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A 25m-year-old eagle fossil discovered on a remote outback cattle station in South Australia has been identified as one of the oldest raptor species in the world.
Palaeontologists discovered the eagle fossil on the shore of a dry lake known as Lake Pinpa in 2016, and have since identified it as a new species, Archaehierax sylvestris, in a study published in the journal Historical Biology.
Ellen Mather, a PhD candidate at Flinders University and the study’s first author, said the ancient bird was slightly smaller than a wedge-tailed eagle, with a footspan of 15cm.
“It would have been probably one of the larger eagles around at the time, based on what we know,” she said. Though Lake Pinpa is now sandy desert habitat, 25m years ago it was likely a temperate rainforest with permanent bodies of water.
“We believe that it would probably have been preying on most of the small-to-medium birds and mammals that were also alive at that time, so things like the ancestors of modern possums and koalas living in the forest,” in addition to other birds such as ducks and possibly flamingos, Mather said.
The eagle had relatively short wings for its size, but long legs. “That’s pretty common in forest eagles,” Mather said. “It’s an adaptation for essentially having to fly through a more cluttered space compared to eagles that live in more open spaces like grasslands or woodlands.”
“We think it would have been an ambush hunter. So while not a very fast flyer, it would have been quite agile, able to make quick turns, and probably would have waited [on] a perch for prey to wander within striking distance.”
Mather said the discovery showed Australia had an endemic lineage of eagles early in the history of the animals’ evolution. The ancient bird belonged to a distinct branch of eagles and was not a direct ancestor of any living species.
“It shows that even 25m years ago, this family [the Accipitridae] was widely globally distributed but also already diversifying, and Australia seems to have been a place of unique diversity even back then.”
The ancient eagle had a number of features unlike any seen among modern hawks and eagles, including a wider foot-span for capturing prey.
“Some of the muscle insertion points on the leg were quite deep compared to most living accipitrids [the family of birds including hawks and eagles] so that could suggest quite a strong grip … considering its quite slender build,” Mather said.
More than 60 bones of the ancient eagle were recovered, making it one of the best preserved fossils found at the Lake Pinpa cattle station. Ancestors of many other Australian species, including wombats, platypus and kangaroos, have also been discovered at the site.
It was likely that the eagle specimen was washed into a lake once it had died. “Those bones would have been buried in the sediment at the time, which is what allowed them to be preserved,” Mather said.