A series of dinosaur tracks discovered on Australia’s southern coast have been identified as belonging to two groups of prehistoric animals: theropods, characterised by their hollow bones and three-toed feet; and ornithopods, a group of bipedal herbivorous dinosaurs.
A total of 24 footprints were found – 18 belonging to theropods and four to ornithopods. The tracks date to the Early Cretaceous, between 120 million and 128 million years ago when Australia was still connected to Antarctica.
The findings, published in the journal Alcheringa, provide strong evidence that large theropod dinosaurs thrived in polar environments, say a team scientists.
“These numerous tracks are the best evidence yet that these former polar environments supported large carnivores,” says Anthony Martin, first author of the study and a professor in Emory University’s Department of Environmental Sciences.
“The large theropods would likely have fed on prey such as smaller dinosaurs, fish and turtles.”
The largest track found was 18.5 inches long, says Martin. “The hip height of that theropod would have been about the same as the full height of a tall, modern-day human – or a bit more than six feet tall."
The Wonthaggi Formation, where the tracks were discovered, marks where the ancient supercontinent Gondwana began to break up around 100 million years ago, separating Australia from Antarctica. At the time, this was a polar environment, particularly bitter in the winter months when freezing temperatures and months of darkness swept across the land.
The area is known for its polar dinosaur fossils, but most of these remains are small fragments of bones and teeth, and their origins are ambiguous – some may have been carried to the site by torrential spring floods.
Marin says the theropod tracks are different, and "confirm that a variety of dinosaurs actually lived and walked on the ground where their bones were found."
Analysing the tracks wasn't easy, admits Martin, explaining that "many of them are underwater twice in a 24-hour period when the tide comes in. All sorts of modern marine life, including algae, tube worms, barnacles and clams have encrusted on, and partially eroded, some of the tracks.”
The team used a range of methods to help identify the footprints, including casts of the tracks, in-field analysis, and the production of 3D digital images.
They eventually concluded that, of the 24 tracks reported in the paper, two are of uncertain origin and 18 are theropod tracks ranging in length from seven to 18.5 inches. They are distinguished by relatively thin toes tipped with sharp claws. Four tracks were made by ornithopods — the first reported from the Wonthaggi Formation — and range in size from four to seven inches.
Martin and the team say the fact that the tracks are preserved in floodplain strata next to channel sandstones suggests they were likely made by dinosaurs travelling through the landscape after spring-thaw flooding, during polar summers.
“Were the dinosaurs living in this environment during the winter? We don’t know,” Martin says. “It would have been frozen over and dinosaurs walking on ice don’t leave tracks.”
The range of sizes of the tracks indicates a mix of juvenile and adult dinosaurs, suggesting that the animals "may have nested and raised their young in the polar environment,” Martin adds.
The researchers say they are continuing to look for more tracks in the area, with more discoveries offering a clearer picture of how dinosaurs interacted with these ancient polar ecosystems.
Learn more about the study: Polar dinosaur tracks of the Wonthaggi Formation (Lower Cretaceous), Victoria, Australia and their palaeontological significance
Main image: A 14-inch-long theropod track encrusted with marine life/Anthony Martin
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