Fossil evidence of ‘winter battle-like’ state in 250 million year old Antarctic animal

Hibernation is a well-known feature on Earth today. Many animals – especially those living near or within polar regions – overwinter to get through the harsh winter months when food is scarce, temperatures drop and days are dark.

According to new research, this type of adaptation has a long history. In an article published August 27 in the magazine Communication Biology, scientists at the University of Washington and its Burke Museum of Natural History and Culture report evidence of a winter-like state in an animal that lived in Antarctica in the early Triassic, about 250 million years ago.

The creature, a member of the genus Lystrosaurus, was a distant relative of mammals. Antarctica lay in the time of Lystrosaurus for the most part within the Antarctic circle, as it does today, and experienced longer periods without sunlight each winter.

The fossils are the oldest evidence of a hibernation-like state in a vertebrate, and indicate that torpor – a general term for hibernation and similar states in which animals temporarily lower their metabolism to get through a difficult season – originated in vertebrates or mammals and dinosaurs evolved.

“Animals that live on or near the poles have always had to deal with the more extreme environments that exist,” said lead author Megan Whitney, a postdoctoral researcher at Harvard University who conducted this study as a UW -doctoral student in biology. “These preliminary findings indicate that introducing a hibernation-like state is not a relatively new type of adaptation. It is an old one.”

Lystrosaurs lived in a dynamic period of our planet’s history, occurring just before the Earth’s largest mass extinction at the end of the Permian period – which wiped out about 70% of the vertebrate species on land and it somehow survived. The naughty, fiery potters lived for another 5 million years during the subsequent Triassic and spread over parts of the then only continent of the earth, Pangea, which encompassed what is now Antarctica.

“The fact that Lystrosaurus survived the Late Permian mass extinction and had such a wide range in the early Triassic made it a very well-researched group of animals for understanding survival and adaptation,” said co-author Christian Sidor , a UW professor of biology and curator of vertebrate paleontology at the Burke Museum.

Paleontologists today find Lystrosaurus fossils in India, China, Russia, parts of Africa and Antarctica. These hardy, blunt creatures – most were roughly pig, but some were 6 to 8 feet long – had no teeth, but carried a few branches in the upper jaw, which they probably used to feed under ground vegetation and to roots. digging and tubers, according to Whitney.

Those teeth made the study of Whitney and Sidor possible. Like elephants, Lystrosaurus teeth grew continuously throughout their lives. The cross-sections of fossil branches may have life-history information on metabolism, growth, and stress as well as stress. Whitney and Sidor compared cross-sections of teeth of six Antarctic Lystrosaurs with cross-sections of four Lystrosaurs from South Africa.

Back in the Triassic, the gathering places in Antarctica were at about 72 degrees south latitude – well within the Antarctic circle, at 66.3 degrees south. The gathering places in South Africa were more than 550 miles north during the Triassic at 58-61 degrees south latitude, far beyond the Antarctic circle.

The teeth of the two regions showed similar growth patterns, with layers of dentine deposited in concentric circles like tree rings. But the Antarctic fossils could have an additional function that was rare or absent in branches further north: dense rings at close range, which probably indicate periods of less deposition due to prolonged stress, according to the researchers.

“The closest analogy we can find to the ‘stress marks’ we have in Antarctic Lystrosaurus teeth are stress marks in teeth associated with hibernation in certain modern animals,” Whitney said.

The researchers can not definitively conclude that Lystrosaurus actually underwent hibernation – which is a specific, week-long reduction in metabolism, body temperature and activity. The stress could be caused by another hibernation-like form of torpor, such as a more short-term decrease in metabolism, according to Sidor.

Lystrosaurs in Antarctica probably need some form of winter-like adaptation to cope with life at the South Pole, Whitney said. Although the Earth in the Triassic was much warmer than it is today – and parts of Antarctica may have been forested – plants and animals under the Antarctic Circle would still experience extreme annual variations in the amount of daylight, with the sun long in ‘ the winter was absent.

Many other high-latitude vertebrates may also have used torpedoes, including hibernation, to cope with the strains of winter, Whitney said. But many famous extinct animals, including the dinosaurs that evolved and spread after the extinction of Lystrosaurus, do not have teeth that grow continuously.

“To see the specific signs of stress and tension caused by hibernation, you need to look at what can be fossilized and grown continuously in the animal’s life,” Sidor said. “Many animals do not have that, but fortunately Lystrosaurus does.”

If analysis of additional Antarctic and South African Lystrosaurus fossils confirms this discovery, it could also spark another debate about these ancient, heart-shaped animals.

“Cold-blooded animals often shut down their metabolism completely during a heavy season, but many endothermic or ‘warm-blooded’ animals that hibernate often activate their metabolism during the winter,” Whitney said. “What we observed in the Antarctic Lystrosaurus teeth fits into a pattern of small metabolic ‘reactivation events’ during a period of stress that most closely resembles what we see today in warm-blooded hibernation.”

If so, this distant cousin of mammals is not merely an example of a hearty creature. It is also a reminder that many features of life today may have existed hundreds of millions of years before humans evolved to observe them.