Antarctica just any time you feel like it and then go searching for fossil tetrapod burrows!"

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