Flight first, sound later

Sources: American Museum of Natural History, Royal Ontario Museum, University of Michigan

The discovery of a well-preserved fossil representing the most primitive bat species known to date - and an entirely new family of bats - demonstrates that the animals evolved the ability to fly before they could echolocate, researchers say.

The new species, named Onychonycteris finneyi, was unearthed in 2003 in southwestern Wyoming in the US and is named for Bonnie Finney, the excavator who found the specimen and first recognised its importance.

It is described in a study written by Nancy Simmons from the American Museum of Natural History, Kevin Seymour of the Royal Ontario Museum, Gregg Gunnell of the University of Michigan and Jorg Habersetzer of the Senckenberg Research Institute in Germany in the February 14 issue of Nature.

"Up until now, all fossil bats looked pretty much like living bats" Seymour said. "Now, for the first time, we have a glimpse of what an ancestral bat looked like, and it surprised us."

Despite the fact that bats are so widespread and include a fossil record that extends over more than 50 million years, the evolutionary timing and development of both echolocation and flapping flight has been widely debated.

The well-preserved condition of the new fossil permitted the scientists to take an unprecedented look at the most primitive known member of the order Chiroptera.

"When we first saw it, we knew it was special," Simmons said. "It's clearly a bat, but unlike any previously known. In many respects it is a missing link between bats and their non-flying ancestors."

"There has been a longstanding debate about how bats evolved, centering around the development of flight and the development of the sonar system they use to navigate and hunt for prey," Gunnell said.

"The three main theories have been that they developed the two abilities together, that flight came first, or that sonar came first. Based on the specimen described in this paper, we were able to determine that this particular animal was not capable of echolocating, which then suggests that bats flew before they developed their echolocation ability."

Dating the rock formation in which the fossil was found put its age at 52 million years. Onychonycteris was not the only bat alive at the time - fossils of Icaronycteris, a more modern bat that could echolocate, are found in the same formations.

A careful examination of the fossil's physical characteristics revealed several surprising features. For example, it had claws on all five of its fingers, whereas modern bats have claws on only one or two digits of each hand. The limb proportions of Onychonycteris are also different from all other bats the hind legs are longer and the forearm shorter - and more similar to those of climbing mammals that hang under branches, such as sloths and gibbons.

The fossil's limb form and the appearance of claws on all the fingers suggests that Onychonycteris may have been a skilled climber. However, long fingers, a keeled sternum, and other features indicate that Onychonycteris could fly under its own power like modern bats. It had short, broad wings, which suggest that it probably could not fly as fast as most bats that evolved later.

Instead of flapping its wings continuously while flying, it may have alternated flapping and gliding while in the air. Onychonycteris's teeth indicate that its diet consisted primarily of insects, just like most living bats.

"We don't know what the initial incentive was to take to the air," Gunnell said. "My thought is that these bats probably were commuters at first - developing the ability to fly allowed them to travel to a particular place to feed, then fly back to their nesting area."

Eventually, selective pressures likely favoured the development of more sustained and agile flight, allowing bats to hunt on the wing.

Despite Onychonycteris's resemblance to animals that evolved subsequently, its skull lacks features in and around the ear seen in bats that use echolocation to navigate and hunt. The structure of its feet and ankles, which include a special, spur-like bone that likely supported a tail membrane, led the researchers to conclude that Onychonycteris had the broad tail that modern bats use to capture prey in flight, but that it was probably used as an airfoil to aid manoeuvring.

Without echolocation, Onychonycteris likely had to make do with visual, olfactory, or passive audio cues to hunt.

"It finally gives us an answer," Simmons said. "Flying evolved first, echolocation second."