50 million-year-old fossil bat skull helps answer long-debated evolution question

A tiny and beautifully preserved 3-dimensional fossil bat skull found in a cave in France could help resolve a long-standing debate about how and when echolocation evolved in bats.

The 1.8cm skull is significant because of its age – about 50 million years old – and its intact 3D form, making it the oldest, uncrushed bat cranium according to the study published in Current Biology.

The skull, from an early Eocene bat species named Vielasia sigei, was found with more than 400 other specimens (including baby bat teeth) from at least 23 individuals, preserved in a limestone cave at Vielase in southwestern France.

The Vielasia sigei is also the first stem (or ancestor) bat described from a cave, rather than other environments.

Lead author of the study is emeritus professor Suzanne Hand from the University of New South Wales, a palaeontologist specialising in bat evolution.

“The beauty of being preserved in a cave deposit is that the fossils tend to be 3-dimentionally preserved, not squashed between layers of rock,” Hand says.

She says the fossil find helps answer a long running debate regarding how echolocation evolved in bats, with two main hypotheses. 

Approximately 86% of modern bats use echolocation, the ability to use sound waves to navigate, forage and hunt. Meanwhile others, including fruit bats and flying foxes, do not.

The first theory is that the common ancestor of all modern bats echolocated, but this capability was later lost in flying foxes and fruit bats. The other school of thought suggests the ability arose later in modern bats.

The well-preserved skull enabled researchers to observe and measure the inner ear of a 50 million-year-old bat and compare those results with modern bats – including those that echolocate and those that don’t.

Hand says the analysis strongly suggests the ability of Vielasia sigei to echolocate: “Our measurements put this really old stem bat right in the middle of the echolocating group.”

She says while there is not 100% certainty, “the evidence is quite compelling that this was a bat that echolocated in a way that was similar to the way many modern bats echolocate today.”

The Vielasia bat dates back to the Eocene / Credit: Maeva Orliac

Based on its skull and the other fossil finds, the likely size of the Vielasia sigei bat was about 18 grams, that’s fairly similar to the average size of modern bats at about 13 or 14 grams, Hand says.

According to the paper, Vielasia sigei appears to have been capable of powered and manoeuvrable flight, ate insects and echolocated (its frequency range is estimated at 30-56 kHz). It lived in small colonies in caves with semi-independent young. 

The fossil find is significant, Hand says, as unlike birds and dinosaurs, there are huge gaps and missing links in the fossil record for bats. She says about 80% of the fossil record relating to bats is missing and there are many unanswered questions about how these mammals evolved.

Apart from this find, there are a handful of fragmentary fossil teeth dating back to 56 million years old. A fossil from Queensland dating to 55 million years old is among the oldest, Hand says.

In the global hothouse conditions of the early Eocene bats rapidly appeared on most continents. The paper notes that bats like Vielasia sigei may have moved into caves in search of a more stable thermal environment. 

Hand says by the time bats are picked up again in the fossil record – largely as flattened specimens – they tend to look pretty well like “a good bat”, more-or-less how you’d expect a modern bat to look.

Hand expects the 3-dimensional skull and fossil find to remain “incredibly useful”. Her colleagues are already undertaking further research into the brain of the ancient mammal.

The bat has been named in honour of French palaeontologist Dr Berard Sigé.

Please login to favourite this article.