In a paper published in the journal Communications Biology, a team of U.S. paleontologists reports evidence of a hibernation-like condition in Lystrosaurus, an early relative of mammals that lived between 253 and 248 million years ago (Early Triassic epoch). The discovery was enabled by high-resolution of incremental growth marks preserved in the fossilized tusks of Lystrosaurus from Antarctica.
The ancient creature was between 1.8 and 2.4 meter (6-8 feet) in length, had no teeth, but bore a pair of tusks in the upper jaw.
The fossilized remains of Lystrosaurus are known from China, Europe, India, South Africa and Antarctica and this geographic distribution was one of the early pieces of evidence used in support of the large supercontinent Pangea.
The animal’s fossils have been found in burrow structures in South Africa and similar burrow trace fossils have been recovered from Antarctica, but not with Lystrosaurus inside them.
Lystrosaurus arose before Earth’s largest mass extinction at the end of the Permian period — which wiped out 70% of vertebrate species on land — and somehow survived.
“The fact that Lystrosaurus survived the end-Permian mass extinction and had such a wide range in the early Triassic has made them a very well-studied group of animals for understanding survival and adaptation,” said Burke Museum’s Professor Christian Sidor, co-author of the study.
The Lystrosaurus fossils from the Fremouw Formation of Antarctica are the oldest evidence of a hibernation-like state in a vertebrate animal and indicate that torpor — a general term for hibernation and similar states in which animals temporarily lower their metabolic rate to get through a tough season — arose in vertebrates even before mammals and dinosaurs evolved.
“Animals that live at or near the poles have always had to cope with the more extreme environments present there,” said lead author Dr. Megan Whitney, a postdoctoral researcher at Harvard University.
“These preliminary findings indicate that entering into a hibernation-like state is not a relatively new type of adaptation. It is an ancient one.”
The Lystrosaurus tusks made the study possible because, like elephants, their tusks grew continuously throughout their lives.
Taking cross-sections of the fossilized tusks revealed information about the animal’s metabolism, growth and stress or strain.
The paleontologists compared cross-sections of tusks from six Antarctic Lystrosaurus to cross-sections of four Lystrosaurus from South Africa.
The tusks from the two regions showed similar growth patterns, with layers of dentine deposited in concentric circles like tree rings.
The Antarctic fossils, however, held an additional feature that was rare or absent in tusks farther north: closely-spaced, thick rings, which likely indicate periods of less deposition due to prolonged stress.
“The closest analog we can find to the stress marks that we observed in Antarctic Lystrosaurus tusks are stress marks in teeth associated with hibernation in certain modern animals,” Dr. Whitney said.
The researchers cannot definitively conclude that Lystrosaurus underwent true hibernation.
The stress could have been caused by another hibernation-like form of torpor, such as a more short-term reduction in metabolism.
“Lystrosaurus in Antarctica likely needed some form of hibernation-like adaptation to cope with life near the South Pole,” Dr. Whitney said.
M.R. Whitney & C.A. Sidor. 2020. Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica. Commun Biol 3, 471; doi: 10.1038/s42003-020-01207-6