Megaraptorid skull fragment sheds light on unique Australian dinosaur evolution

A megaraptorid dinosaur skull fossil fragment has been dug up by palaeontologists in southeastern Australia. It might help place the group in the evolutionary tree of meat-eating dinosaurs including Tyrannosaurus rex.

The specimen is known from only one fossil, so hasn’t been designated a species. The discovery is detailed in a paper published in Cretaceous Research.

Lead author Jake Kotevski, a Monash University PhD candidate, tells Cosmos it is difficult to tell the animal’s size from one bone, but it has similar proportions to a juvenile Argentinian Megaraptor. The Australian megaraptorid was also not fully grown and probably about 3 metres long.

Megaraptorids like the 6m Queensland dinosaur Australovenator, meaning southern hunter, would have been the top predators in Cretaceous Australia.

“I compared it to pretty much anything they had and saw the most similarities in megaraptorid dinosaurs from South America,” Kotevski says. “Their ones are a bit evolved and specialised.”

The fossil was found on eastern Victoria’s Bass Coast. Deposits from this region contain remains from the early to middle Cretaceous period (145–66 million years ago). Palaeontologists who uncovered the megaraptorid skull element suggest it lived about 120 million years ago.

Kotevski explains that southeastern Australia at this time was below the polar circle, but Earth was warmer. There were no polar ice caps, but temperate rainforests at the poles.

The area was the site of a large river about a kilometre across and 30 metres deep. The region was home to other dinosaurs including small plant-eating Leaellynasaura and Qantassaurus, as well as turtles, birds and crocodile-sized amphibians called Koolasuchus.

Only a few frontal skull fossils exist in the world. But Kotevski says the Victorian bone is the oldest by about 30 million years. It could help explain the evolution and dispersal of megaraptorids. “The group is known to have relatively small teeth and large claws on their forearms,” says Kotevski, comparing it to other therapods – dinosaurs with 3 toes and claws on each limb. While difficult to judge from the sparse fossil record, these large forearm claws could have been the megaraptorids’ main way of catching prey.

The carnivorous group may have first emerged in Australia before spreading across the ancient southern supercontinent of Gondwanaland. “What we’re seeing is that, compared to older megaraptorids in Victoria, Argentinian megaraptorids developed features that seem to support elongation of the snout.”

Megaraptorids belong to a clade of theropods called coelurosaurs which includes tyrannosaurs and raptors like Velociraptor and Utahraptor.

“It seems the groups that spring out of coelurosaurs can take several different evolutionary paths,” Kotevski says. “It’s thought currently that megaraptorids are probably related to tyrannosaurs, or at least they’re part of a wider group of tyrannosauroids. That shows tyrannosauroids can take two routes: they get large, shrink their forearms and grow a large head like T. rex; or, in the case of megaraptorids, the head stays thin and elongate, and the forearms stretch out rather than shrink.”

“It’s just a theory so far. This is just one exciting find on the Bass Coast,” Kotevski says. “This fossil had been sitting in the museum since 2008. We didn’t have comparative material from around the world. There’s quite a lot still in the museum and so much more still to be found that’s going to help us piece together the story of Australian dinosaurs.”