A dinosaur relative of Tyrannosaurus rex and Velociraptor has been unearthed in the Australian state of Victoria. It has an unusually long neck and may have transitioned from predator to plant eater as it reached adulthood.
The elaphrosaur was a member of the theropod family of dinosaurs, which included all of the predatory species. It stood about the height of a small emu, measuring two metres from its head to the end of its long tail. It had short arms, each ending in four fingers.
A puzzle with only one piece
Stephen Poropat, a palaeontologist at Swinburne University of Technology in Melbourne, was the lead researcher behind the find. He says elaphrosaurs were “really rare”, with just three named species, from Tanzania, China and Argentina. “This is the first record of the group in Australia and only the second Cretaceous record worldwide.”
“These are some of the most puzzling theropod dinosaurs, as they are known from such few fossils,” says Steve Brusatte, a palaeontologist at the University of Edinburgh, who was not involved in the research. “They seem to have been lightly built, fast-running, long-necked theropods which traded the carnivorous diet of their ancestors and become omnivores.”
Introduced in the journal Gondwana Research, the fossil — which consists of a single vertebra — was discovered in 2015 at a dig site called Eric the Red West near Cape Otway, about three hours’ drive south-west of Melbourne. The digs here are led by the Melbourne Museum. Rather than the typical desert environments where fossils are often found, fossils from digs here are extracted from immense hard grey slabs of rock along the coastline, which are frequently flooded with seawater. “One of the unusual things about this site is that the first hour or two on a dig here is spent shovelling wet sand,” says Swinburne University PhD student Adele Pentland, a co-author of the study.
Found by volunteer participant Jessica Parker in the dig site, the unusual five-centimetre-long vertebra ended up in the collection of the Melbourne Museum. Based on the shape of the delicate bone, it was first thought to have been from a flying pterosaur, rather than a dinosaur. It was only several years later, when Pentland came to study the bone as part of her research on Australian pterosaurs, that the researchers realized it was something else entirely.
“I’d heard about this beautiful-looking pterosaur vertebra in the collection. It said pterosaur on the label and had been identified [as such] by the person who had prepared it,” she says. But pterosaur neck vertebrae are very distinctive, Pentland says. They all have a ball at the head end and a socket at the tail end. But this bone had sockets, or concave surfaces, at both ends, meaning it could not have belonged to a pterosaur.
The story behind the bone
“So, we went back to square one and started going through textbooks, trying to work out what kind of vertebra it was,” Poropat says. After establishing that the bone was from a theropod, they eventually discovered an African dinosaur from the Jurassic called Elaphrosaurus, which has neck vertebrae “about four times as long as they are tall, which is unusual for theropods”. Poropat adds that “the beauty of this neck vertebra is that it was particularly informative, just because elaphrosaur neck vertebrae are so weird compared to other dinosaurs”.
Although the Australian elaphrosaur is known from just a single bone, its Jurassic-era Chinese relative Limusaurus is known from a whole series of fossil skeletons from babies up to full adults. These show it had the sharp teeth of a predator as a baby, which it lost upon maturity, perhaps transitioning to a diet of mostly plants.
“Among elaphrosaurs, skulls are only known for Limusaurus, and they show that the juveniles had teeth, whereas the adults had beaks,” Poropat says. “Presumably, this indicates a dietary shift, but from what to what is unclear. I’d speculate that it was primarily herbivorous [as an adult] but might have been an opportunistic predator of small animals.”
The new find is “a great example of how one fossil bone can tell a huge story,” says Steve Brusatte. “This discovery greatly expands the range of these animals. … They were probably a widespread, and perhaps even global, group of dinosaurs, which we haven’t yet appreciated because of the scanty clues they left behind.” The new find also shows that these dinosaurs ranged even farther than previously thought — even into southern polar environments.
Because Australia was much further south 110 million years ago, these Victorian dinosaurs would have been living within the Antarctic Circle. And although the world was much warmer during the Cretaceous, the dinosaurs would still have had to tolerate months of darkness during winter and temperatures that periodically fell below freezing. Nevertheless, other fossils show there was a great diversity of life there. Forests of conifers similar to monkey puzzles and ginkgo trees had understories carpeted with cycads, ferns and horsetails. Animals included a variety of dinosaurs, as well as turtles, fish and marine reptiles.
Further expeditions were postponed due to bushfire and Covid-19. But Poropat says many fossils awaited excavation, and there was a “high chance” there were more elaphrosaur bones to be found.
© Guardian News & Media 2020