50 million year old fossil skull uncovers evolutionary history of bat echolocation

Bats are the only mammals capable of flight, and they are among the most recognisable, numerous and widespread of all mammals, with more than 1400 species alive today. But much of their fossil record is missing, and the early stages of bat evolution remain poorly understood.

A particularly controversial question is when echolocation evolved in bats. Echolocating bats “see” the world around them by emitting high frequency sounds through their mouth or nose, and then detecting the echoes that bounce off objects using a specialised and sensitive hearing system. Among modern bats, Old World fruit bats and flying-foxes do not echolocate, and it has been debated whether echolocation was already present in the common ancestor of modern bats, or whether it evolved multiple times after modern bats began to diversify.

Now, a newly discovered bat fossil provides key information that may help solve this controversy, by showing that bats had evolved advanced echolocation abilities by at least 50 million years ago, long before the origin of modern bat families. The fossil belongs to a previously unknown species that has been named Vielasia sigei, and was collected from the Quercy Phosphorites in southwest France, which preserve fossils from ancient cave systems.

Co-author Prof. Robin Beck of the University of Salford, UK, said: “Vielasia is remarkably well preserved – other fossil bats of this age are either known from a few teeth and jaws, or are squashed flat. The uncrushed skull of Vielasia provides a wealth of information that we never had before, including regarding its ability to echolocate – it’s the Rosetta Stone of bat echolocation!”

Vielasia is the oldest three-dimensionally preserved bat skull ever found, and is the oldest fossil bat known to have lived in caves. Co-author Prof. Maeva Orliac of the University of Montpellier, France, said “The outstanding preservation of fossils from Quercy gives a unique window on past biodiversity in Europe”.

CT scanning of the fossil enabled the researcher to see the internal structure of the skull and reconstruct its inner ear. The researchers found that the shape of the inner ear of Vielasia was very similar to that of echolocating bats, but quite different from non-echolocating bats and other mammals, and that it was capable of high frequency hearing, which is necessary for echolocation. University of Montpellier co-author Dr Jacob Maugoust said: “Our analyses show that Vielasia had similar hearing to modern echolocating bats, and so presumably also emitted high frequency calls for echolocation. This remarkable ability clearly evolved early in bat evolutionary history."

The researchers used anatomical evidence, together with molecular data from living species, to show that Vielasia branched off before the origin of modern bats, but was nevertheless closely related.

Lead author Prof. Sue Hand of the University of New South Wales, Australia, said: “Vielasia’s position as a close relative of modern bats means that it helps give us a really good idea of what the common ancestor of modern bats was like: an insect-eating echolocator that weighed about 20 grams and that lived in caves. It gives us direct insight into bat evolution in a way that studying modern bats cannot.”