A collection of tiny fossilised thigh bones, some just 2.5 centimetres in length, are the first remains of baby dinosaurs ever found in Australia.
Discovered in the eastern states of New South Wales (NSW) and Victoria, the bones belonged to baby herbivores that were small enough to sit in the palm of a hand.
As Australia was much further south 100 million years ago when these babies were tottering around in their nests, they are also proof that dinosaurs were breeding in southern polar environments within the Antarctic Circle.
The discovery adds to evidence that “dinosaurs were remarkably climate-tolerant, thriving from equatorial to polar latitudes,” write the authors of a paper in the journal Scientific Reports.
The NSW fossils, from the outback town of Lightning Ridge, were so small they may have belonged to embryonic dinosaurs that weighed 150 grams and were less than 20 centimetres in length from head to tail.
“They were just about at, or prior to, the point of hatching from the egg,” says co-author Phil Bell from Australia’s University of New England.
“Hatching lines” in the internal structure of the leg bones from Dinosaur Cove, on the coast of Victoria, reveal a change in bone development after leaving the egg, which hints that these animals were a few months old and weighed up to 230 grams.
These baby dinosaurs were the young of wallaby-sized dinosaurs called ornithopods that walked on two legs and had beaks for cropping vegetation.
They may have lived in herds, and while the scientists can’t be sure which specific species these babies belonged to, they would have been animals like Weewarrasaurus from Lightning Ridge, or Diluvicursor or Galleonosaurus from Victoria, Bell says.
At this time in the Cretaceous Period, Victoria was well within the Antarctic Circle, and a similar distance from the South Pole as Greenland is from the North Pole today. Though the world was warmer then, there would still have been months of complete darkness and potentially freezing temperatures in winter.
While many dinosaurs have been found with fossilised feathers, which could have been useful for polar insulation, there is little evidence so far for feathers in the ornithopod group.
Instead these particular dinosaurs “were probably scaly skinned, meaning that they were exposed to the elements”, says Bell. “They were also too small to migrate and couldn’t leave these polar realms in the dead of winter, so had to find another way to endure the conditions.”
It’s likely the eggs and babies were protected in nests mounded with composting vegetation to generate warmth, similar to what we see in crocodiles and some birds today.
Another way these dinosaurs might have protected themselves was snuggling up in the warmth of burrows, he argues – a behaviour we have evidence for from North America.
“We’ve found skeletons of ornithopods and their youngsters in their burrows, so this seems like a viable way of dealing with the harsh conditions Australia was subjected to in the Cretaceous,” Bell says.
Anthony Fiorillo, a palaeontologist at the Perot Museum of Nature and Science in Dallas, US, who studies Arctic dinosaurs, says it’s exciting to finally have baby dinosaurs from Australia.
“Australia has long been known for having one of the best polar dinosaur records, but the question in the south has been the same as the question in the north – did those dinosaurs live in the polar region year-round?” he says.
While some research has argued that Arctic dinosaurs migrated to warmer climes for the winter months as caribou do today, the new finds from Australia add to a growing consensus that “dinosaurs were highly adapted across a variety of environments, and some dinosaurs did just fine in the more extreme environments of the high latitudes,” Fiorillo says.
It’s a miracle we have these fossils at all. The fact Bell’s team only found thigh bones, is because they were the largest and most robust in the babies’ skeletons, and therefore the most likely to be preserved as fossils.
“The bones of these tiny little animals from around the world are incredibly rare as they are so small and fragile,” he says. “Many never become fossils, so this is a pretty singular insight into this particular group of dinosaurs.”