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A species of eagle that may have eaten ancient koalas over 25 million years ago has been described from Australia.
The new species Archaehierax sylvestris was described from a fossil uncovered in the south of the country in 2016. It is currently the largest eagle species of its time period and would have been one of the top predators in the forests that once covered the area.
Ellen Mather is a PhD student at Flinders University, Adelaide, who led the paper describing the ancient eagle.
'The foot span was nearly 15 centimetres long, which would have allowed it to grasp large prey,' says Ellen. '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.'
The paper has been published in the journal Historical Biology.
The new species is a member of the Accipitridae, which includes eagles, hawks and vultures. As they are often the top predator in their food chain, there are relatively few individuals around. This, coupled with the fragile nature of bird bones, means there aren't many fossils around.
The earliest fossils from this group are found in Europe and date to around 40 million years ago. While Europe and North America have a range of eagle fossils in the following millions of years, other continents like Australasia are poorly represented.
Before the Pleistocene, some 2.5 million years ago, only two species from Australia are known - Pengana robertbolesi and Aquila bullockensis. While these only have a few bones between them, 63 have been found for A. sylvestris, including parts of the head, wings and feet.
'It's rare to find even one bone from a fossil eagle,' says Dr Trevor Worthy, who co-authored the paper. 'To have most of the skeleton is pretty exciting, especially considering how old it is.
'I have studied this system for many years now, and this is the most exquisite fossil we have found to date.'
Looking at the features of the bones allows researchers to assess the species' relationship with other members of the Accipitridae. In the case of A. sylvestris, the combination of features is unlike any seen in modern groups, though not all characters could be assessed.
'We found that Archaehierax didn't belong to any of the living genera or families,' says Ellen '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.'
Alongside evidence from where the fossils were found, the features do, however, give scientists an insight into how A. sylvestris may have lived.
While the region is now a dry lake in a desert, pollen found in the fossil record suggests that there was once a lakeside forest. Forests require hunting birds to be agile to avoid the various trunks, branches and other obstacles that could get in their way.
The bones of A. sylvestris are short for its overall size, an adaptation seen in forest eagles today which allows them to manoeuvre in tight spaces.
While this offers it an advantage in forests, it would also have made the birds slow in flight, suggesting that it ambushed prey by diving from a forest perch.
Even though its wings may have been short, its feet were incredibly long. This would have given the eagle a 'considerable reach', allowing it to hunt species such as Madakoala devisi, an early relative of modern-day koalas.
While it couldn't have caught full grown adults, the birds would have been able to hunt juveniles and young adults.
As one of the largest predators in the forests, other potential prey on the menu outside koalas would have included possums and birds.
But like its short wings, the eagles' long feet would have resulted in a trade-off. Its feet and ankle had a different structure to most other eagles, which would allow it to extend its claws further but makes it less able to move some of its toes. This would have prevented it from hunting for fish in the nearby lake.
The description of A. sylvestris is significant in the understanding of ancient eagles and the lives they led. As part of her PhD, Ellen hopes to find out more about where undescribed eagles fit into the tree of life to improve our understanding of these mighty birds.