Holy bat skull! Fossil adds vital piece to bat evolution puzzle

 Of all the mammals, bats have one of the poorest fossil records, with palaeontologists estimating that about 80 per cent of it is missing.

This has made it difficult to pinpoint exactly when they first began to fly, or began roosting in caves, or developed their unique way of 'seeing' their surroundings in the dark using sound -- called echolocation.

But a near-perfectly preserved bat's skull discovered by French palaeontologists in a cave that dates back about 50 million years has shed new light on what we thought we knew about this ancient, hypothetical creature.

Emeritus Professor Sue Hand from UNSW Sydney's School of Biological Earth and Environmental Sciences is a leading palaeontologist with expertise in bat evolution. She led an analysis of the skull, published today in the journal Current Biology, that involved Dr Jacob Maugoust and Professor Maeva Orliac from University of Montpellier in France, and Professor Robin Beck from the University of Salford, UK.

Prof. Hand says prior to the discovery of this skull -- which was among 23 separate fossilised individuals found in the cave belonging to the extinct species Vielasia sigei -- only fragments or completely flattened skeletons of early bats had existed in the fossil record.

"We don't know very much about the beginnings of bats because we don't have the missing links like we do, say, between dinosaurs and modern birds," she says.

"The oldest bat fossil is about 57 million years old, and it's a single tooth from a site in Portugal -- that's all we know about it. The first bats are all just known from fragmentary fossils, mostly teeth. When bats appear in the fossil record a little later, about 52 million years ago, some are wonderfully complete bats, but they're flattened."

While these flattened specimens are, in Prof. Hand's words, "beautifully preserved," the fact that they've been flattened by layers of rock deposited over millions of years makes it difficult to decide with conviction, the exact positioning of bones in their three-dimensional anatomy. And when it comes to determining whether a fossil is from a species of bat that is already using echolocation, detailed and precise anatomy of the skull is crucial.

"In modern bats, between the voice box and the ear, there are some bones called the hyoid bones. In all modern bats that echolocate, one of these bones directly contacts the middle ear bones and appears to be involved in transmitting high frequency sound.

"But in the flattened fossils, while we can see these various bones, there is a question about their precise relationships to each other. This has led to a lot of debate among scientists about whether or not a species used echolocation."

Uncrushed skull

But in the case of Vielasia sigei, not only is the skull almost entirely intact, but it has been preserved in limestone in its original three-dimensional shape which the scientists describe as 'uncrushed'.

"In this particular bat, we can see more directly what's going on deeper, in the inner ear," Prof. Hand says.

"We took fine measurements of that inner ear bone and compared it with that in the bats that do echolocate today and bats that don't, and it sits in the middle of the ones that echolocate."

Not all bats echolocate, Prof. Hand explains. Flying foxes regularly seen in the Sydney night sky around the Botanical Gardens, Centennial Park and the Royal National Park rely on their very good eyesight to navigate and find fruit, without echolocation. Meanwhile, Sydney microbats such as the Eastern Bentwing Bat, Gould's Wattled Bat and the Chocolate Wattled Bat, are well known for navigating and catching insects using feedback from the high frequency sound they emit.