Ancient Lystrosaurus tusks may show the oldest signs of a hibernation-like state

The earliest fossil evidence of the metabolic slowdowns known as torpor may come from tusks of ancient creatures called Lystrosaurus.

Fossil signatures of hibernation, a form of torpor, have turned up in rodent teeth several million years old. Lystrosaurus species, however, flourished from about 252 million to 248 million years ago.

These ancient relatives of mammals were “totally bizarre animals,” says paleontologist Megan Whitney at Harvard University’s Museum of Comparative Zoology. With short-legged bodies like a corgi, they sported tusks plus a bony turtlelike beak instead of a mouthful of teeth (SN: 12/13/69). Species ranged in size from smaller, doggish animals to somewhat cowlike creatures.

Lystrosaurus lived in some of the most dramatic times on Earth. Unlike many creatures, it survived the massive volcanic eruptions in what’s now Siberia that upset the chemistry of Earth’s atmosphere and oceans and probably triggered the Permian mass extinction about 252 million years ago. Some 70 percent of species on land went extinct (SN: 8/28/15).

Tusks of Lystrosaurus, like those of modern elephants, grow throughout the animals’ lives, recording a biography of sorts. Whitney examined six fossil tusks found in what is now Antarctica. In the Lystrosaurus heyday, those animals would have been far enough south to experience months of darkness in winter.

She found some zones of closely spaced wide dark bands in the tusks that might indicate stress and seasons of torpor. Cells that normally add light-colored dentine material may have stalled for some period of time, creating only a dark mess. Thin, light stripes between dark ones indicate when cells may have been active again, and could signal brief warm-ups like those of modern hibernators, Whitney and paleontologist Christian Sidor of the University of Washington in Seattle report August 27 in Communications Biology.

For comparison, Whitney examined four other tusks, from Africa’s Karoo Basin, which would not have lost sunlight during winter. These fossils didn’t show the big clusters of dark stress bands. One way to explain the difference is that the zones of wide dark bands indicate torpor, and animals with milder winters didn’t need to enter that state.