Eagle fossil from lake/ forest 25 million years ago found by Flinders University team in South Australia's north

Flinders University palaeontology team members Aaron Camens, Amy Tschirn, Jacob Blokland and Kailah Thorn digging for fossils in the arid north of South Australia at the dried Lake Pinpa, former home to a lush forest ecosystem with creatures such as the ancient eagle Archaehierax sylvestris (inset).
Main image by Trevor Worthy, inset by Jacob Blokland, Flinders University.

The fossil of an eagle, apex predator of a lush ecosystem 25 million years ago at Lake Pinpa, 400 kilometres north of Adelaide in South Australia, was discovered in 2016 by Flinders University palaeontologists.

The eagle fossil found under the dried lake was identified 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 63 bones found showed Archaehierax sylvestris (ancient hawk belonging to the forest) clearly belonged to the raptor family, including most hawks and eagles.

It was slightly smaller than a wedge-tailed eagle, with talons spanning 15 centimetres, allowing it to grab prey the size of a koala or possum. Its short robust wings were adapted to fly in the confines of a forest and was likely an ambush hunter, rather than a soaring forager. In the forest, it probably preyed on medium-sized marsupials. From a high perch, it also would have made forays over the lake to catch ducks and flamingos.

Since the 1970s, the barren salt-crusted sediments in South Australia’s arid north had yielded bone fragments, teeth and other fossils of animals ­– many of them prey for Archaehierax. These fossils included mammals, from wombat ancestors the size of a cow through to tree-dwelling herbivores such as possums and koalas, to small terrestrial carnivores no bigger than a mouse. These animals lived around a large lake where crocodiles and turtles abounded, and freshwater dolphins played.

Waterbirds were abundant, including cormorants, several types of flamingo, four species of duck, and Presbyornis, a bizarre long-legged fowl that went extinct elsewhere in the world 20 million years earlier. Many smaller forest birds such as songbirds, parrots and rails were also found but most weren't yet described. Archaehierax sylvestris was n't the only raptor found at Lake Pinpa. Isolated bones show a smaller eagle also lived in these forests but the fossils were too fragmented to give it a name.

Archaehierax represented an ancient lineage that split off near the base of the raptor family tree. This was consistent with previous genetic analysis suggesting most living groups of hawks and eagles evolved only in the past 20 million years — roughly five million years after Archaehierax lived and died. Archaehierax sylvestris and its smaller contemporary showed Australia was important in early global evolution of raptors – as it was with songbirds.

These raptors and the earliest songbirds lived in temperate rainforests. Back then, the area around what became Lake Pinpa was more than 1,100km south of modern Adelaide, at a latitude equivalent to present-day Fiordland on New Zealand’s southwest tip. In the 25 million years since, continental drift pushed Australia and the fossils north at six centimetres per year – more than 1,500km.

The rainforest where those birds lived became arid outback still containing many fossils to be found.