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The fossilized remains of a pair of 53-million-year-old bats may have solved the biggest mystery in bat evolution: which came first—flight or echolocation? Researchers report in Nature this week that the fossils, each measuring less than four inches (10 centimeters) long, represent the oldest and most primitive bat species yet. The creatures had wings much like those of modern bats—except for tiny claws capping their toes and elongated fingers—meaning they likely fluttered through the air.
"You look at it and say, 'it's a bat, no question,'" says evolutionary biologist Nancy Simmons of the American Museum of Natural History in New York, lead author of the study. She and her co-authors, however, report that the animal's cochlea (the part of the inner ear that detects air vibrations) is too small for it to have navigated by listening to the echoes of its high-pitched squeals, called echolocation. Also missing are two other bony features that mark echolocating bats: a large protuberance off of the middle ear bone, and a flared tip at the end of a long, skinny bone in the back of its skull.
"Given the fact that those are missing, we feel pretty confident this was not an echolocating bat," Simmons says. The finding is "spectacular," writes bat researcher John Speakman of the University of Aberdeen, Scotland, in an accompanying editorial.
Some biologists have proposed that bats evolved echolocation to aid in hunting insects before they acquired flight. Ancient bat fossils dating to around 50 million years ago looked much like existing bats, down to the enlarged cochlea necessary for echolocation.
Speakman and other bat researchers, however, observed in the late 1980s that echolocation requires a lot of energy. That is because bats have to force air out of their lungs to make an ultrasonic pulse. When bats are in flight, however, their beating wings compress and expand the rib cage, which powers the lungs.
Speakman speculates that the first bats were active in the daytime, using their eyes to get their bearings in the air and to hunt insect prey, but were driven into nocturnal activity by feathered predators that emerged after the dinosaurs died out 65 million years ago. Unfortunately, researchers cannot tell whether the fossil bat's eyes were adapted for searching the dark, as in a few living bats: Their upper skulls, including the eye sockets, are crushed beyond reconstruction.