UW News

August 27, 2020

Frequently asked questions: torpor in Antarctic Lystrosaurus

Prepared by Megan Whitney with Harvard University and Christian Sidor with the University of Washington and the Burke Museum of Natural History and Culture. University of Washington press release here.

Reference: “Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica” by Whitney MR and Sidor CA. Communications Biology. DOI: 10.1038/s42003-020-01207-6


Major findings

We provide evidence for a hibernation-like condition in Lystrosaurus, a mammal relative that lived in the Antarctic portion of Pangea about 250 million years ago. This discovery was enabled by high-resolution of incremental growth marks preserved in the tusks of Lystrosaurus.

An artist's rendition of an ancient vertebrate called Lystrosaurus

Life restoration of Lystrosaurus in a state of torpor.Crystal Shin

Frequently Asked Questions

What is Lystrosaurus?

  • Lystrosaurus is a type of dicynodont, a major group of primarily herbivorous vertebrates that were common during the Permian and Triassic periods. Dicynodonts are characterized by their turtle-like beaks and ever-growing tusks, which are present in most species. They are distantly related to modern mammals.
  • Lystrosaurus is known from rocks from about 253-248 million years ago and ranged from about the size of a corgi to slightly smaller than a cow.
  • Fossils of Lystrosaurus are known from China, Russia, India, South Africa and Antarctica and this geographic distribution was one of the early pieces of evidence used in support of a large supercontinent called Pangea.
  • Fossils of Lystrosaurus 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.

What is torpor?

  • Torpor is a term that refers to the general pattern of vertebrate physiology where there are periodic reductions in metabolic activity. These periods of slower metabolism are often related to seasonal changes in the environment that create unfavorable conditions.
  • Hibernation is one form of torpor and is found in warm-blooded animals today. Hibernation is marked by reduction of metabolic activity with periodic small reactivations of activity throughout the hibernation period. This is in contrast to a different kind of torpor called brumation which is common in cold-blooded animals. In brumation, metabolic activity is completely inactive for the entirety of the torpor period.
  • Modern examples of hibernators include many North American bear, echidnas, many rodents, hedgehogs, badgers and some lemurs during dry seasons. Modern examples of daily torpor, a form of torpor characterized by daily reductions in metabolic activity, include many birds and bats, as well as bushbabies. Modern examples of brumation include many reptiles such as lizards, turtles and snakes.
A map of Earth's surface during the Early Triassic, some 250 million years ago

A map of Pangea during the Early Triassic, showing the locations of the Antarctic (blue) and South African (orange) Lystrosaurus populations compared in this study.Megan Whitney/Christian Sidor

What was Antarctica like in the Early Triassic?

  • In general, the Early Triassic (252-247 million years ago) was a warm period in Earth history.
  • The climate of Antarctica during the Early Triassic is still a subject of active research, but it is clear that the continent was NOT under a thick ice sheet like today. The discovery of fossil plants — including fossil forests — as well as a wide variety of land-living vertebrate fossils demonstrate that the continent was habitable for at least part of the year.
An image of a thin cross-section of the tusk of an ancient vertebrate that lived 250 million years ago in South Africa.

In this image of the thin-section of a fossilized tusk from a South African Lystrosaurus, black arrowheads show layers of dentine deposited in layers of growth, with no signs of a hibernation-like state. Scale bar is 0.1 millimeters.Megan Whitney/Christian Sidor

What is thin-sectioning?

  • We make thin-sections of fossil bones and teeth so that we could study the fine, inner details that are preserved in these hard tissues. These small, microscopic details act as storybooks, preserving a lot of information about the biology of these animals while they were alive. As is easy to imagine, studying the biology of animals that lived millions of years ago can be challenging. These details, in this case the tree-ring-like growth marks, preserve critical clues into the biology of fossilized animals.

How did you analyze the tusks?

  • The way that these tusks grow is layer by layer, growing inward toward the pulp cavity.
  • Growth of the tusk happens periodically during both normal and stressful times for the animal. Each increment of growth will leave behind a ring. We looked at both normal growth mark rings and growth marks that were especially thick, which represent a stressful time for the animal.
  • We counted how much growth had occurred between rings, and also measured the thickness of the stressful rings.
  • We compared periods of regular growth and stress in polar Antarctic tusks to those from non-polar South African localities from the same time period.
An image of a cross section of the tusk of an ancient vertebrate, showing rings of dentine deposited throughout its lifespan and evidence of a hibernation-like state.

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

Has torpor been found in the fossil record before?

  • Torpor has been reported in some fossil rodents, where hibernation marks were found in their ever-growing incisors. But these relatively recent fossils are from the Pleistocene, on the order of hundreds of thousands of years old.
  • Our study is by far the oldest evidence of torpor.
  • Given how widespread torpor is in modern vertebrates, it is expected that this is not a trait that evolved recently, and instead has likely been widespread throughout the vertebrate evolutionary history. However, torpor is a difficult feature to study in the fossil record.
  • This study provided a unique opportunity to study torpor in the fossil record. First, Lystrosaurus had ever-growing tusks, which provide a lengthy record of regular and stress growth rings. Second, we have two populations to compare, a polar population from Antarctica and a non-polar population from South Africa.
  • This study suggests torpor was present even 250 million years ago and lends support for the idea that having a flexible physiology may serve as a key feature in surviving mass extinctions.

What this paper does NOT say:

  • This paper does not say that Lystrosaurus was a reptile — or a dinosaur! Though it is only a distant relative to mammals, Lystrosaurus is actually more closely related to mammals, including humans, than to any reptile. Lystrosaurus is a member of a very early branch on the lineage that eventually gave rise to mammals.
  • This paper does not say that Lystrosaurus was the only animal experiencing torpor in the Early Triassic. This is the first study of its kind. There is a rich assemblage of vertebrates from the Early Triassic of Antarctica and similar studies on their seasonal physiologies have not yet been launched.
  • This paper does not prove that Lystrosaurus was hibernating. This is a preliminary study that puts forward a hypothesis. Our hope is that there is continued testing of this hypothesis and additional sampling of Lystrosaurus and other polar vertebrates to look for signals of hibernation or other forms of torpor.

For additional information, contact Christian Sidor at casidor@uw.edu and Megan Whitney at meganwhitney@fas.harvard.edu. University of Washington press release here.