An auditory time capsule
In the limestone deposits of the Riversleigh World Heritage Area, palaeontologists have found one of the most diverse and most continuous fossil records in the world. The site documents life in Australia from the late Oligocene to the Miocene, roughly 25 to 15 million years ago. During this period, the now semi-arid landscape was a lush rainforest with freshwater pools rich in lime. This calcium-carbonate-charged water was essential for the area's incredible preservation. As animals died in or near the water, their bones were quickly coated and cemented into the limestone, creating perfectly preserved, three-dimensional fossils.
Among the most numerous and revealing fossils are those of bats. The limestone cave deposits have yielded thousands of bat specimens, representing what is considered the most diverse fossil bat record in the world. At least 44 previously unknown extinct bat species have been identified from Riversleigh. The preservation is so fine that it allows for detailed study of fragile structures, including the skulls and the tiny bones of the inner ear. This has provided a record of the sensory world of ancient bats.
Echoes from the Miocene
The delicate inner ear bones, specifically the cochleae, are the anatomical hardware for hearing and echolocation. Their size and shape in the Riversleigh fossils offer direct evidence of the bats' auditory capabilities. Analysis of these structures shows that by 25 million years ago, some bats had already evolved the sophisticated high-frequency hearing necessary for echolocation. The fossils include species from the Hipposideridae family (Old World leaf-nosed bats), such as Brachipposideros nooraleebus and Riversleigha williamsi. These ancient bats had elaborate, fleshy nose-leafs to help focus the ultrasonic pulses they emitted through their nostrils.
The fossil record at Riversleigh captures a diversification in bat evolution. The cochleae of different species show variations that correspond to different hunting techniques still seen today. Some bats had anatomy suited for "aerial hawking"—catching insects on the wing in open spaces. Others were adapted for "gleaning"—plucking stationary insects from leaves or the ground in cluttered environments. This evidence shows that the division of bats into different acoustic and foraging niches was well-established millions of years ago. The fossils provide a detailed timeline of how these predators specialized to avoid competing with one another in the dense Miocene rainforest.