Bats, birds and bones: a view to a kill

The scene: an ancient limestone cave that opened onto a vertical cliff facing the Gregory River. At the other end of the cave was a second, larger entrance that opened out to a rocky terrain. The cave roof has eroded away, but remnants of the walls and floor are still in place today. The pinkish limestone cave floor is rich with tiny broken fossils, some of which have been punctured by small but powerful teeth.

This cave site is called Rackham's Roost and is part of the Riversleigh World Heritage Area in Queensland, one of the richest fossil regions in Australia. Rackham's Roost preserves fossils that are at least early Pleistocene in age (about 2.7¬-1.1 million years old). The site has produced thousands of fossils of rodents and many fossils of at least ten species of bats. The remains of frogs, lizards, snakes, budgies, dasyurids, bandicoots, possums, and kangaroos have also been uncovered here.

In a recent study we identified 38 fossils of songbirds from Rackham's Roost, including the oldest known fossils of fairywrens, thornbill-like birds, grassfinches and Australasian babblers and robins. We also discovered ancient remains of honeyeaters, songlarks and reedwarblers. Together, the presence of these songbirds at Rackham’s Roost indicates that the surrounding environment was an open grassy woodland, perhaps next to a denser forest bordering the Gregory River.

The bones at Rackham’s Roost were mainly brought in by an early population of the still-surviving carnivorous Ghost Bat (Macroderma gigas). Ghost Bats feed on their prey at the point of capture or carry them back to roosting caves. Small mammals are eaten whole, but parts of larger animals, such as skulls, feathers, legs and tails, are dropped to the ground. These discarded remains accumulate on the floor of the cave and provide a gruesome record of what the Ghost Bat feasted on.

These fossil discoveries add to our growing understanding of Australia's past songbird fauna and their environments. They also reveal the past diet of the endangered Ghost Bat in this part of northern Australia, an area from which it has recently disappeared.

Dr Jacqueline Nguyen, Chadwick Biodiversity Fellow

More Information

• Nguyen, J.M.T., Hand, S.J. and Archer, M. (2016). The late Cenozoic passerine avifauna from Rackham’s Roost Site, Riversleigh, Australia. Records of the Australian Museum 68(5), 201–230.