POLAR winters weren’t as cold in the Cretaceous as they are today, but they were long and dark. That has posed a puzzle: how did small dinosaurs living in polar areas survive the lean months when little food was available? The discovery of three fossil burrows in south-eastern Australia suggests they may have dug in for the winter.
Fossils from “Dinosaur Cove” in Victoria show that small plant-eating dinosaurs called hypsilophodontids were common in the area about 110 million years ago, a time when it was within what is today’s Antarctic circle. The region was forested, with temperatures 6 or 7 °C warmer than such latitudes are today. But with the sun below the horizon for weeks or months of the winter, fresh vegetation would have been lacking.
Plant-eating dinosaurs had been thought to migrate long distances since fossils from a much larger related dinosaur, Edmontosaurus, have been found from northern Alaska to Montana. But last year, Eric Snively of the University of Alberta in Edmonton, Canada, showed that the fossils were not from a single species at the same time, so if the animals were moving it was over a much smaller distance. “We don’t think the evidence for migration is there,” he told 91av. Geography would have made north-south migration difficult or impossible in Australia and New Zealand.
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Burrowing would have given small dinosaurs a refuge for the winter where they could rest and conserve energy, but evidence for this had been lacking. Then, two years ago, David Varricchio of Montana State University in Bozeman described a fossil burrow in Montana that contained the remains of one adult and two juvenile hypsilophodontids similar to those found in Australia.
“Burrowing would have given dinosaurs a refuge for the winter where they could conserve energy”
Tony Martin of Emory University in Atlanta, Georgia, says that working with Varricchio on the Montana burrow primed him to spot the new burrows when he went to Australia looking for dinosaur tracks. “I turned around a corner at the outcrop, and I saw a structure identical to the one I saw in Montana,” he says. Further investigation uncovered a total of three burrows in the area, each very similar to those in Montana (Cretaceous Research, ).
Although he didn’t excavate the burrows or find any fossils, cross-sectional measurements of a corkscrew-shaped burrow 2.1 metres long suggest that it was dug by dinosaurs weighing 10 to 20 kilograms – the same size as the Montana burrow-diggers.
Martin says the burrows were dug in soil deposited in a valley by massive spring floods of meltwater similar to those seen today in northern Alaska. The dinosaurs could have lived in the burrows in autumn and winter, but would have had to move out before the following year’s spring floods filled them with fresh sediment.
“His interpretation seems quite plausible,” says palaeontologist Tom Rich of Museum Victoria in Melbourne, who visited the burrows with Martin. He hopes Martin will return to Australia and search for more burrows along the Victoria coast, which Rich would then excavate to hunt for fossil bones that might help to identify the occupants.