Large theropod dinosaurs thrived in polar environments

A recent discovery of dinosaur tracks along Australia’s southern coast, dating back to the Early Cretaceous when the continent was still connected to Antarctica, suggests that large theropod dinosaurs thrived in this polar environment.

These carnivorous dinosaurs roamed the river floodplains during the summers when the ice thawed.

Evidence of theropods thriving in polar regions

The experts recently published an analysis of the dinosaur tracks, found in the Wonthaggi Formation, located south of Melbourne.

The tracks, made between 120 and 128 million years ago, include 18 belonging to theropods, alongside four from ornithopods, small herbivorous dinosaurs that may have been prey for the theropods.

“These numerous tracks are the best evidence yet that these former polar environments supported large carnivores,” said Anthony Martin, lead author of the study and a professor at Emory University’s Department of Environmental Sciences.

“The large theropods would likely have fed on prey such as smaller dinosaurs, fish, and turtles.”

Traces of ancient life

Theropods, meaning “wild beast foot” in Greek, are a clade of bipedal dinosaurs with three clawed toes. They belong to the same evolutionary group as famous species like Allosaurus, Tyrannosaurus rex, and Velociraptor.

The largest theropod track in this discovery measures 18.5 inches in length. “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,” Martin said.

Martin is a geologist and paleontologist with a particular focus on ichnology – the study of traces left behind by ancient life, such as tracks, burrows, nests, and tooth marks.

The research team also included Patricia Vickers-Rich, a paleontology professor at Monash University, and Thomas Rich, a vertebrate paleontology curator at Museums Victoria Research Institute. Since the 1970s, the couple has spearheaded efforts to uncover fossils in Victoria, Australia.

Extreme polar winters

Victoria’s coastal rock layers mark where Gondwana, the ancient supercontinent, began to break apart roughly 100 million years ago, separating Australia from Antarctica. At that time, the polar environment was a rift valley crisscrossed by braided rivers. 

Although the average air temperature was warmer during the Cretaceous, the polar winters experienced freezing temperatures and months of darkness.

Polar dinosaur fossils 

The Wonthaggi Formation is one of the richest sources of polar dinosaur fossils in the Southern Hemisphere, though most finds have been small fragments of bones and teeth, likely transported by spring floods before burial.

“Our find of so many theropod tracks, however, confirms that a variety of dinosaurs actually lived and walked on the ground where their bones were found,” Martin noted. “Dinosaur tracks are actually much more common at the site than we previously realized.”

Discovery of the theropod tracks

In 2007, Martin and his colleagues announced the discovery of three theropod tracks in the Wonthaggi Formation. However, it wasn’t until the social disruptions caused by the COVID-19 pandemic that more tracks were uncovered, thanks to the dedication of co-author Melissa Lowery. 

A volunteer fossil hunter for Monash University, Lowery, often referred to as “the doyenne of dinosaur discovery,” has made hundreds of finds over the years.

“Melissa had a lot more time to go prospecting during the height of the pandemic,” Martin said.

“While many of us were home watching reruns of Jurassic Park, she was out finding dinosaur tracks. She has this incredible eye that allows her to pick out distinctive patterns from surrounding materials. I call it ‘ichno-vision.’ It’s her superpower.”

Challenges in identifying the tracks

The newly discovered tracks likely formed when the dinosaurs walked on wet sand or mud in the floodplain.

Mud, being soft and malleable, can collapse into fresh footprints, often creating messy fossil tracks that might be missing a toe or other identifiable features, making them harder to spot.

“Another problem with identifying these tracks is that many of them are underwater twice in a 24-hour period when the tide comes in,” Martin said. “All sorts of modern marine life, including algae, tube worms, barnacles, and clams, have encrusted on, and partially eroded, some of the tracks.”

Of the 24 tracks detailed in the paper, 18 belong to theropods, ranging in size from seven to 18.5 inches. These tracks are recognized by their relatively thin toes ending in sharp claws. 

Discovery of ornithopod tracks

Four tracks, ranging in size from four to seven inches, were made by ornithopods – the first such tracks found in the Wonthaggi Formation.

Ornithopods, whose name comes from the Greek for “bird feet,” were small herbivorous dinosaurs with thicker toes capped by blunt claws. These bipedal creatures grew to about three feet in length.

To preserve the newly discovered track specimens, Peter Swinkels, a taxidermist at Museums Victoria Research Institute, made field moldings and casts. Martin conducted further analysis at the site in May 2022 after pandemic travel restrictions were lifted.

Helping with data collection and mapping of the tracks was Doris Seegets-Villiers, a paleontologist from Swinburne University of Technology in Victoria. Additionally, John Broomfield, media production manager at Museums Victoria, created 3D digital images of the tracks for further analysis.

Dinosaur movement during the polar summer 

The tracks were preserved in floodplain deposits near channel sandstones, suggesting the dinosaurs traveled through the area after spring-thaw flooding during the polar summer.

“Were the dinosaurs living in this environment during the winter? We don’t know,” Martin said. “It would have been frozen over and dinosaurs walking on ice don’t leave tracks.”

Future research directions

The variation in track sizes hints at a mix of juvenile and adult theropods and ornithopods, leading Martin to believe that some dinosaurs may have nested and raised their young in these polar environments.

This paper follows a 2023 report by the same authors documenting bird tracks from the Early Cretaceous at the same site, which represents the oldest-known evidence of birds so far south.

“We’re continuing to discover more tracks in the area. The more we find, the clearer picture we hope to get of these ancient polar ecosystems,” Martin concluded.

The study is published in Alcheringa: An Australasian Journal of Paleontology.

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