The tracks of my tetrapod

Fossils discovered nearly 80 years ago in what is now suburban Sydney have been identified as traces of a four-legged animal swimming nearly a quarter of a billion years ago.

And that makes them the oldest record of a swimming tetrapod in Australia, according to researchers from the University of New England.

“The foot and hand prints, along with the gaps between the sequence of traces, were unlike anything I had seen before. This led me to believe the animal was swimming in water,” says Roy Farman, co-author of a paper in the Journal of Palaeontology.

He and University of New England colleague Phil Bell later confirmed the animal was merely brushing the river bottom with its hands and feet rather than making full contact, as would be expected if it was walking.

Farman came across the fossil trackway in a storage facility belonging to the Australian Museum. It was unearthed in the 1940s from what was then a sandstone quarry in Berowra, about 40 kilometres north of Sydney’s CBD.

Large rivers covered the region in the Middle Triassic period (around 240 million years ago) and the sand at the bottom of these rivers eventually turned to stone, which in recent years has been used as a building material.

Somewhat surprisingly, animal fossils are rare in what is known as “Sydney sandstone”.

The trackway is 4.2 metres long and includes at least 35 prints. Only two digits from each made impressions in the sand, making the identity of the animal difficult to establish. 

However, the researchers believe it was likely a temnospondyl (an extinct group of salamander-like amphibians) between 0.8 and 1.35 metres long.

Tetrapods emerged onto land 360 million years ago, roughly 100 million years before the Berowra footprints were made. 

Most tetrapod footprints are found on land, so the discovery of tracks made by a swimming animal is a valuable insight into how these animals moved, the researchers say.