For the first time in Western Australia, scientists have unearthed a starfish-like creature – an ophiuroid – on Yamatji country in the state’s North West. The fossil is 275 million years old, from before the great Permian/Triassic mass extinction event, known as “The Great Dying”.
The research was team led by Dr Aaron Hunter, an Adjunct Research Fellow in at The University of Western Australia’s School of Earth Sciences and was a join project with researchers at Curtin University.
The fossil site is one of the most spectacular in Australia, with vast bedding surfaces covered by sea lilies and starfish the size of dinner plates, according to Dr Hunter.
“All these animals are perfectly preserved, frozen in time, in the very spot they died submerged by sediment during an underwater storm,” Dr Hunter said.
“Although it’s a dry bush landscape today, during the Permian period this was a polar seabed close to modern-day Antarctica. It gives us a unique window to the seas of the polar past, and how, like animals in the Southern Ocean today, they responded to climate change and global warming.”
The brittle stars are large (saucer-sized), more than 10 times the size of other Permian brittle stars, much the same as in Antarctica today.
Dr Hunter, currently a visiting postdoctoral researcher at Cambridge University, said the new form was the last known “archaic” brittle star, despite the majority of these animals had been previously thought to have disappeared 25 million years earlier.
“It shows that they overlapped with the ‘modern’ brittle stars, that are still with us today but the ‘modern’ ones evolved a much more flexible body. Unlike most marine animal groups in which their evolutionary clock was reset or stopped at the end-Permian mass extinction event, our fossils show that the major evolutionary change in these animals occurred well before this event, with the ‘modern’ brittle stars being equipped to survive the mass extinction and thrive in the new world following the event.”
The researchers believe the evolution of a flexible body in “modern” brittle stars allowed them to cope better with the pressure of predators. During the late Palaeozoic period, when they evolved, there was an expansion of shell-crushing animals, especially fish. The more flexible “modern” types were able to either burrow in the sediment or move more rapidly than the clunky “archaic” forms.
The researchers suggest the “archaic” form lasted for such a long time in these specimens because 275 million years ago Western Australia was in a higher latitudinal position, close to the South Pole.
“Our ‘archaic’ forms sought refuge and hung on to existence in the harsh high latitudes of this polar world, where there were far fewer shell-crushing predators,” Dr Hunter said.
“There are modern analogues for these types of Permian echinoderm meadows today around Antarctica, where there are similar examples of gigantism, and a dominance of slow-moving invertebrates and few shell-crushing fishes and crabs,” he said.