Rare eagle fossil unearthed in remote SA

Archaehierax sylvestris is thought to be one of the world's oldest species of raptors.
Archaehierax sylvestris is thought to be one of the world's oldest species of raptors.

A 25-million-year-old eagle fossil, thought to be one of the world's oldest species of raptors, has been unearthed on a remote South Australian cattle station.

Flinders University palaeontologists discovered Archaehierax sylvestris, on the barren shore of a dry lake in a desolate sandy desert habitat during ongoing investigations into a lost ecosystem, when Australia's interior was covered in trees and verdant forests.

"This species was slightly smaller and leaner than the wedge-tailed eagle, but it's the largest eagle known from this time period in Australia," PhD candidate Ellen Mather said.

"The foot span was nearly 15 cm long, which would have allowed it to grasp large prey.

"The largest marsupial predators at the time were about the size of a small dog or large cat, so Archaehierax was certainly ruling the roost."

Associate Professor Trevor Worthy said eagles at the time were few in numbers so were infrequently preserved as fossils.

"It's rare to find even one bone from a fossil eagle," he said.

"To have most of the skeleton is pretty exciting, especially considering how old it is."

Ms Mather said the Australian environment of the time would have been challenging for flight given the forest coverage.

"The fossil bones reveal that the wings of Archaehierax were short for its size, much like species of forest-dwelling eagles today," she said.

"Its legs, in contrast, were relatively long and would have given it considerable reach.

"The combination of these traits suggest Archaehierax was an agile but not particularly fast flier and was most likely an ambush hunter.

"It was one of the top terrestrial predators of the late Oligocene, swooping upon birds and mammals."

The raptor would have hunted koalas, possums and other animals in trees surrounding a vast shallow lake, on which waterfowl, cormorants and flamingos would have been abundant, the researchers said.

They said the Archaehierax partial skeleton, made up of 63 bones, was one of the best-preserved from the SA site.

"The completeness of the skeleton allowed us to determine where it fits on the eagle family tree," Ms Mather said.

"It shows a range of features unlike any seen among modern hawks and eagles.

"We found that Archaehierax didn't belong to any of the living genera or families. It seems to have been its own unique branch of the eagle family.

"It's unlikely to be a direct ancestor to any species alive today."

Details of the find have just been published in the journal Historical Biology.

Australian Associated Press