RESEARCHERS FIND BATS EVOLVED ABILITY TO FLY BEFORE ECHOLOCATION
The discovery of a remarkably well-preserved fossil representing the most primitive bat species known to date—and an entirely new family of bats—demonstrates that these animals evolved the ability to fly before they could echolocate, or use high-pitched sounds to find the location of objects by the sounds reflected from them. The new species, named Onychonycteris finneyi, was unearthed in 2003 in southwestern Wyoming and is described in a study led by Nancy Simmons, Chair of the Division of Vertebrate Zoology and Curator at the American Museum of Natural History in New York, that appears in the February 14, 2008 issue of the journal Nature.
Bats represent one of the largest and most diverse orders of mammals, accounting for one-fifth of all living mammal species. 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,” said Dr. Simmons. “It’s clearly a bat, but unlike any previously known. In many respects it is a missing link between bats and their nonflying ancestors.”
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 Onychonycteris’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.
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 maneuvering. Without echolocation, Onychonycteris likely had to make do with visual, olfactory, or passive audio cues to hunt.
“It finally gives us an answer,” said Dr. Simmons. “Flying evolved first, echolocation second.”
Coauthors on the study include Kevin L. Seymour of the Royal Ontario Museum in Canada, Jörg Habersetzer of the Senckenberg Research Institute in Germany, and Gregg F. Gunnell of the University of Michigan Museum of Paleontology in Ann Arbor, Michigan.
Funding for the work was provided by the National Science Foundation (NSF) and Deutsche Forschungsgemeinschaft (DFG).
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