What was thought to be the biggest meat-eating dinosaur of its time has been reclassified as a more petite vegetarian.
That’s the conclusion drawn by palaeontologists from Australia and Germany, who reanalysed a plaster cast of a footprint left by the beast as it traipsed across a swamp more than 200 million years ago.
The study, published in the journal Historical Biology, overturns a decades-long theory that the Triassic beast was a predator.
Anthony Romilio, a co-author from the University of Queensland, said the footprint, which was discovered in an Ipswich coal mine, might well be the oldest-known evidence of a sauropodomorph dinosaur in Australia.
“For me, it’s one of the most important specimens we have of dinosaurs in Australia, because it’s the only dinosaur fossil that we have from the Triassic period in Australia.
“But it’s a specimen that has been sitting in the [Queensland] Museum, tucked away, overlooked, and I thought, ‘Hey, there’s a lot more to it.’”
Strides through a swamp
The plaster cast may be on display in the Brisbane museum today, but back when the footprint was stamped into the ground, what is now Queensland was a completely different place.
The Australian continent was down around the south pole.
“Places like Ipswich were well and truly within the Antarctic Circle,” Dr Romilio said.
But far from being cold and icy, the late Triassic climate of 220 million years ago was warm and wet. Ferns, pines and mosses covered the land.
Dinosaurs had only been around for a short time, relatively speaking — only 10 to 20 million years or so.
One day (or night), one of those early dinosaurs walked over a patch of swampy vegetation on its hind legs.
Muddy water trickled into the indentations left by its big, clawed, bird-like feet, filling the imprints with silt and sand.
Over millions of years, the sediment turned to siltstone and sandstone, the plants beneath became coal, and the whole lot was buried under yet more layers of stone and coal.
In the 1960s, miners in the Ipswich suburb of Dinmore tunnelled more than 200 metres below the surface and into that 220-million-year-old coal layer.
When they looked up, they saw three fossilised footprints hanging from the stone ceiling.
From big predator to smaller vegetarian
Shortly after the discovery, scientists descended the mine, took photos of the dinosaur track, and made a plaster cast impression of one footprint, which is at the Queensland Museum today.
Palaeontologists, analysing the size of the prints and step length, deduced the track maker was a carnivorous creature that stood more than 2 metres tall at the hip.
That made it the biggest Triassic predator known.
But, Dr Romilio said, many of those analyses relied on drawings or photographs that lacked detail, and their estimates of something as fundamental as foot length varied wildly.
So he made a 3D model of the footprint plaster cast and sent it to palaeontologists in Germany.
“By having a 3D model, we can change the visualisation perspective, and actually see what is a footprint and what is not,” Dr Romilio said.
This let his crew get a closer look at the footprint’s detail — such as the curve of its long middle toe and the edge of its heel — and get a better estimate of the track maker’s size.
They measured the foot to be 34 centimetres from heel to toe-tip — shorter than previous estimates of up to 46cm.
Those early studies, which overestimated foot size, probably included features like the gouged drag marks made by the animal’s claw as it lifted its foot to take another step, Dr Romilio said.
The new analysis put the dinosaur around the height of an average adult.
Sauropodomorphs around the world
Other sauropodomorph fossils have been found in Australia, such as a specimen also found in Queensland last century, but they’re a few tens of millions of years younger than the subject of the new study.
The Triassic track maker’s reclassification from meat-eater to plant-eater makes sense when you think about how the continents were arranged when it was alive, said Steve Poropat, a palaeontologist with the Australian Age of Dinosaurs Museum and Swinburne University.
In the late Triassic, the world’s continents were still clustered together in one massive landmass called Pangaea.
Animals, including sauropodomorphs, spread across the supercontinent. Their traces have been found in South Africa, South America and India — all of which were butted up against the Australian continent at the time.
“So it’s been safely presumed that these sauropodomorphs were everywhere, and there’s no doubt they were here [in Australia] too,” Dr Poropat said.
The study does hinge on a 3D model of just one footprint though, and relies heavily on the plaster-casting skills of the scientists in the coal mine.
“This paper is a very valiant effort based on the meagre evidence available,” Dr Poropat added.
“If the features that they identify are correct, which are the turned-in toe and the lack of a heel pad, then it probably is a sauropodomorph. And that’s cool.
“What would also be cool is to find bones of these things somewhere in the Triassic of Australia, but whether that’s going to happen, considering the lack thereof so far, is another question.”
So could Dr Romilio take a trip into the coal mine and examine the tracks in the ceiling?
“No. Not a chance,” he said.
The mine in which the tracks were found has long since closed, as have other mines nearby housing similar prints.
“In the same area around Ipswich, there are older coal deposits, and there were a lot more fossil footprints found in mines,” Dr Romilio said.
“But most of them are closed, and some have been converted into open-pit mines, and then those layers with the footprints are actually physically removed.”