(CN) — Bears curled up in their dens as the year’s first snow falls is the classic picture of hibernation. Did it start with bears? No. In a new report, scientists say the tusks of an animal that roamed Earth 250 million years ago may show it too hibernated.
Ranging in size from a small dog to a cow, Lystrosaurus were herbivores with barrel chests, beaks like turtles and powerful forelegs they used to dig out nesting burrows.
A distant relative of mammals, they survived an extinction event at the end of the Permian period 251 million years ago that killed off 70% of land-dwelling vertebrates and thrived for another 5 million years into the Triassic Period when dinosaurs came to dominate.
Earth had only one continent then called Pangea, and the Antarctic region was not perpetually covered in ice, as evidenced by plant and vertebrate fossils found by scientists on Antarctica.
Lystrosaurus made the most of their anatomy, digging for roots and tubers with two large tusks that were their only teeth.
In the Early Triassic Period, 251 to 247 million years ago, Lystrosaurus flourished and accounted for 95% of all land vertebrates. Their fossils have been found in China, India, Western Russia, South Africa and Antarctica.
Because their tusks grew continuously throughout their lives, forming rings for each growth increment, scientists can study them for insights into their lives, just as rings on old trees can show when they were stricken by droughts and wildfires.
Comparing fossilized Lystrosaurus tusks found in South Africa to those discovered in Antarctica, researchers noticed something odd. The Antarctic tusks had thick, closely spaced rings missing in the South African ones.
Harvard University postdoctoral fellow Megan Whitney said she and her research partner found similar “stress marks” in the ever-growing incisors of hibernating rodents.
Whitney’s and University of Washington paleontologist Christian Sidor’s findings were published Thursday in the journal Communications Biology.
While the fossils do not prove Lystrosaurus hibernated, Sidor and Whitney say, they are the oldest evidence of a hibernation-like state in a vertebrate, generally known as a state of torpor.
Today, bears are one of many warm-blooded animals that hibernate including some species of bats, birds, hedgehogs, badgers, lemurs and ground squirrels. Their metabolism slows down dramatically in this state.
The heartrates of black bears, for instance, can fall from 50 to 8 beats per minute. Some bats’ heartrates drop from 1,000 to 25 beats per minute and they take one breath every two hours.
Hibernation is a survival tactic of animals that live near the poles, which only have two seasons — summer and winter. Due to the tilt of the Earth, the sun does not set in the six months of summer and disappears for the entirety of winter at the poles.
The dual seasons were no different in these regions in Lystrosaurus’ time.
“Though Earth was much warmer during the Triassic than today – and parts of Antarctica may have been forested – plants and animals below the Antarctic Circle would still experience extreme annual variations in the amount of daylight, with the sun absent for long periods in winter,” Whitney said in a statement.
Whitney and Sidor say their study is the first of its kind and they hope more research is done looking for signs of hibernation in Lystrosaurus fossils and those of other early polar vertebrates who lived in the Early Triassic.
Whitney told Courthouse News they collected fossils from many different animals during their Antarctic expedition three years ago, a painstaking process in which they used saws to cut out chunks of rock, then shipped them back to their universities where they chipped the fossil-laden rock out.
She said the closest living relatives of Lystrosaurus are mammals.
“Lystrosaurus is a mammalian forerunner that was one of many animals that existed after the split of the reptile and mammal lines (more than 300 million years ago) but before the rise of modern mammals (about 170 million years ago),” she said in an email.
It is unknown whether they laid eggs or gave birth. Today only two mammals lay eggs, the duck-billed platypus and echidna, or spiny anteater.
“This is still one of the great mysteries left in mammalian evolutionary history!” Whitney said. “We do not have evidence of eggs along the mammalian line including for Lystrosaurus. That could mean they were having live birth, or it could mean those eggs didn’t preserve in the fossil record which is very likely given the often delicate nature of eggshells.”
Experts say Lystrosaurus may have been driven to extinction by being outcompeted by early dinosaurs. Regardless, they had a good run.
“Lystrosaurus…existed for nearly 5 million years. That’s a long time if you consider we, Homo, have only been around for around 2 million years,” Whitney said.
The researchers said if their hypothesis is correct — that Lystrosaurus hibernated — it is a humbling reminder that some animal behaviors today existed for millions of years before humans evolved to observe them.