25 million-year-old Aussie eagle ‘unique’

Australia’s oldest eagle fossil, Archaehierax sylvestris, has been discovered in the South Australian outback.

Archaehierax sylvestris, a newly described raptor fossil species which lived during the late Oligocene in Australia’s interior. Picture: J. Blokland / Flinders University.

Australia’s oldest eagle fossil, ­Archaehierax sylvestris, has been discovered in the South Australian outback.

After extensive comparisons between the 25 million-year-old raptor skeleton and living species, palaeontologists believe it is unique. They also think the so-called “ancient hawk of the forest” is one of the oldest eagle-like raptors in the world.

“It’s unlikely to be a direct ancestor to any species alive today,” lead researcher and Flinders University PhD candidate Ellen ­Mather said. “It seems to have been its own unique branch of the eagle family.”

Differences such as more widely spaced toes, deeper insertion points of the muscles into its legs and possibly smaller shoulders than other raptors alerted the researchers that it was ­unusual.

“We don’t really know what significance those differences hold but based on a lot of little things like that, along with the age of the fossil, we felt satisfied that it’s a new subfamily,” Ms Mather said.

The 63 bones, which comprise more than half the complete skeleton, were found on the shore of the dry Lake Pinpa about 400km north of Adelaide, which is part of a sandy desert habitat the Flinders scientists have been investigating and part of an ecosystem once covered in forests.

Doctoral supervisor and co-author Trevor Worthy, who has studied the system for many years, said the skeleton was “the most exquisite fossil we have found to date”.

“It’s rare to find even one bone from a fossil eagle,” Associate Professor Worthy said.

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

Its 25 million-year age dates it to the geological Oligocene epoch of the Palaeogene period, post-dinosaur, when mammals were emerging.

So far, researchers have gleaned that Archaehierax had wings that were short for its size but legs that were comparatively long, and would have given it considerable reach.

Its 15cm foot span, while not overly large, would have allowed it to grasp large prey.

Its habitat gave it access to a varied diet, including forest dwellers such as koalas and ­possums and the large lake’s birdlife, including cormorants and ­flamingoes.

The raptor was likely to have been an agile but not particularly fast flyer that ambushed its prey rather than hunting it down.

“It was one of the top terrestrial predators of the late Oligocene, swooping upon birds and mammals that lived at the time,” Ms Mather said.

It was probably slightly smaller and leaner than today’s wedge-tailed eagle but “the largest eagle known from this time period in Australia”.

“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,” Ms Mather said.

The research has just been published in Historical Biology. .