Evidence of “Hibernation-Like” State Discovered in Early Triassic Creature

Torpor in Lystorsaurus

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Life repair of Lystrosaurus in a state of torpor. Credit: Crystal Shin

Hibernation is a familiar function on Earth today. Many animals — particularly those that live near or within polar areas — hibernate to make it through the difficult cold weather when food is limited, temperature levels drop and days are dark.

According to brand-new research study, this kind of adjustment has a long history. In a paper released today (August 27, 2020) in the journal Communications Biology, researchers at the University of Washington and its Burke Museum of Natural History and Culture report proof of a hibernation-like state in an animal that resided in Antarctica throughout the Early Triassic, some 250 million years back.

The animal, a member of the genus Lystrosaurus, was a far-off relative of mammals. Antarctica throughout Lystrosaurus’ time lay mostly within the Antarctic Circle, like today, and experienced extended durations without sunshine each winter season.

Pangea Map Early Triassic

A map of Pangea throughout the Early Triassic, revealing the places of the Antarctic (blue) and South African (orange) Lystrosaurus populations compared in this research study. Credit: Megan Whitney/Christian Sidor

The fossils are the earliest proof of a hibernation-like state in a vertebrate animal, and suggests that torpor — a basic term for hibernation and comparable states in which animals briefly lower their metabolic rate to make it through a hard season — occurred in vertebrates even prior to mammals and dinosaurs developed.

“Animals that live at or near the poles have always had to cope with the more extreme environments present there,” stated lead author Megan Whitney, a postdoctoral scientist at Harvard University who performed this research study as a UW doctoral trainee in biology. “These preliminary findings indicate that entering into a hibernation-like state is not a relatively new type of adaptation. It is an ancient one.”

Antarctic Lystrosaurus Tusk

This thin-section of the fossilized tusk from an Antarctic Lystrosaurus reveals layers of dentine transferred in rings of development. The tusk grew inward, with the earliest layers at the edge and the youngest layers near the center, where the pulp cavity would have been. At the leading right is a close-up view of the layers, with a white bar highlighting a zone a sign of a hibernation-like state. Scale bar is 1 millimeter. Credit: Megan Whitney/Christian Sidor

Lystrosaurus lived throughout a vibrant duration of our world’s history, emerging right before Earth’s biggest mass termination at the end of the Permian Period — which erased about 70% of vertebrate types on land — and in some way enduring it. The stout, four-legged foragers lived another 5 million years into the subsequent Triassic Period and spread out throughout swathes of Earth’s then-single continent, Pangea, that included what is now Antarctica.

“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,” stated co-author Christian Sidor, a UW teacher of biology and manager of vertebrate paleontology at the Burke Museum.

Paleontologists today discover Lystrosaurus fossils in India, China, Russia, parts of Africa and Antarctica. These squat, stubby, animals — most were approximately pig-sized, however some grew 6 to 8 feet long — had no teeth however bore a set of tusks in the upper jaw, which they likely used to forage amongst ground plant life and dig for roots and roots, according to Whitney.

South African Lystrosaurus Tusk

In this picture of the thin-section of a fossilized tusk from a South African Lystrosaurus, black arrowheads reveal layers of dentine transferred in layers of development, without any indications of a hibernation-like state. Scale bar is 0.1 millimeters. Credit: Megan Whitney/Christian Sidor

Those tusks made Whitney and Sidor’s research study possible. Like elephants, Lystrosaurus tusks grew constantly throughout their lives. The cross-sections of fossilized tusks can harbor life-history details about metabolic process, development and tension or pressure. Whitney and Sidor compared cross-sections of tusks from 6 Antarctic Lystrosaurus to cross-sections of 4 Lystrosaurus from South Africa.

Back in the Triassic, the collection websites in Antarctica were at about 72 degrees south latitude — well within the Antarctic Circle, at 66.3 degrees south. The collection websites in South Africa were more than 550 miles north throughout the Triassic at 58-61 degrees south latitude, far outside the Antarctic Circle.

The tusks from the 2 areas revealed comparable development patterns, with layers of dentine transferred in concentric circles like tree rings. But the Antarctic fossils harbored an extra function that was uncommon or missing in tusks further north: closely-spaced, thick rings, which likely suggest durations of less deposition due to extended tension, according to the scientists.

Megan Whitney

Megan Whitney, then a University of Washington doctoral trainee, excavating fossils in Antarctica in 2017. Whitney is now a paleontologist at Harvard University. Credit: Christian Sidor

“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,” stated Whitney.

The scientists cannot definitively conclude that Lystrosaurus went through real hibernation — which is a particular, weeks-long decrease in metabolic process, body temperature level and activity. The tension might have been brought on by another hibernation-like kind of torpor, such as a more short-term decrease in metabolic process, according to Sidor.

Lystrosaurus in Antarctica most likely required some kind of hibernation-like adjustment to manage life near the South Pole, stated Whitney. Though Earth was much warmer throughout the Triassic than today — and parts of Antarctica might have been forested — plants and animals listed below the Antarctic Circle would still experience severe yearly variations in the quantity of daytime, with the sun missing for extended periods in winter season.

Christian Sidor

University of Washington paleontologist Christian Sidor excavating fossils in Antarctica in 2017. Credit: Megan Whitney

Many other ancient vertebrates at high latitudes might likewise have actually utilized torpor, consisting of hibernation, to manage the stress of winter season, Whitney stated. But numerous popular extinct animals, consisting of the dinosaurs that developed and spread out after Lystrosaurus passed away out, don’t have teeth that grow constantly.

“To see the specific signs of stress and strain brought on by hibernation, you need to look at something that can fossilize and was growing continuously during the animal’s life,” stated Sidor. “Many animals don’t have that, but luckily Lystrosaurus did.”

If analysis of extra Antarctic and South African Lystrosaurus fossils validates this discovery, it might likewise settle another dispute about these ancient, hearty animals.

“Cold-blooded animals often shut down their metabolism entirely during a tough season, but many endothermic or ‘warm-blooded’ animals that hibernate frequently reactivate their metabolism during the hibernation period,” stated Whitney. “What we observed in the Antarctic Lystrosaurus tusks fits a pattern of small metabolic ‘reactivation events’ during a period of stress, which is most similar to what we see in warm-blooded hibernators today.”

If so, this far-off cousin of mammals isn’t simply an example of a hearty animal. It is likewise a pointer that numerous functions of life today might have been around for numerous countless years prior to people developed to observe them.

Reference: “Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica” by Megan R. Whitney and Christian A. Sidor, 27 August 2020, Communications Biology.
DOI: 10.1038/s42003-020-01207-6

The research study was moneyed by the National Science Foundation.

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