A 14-inch-long theropod track encrusted with marine life. Credit: Anthony Martin.

Polar theropod footprints found in Australia have provided researchers with new clues as to life in the Early Cretaceous.

A discovery of dinosaur tracks on Australia’s southern coast – dating back to the Early Cretaceous when Australia was still connected to Antarctica – indicates that large theropod dinosaurs thrived in this polar environment, prowling the river floodplains when the ice thawed during the summers.

Analysis was made of the tracks in the Wonthaggi Formation south of Melbourne between 120 million and 128 million years ago. The find includes 18 theropod tracks and four tracks made by ornithopods — small, herbivorous dinosaurs that may have been prey for the theropods.

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

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

Theropods, from the ancient Greek for ‘wild beast foot’, are a dinosaur clade characterised by walking on two legs and feet with three, clawed toes. They belong to the same evolutionary group as Allosaurus, Tyrannosaurus rex and Velociraptor.

The largest track from the current find was 18.5 inches long (~47cm), suggesting the hip height of the theropod would have been around six feet (~1.83m).

Fragments

The Wonthaggi Formation in the Australian state of Victoria, the coast of which marks where the ancient supercontinent Gondwana began to break up around 100 million years ago, separating Australia from Antarctica, has yielded one of the best assemblages of polar dinosaur body fossils in the Southern Hemisphere, but most of these remains are small fragments of bones and teeth.

These fragments may have been carried to the site where they were buried by torrential spring floods.

Martin said: “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. Dinosaur tracks are actually much more common at the site than we previously realised.”

The tracks discovered for the current paper were likely made when the dinosaurs were walking on wet sand or mud in the floodplain. Gooey substances like mud can collapse into a fresh footprint leading to messy fossil tracks that are missing a toe or other identifying mark, adding to the challenge of spotting them.

Martin explained: “Another problem with identifying these tracks is 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.”

Of the 24 tracks reported in the paper, two are of uncertain origin and 18 are theropod tracks ranging in length from 7 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.

Ornithopod is from the Greek for ‘bird feet’, in reference to their three toes, which are thicker than those of theropods and capped with blunter claws. These early, bipedal herbivores grew to about three feet in length.

A 4.5-inch ornithod track. Credit: Anthony Martin.

Polar summer

The tracks are preserved in floodplain strata next to channel sandstones, which suggests they were like made by dinosaurs traveling through the landscape after spring-thaw flooding, during polar summers.

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

The range in sizes of the tracks suggests a mix of juvenile and adult ornithopods and theropods, which indicates the dinosaurs may have nested and raised their young in the polar environment, according to Martin.

The current paper follows on the heels of the authors’ 2023 report of bird tracks from the Early Cretaceous found at the same site – the oldest-known evidence for birds so far south.

Martin said: “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.”

The study is published in Alcheringa.