250 million years ago, they hibernated at the bottom of the world

How to tell if something that died 250 million years ago hibernated when it was alive?

After all, hibernation — a state of reduced metabolism — is a good strategy for making it through long, harsh winters when food can be scarce. Biologists would not be surprised that evolution figured this out early in the history of life. But uncovering convincing evidence of that is hard.

“As a paleontologist, what you’re presented with is a pile of bones,” said Christian A. Sidor, a professor of biology at the University of Washington and curator of vertebrate paleontology at the Burke Museum in Seattle. “And that just tells you where the animal died. It doesn’t even tell you where the animal lived.”

But Sidor and Megan R. Whitney, a former graduate student who is now a postdoctoral researcher at Harvard, believe they have good evidence of hibernation behavior in an animal that lived in Antarctica a quarter of a billion years ago — before the age of dinosaurs.

This was a tumultuous time for life all around the planet, which was recovering from the largest mass extinction ever on Earth, marking the end of the Permian geologic period and the beginning of the Triassic. Antarctica, then as now, was near the South Pole, and might have provided something of a haven from the cataclysm, often called the Great Dying. (The cause of this extinction is still being debated.)

Whitney said this animal, Lystrosaurus, was about the size of a medium dog with a beak like a turtle and two small tusks, and it was one of the species to make it through the mass extinction.

“It’s an odd animal,” she said. “It’s kind of a sausage shape. And it had no teeth except for the two tusks that came out from the face.”

Despite its dinosaur-sounding name — it means “shovel lizard” in Greek — this creature was more closely related to mammals.

The tusks — just a few inches long, probably used to dig up roots and tubers to eat — provided the telltale signs that the metabolism of Lystrosaurus periodically slowed down.

As with modern-day elephants, the Lystrosaurus tusks grew continuously. Thus, cutting a thin cross section of a tusk provided a record of the animal’s life, much like tree rings, with alternating dark and light circles. Whitney and Sidor compared the patterns in the tusks of six Lystrosaurus that lived in Antarctica with four from South Africa.

The Antarctic tusks included closely spaced, thick rings — likely periods where growth of the tusks slowed, maybe stopped, because of stress — while the South African ones did not.

Although all of Earth’s land at the time was combined into the supercontinent Pangea, the part that is now Antarctica was still near the South Pole and the part that is now South Africa was still hundreds of miles to the north.

Temperatures were warmer then, so Antarctica was not draped with ice sheets. But Earth was tilted about the same as it is now, leading to short days during winter. The dark days would have slowed the growth of plants, leaving little in the way of food for herbivores such as Lystrosaurus to eat.

Thus the researchers interpreted the thick, dark rings as a result of hibernation-like metabolism. The patterns are similar to what is seen in the teeth of modern-day mammals that hibernate in winter.

The findings also suggest that Lystrosaurus was warm-blooded. While the metabolism of coldblooded reptiles can often shut down entirely, hibernating mammals periodically rouse themselves.

The findings were published on Thursday in the journal Communications Biology.

“The idea that they’re saying, OK, there’s actually some interesting variation in the size of these features that tells us about the life history of the animals,” said Kenneth Angielczyk, curator of paleomammalogy at the Field Museum of Natural History in Chicago. “That’s something kind of new and quite interesting.”

Angielczyk was not involved with the Lystrosaurus research, although he is collaborating on other projects with Sidor and Whitney.

Whether Lystrosaurus actually hibernated or otherwise slowed its metabolism — biologists refer to the strategies as torpor — may never be known.

“This is a first study of its kind,” Whitney said, “so it’s going to be preliminary.”