Flying Deaf? Earliest bats probably didn’t echolocate

Fossils of a cardinal-sized creature recently unearthed in western Wyoming suggest that primitive bats developed the ability to fly before they could track their prey with biological sonar.

More than one-fifth of living mammal species are bats, and most of those use echolocation to track prey or avoid obstacles. The fossil record of these delicate-boned creatures is sparse, but analyses hint that even the earliest known bats—those flitting through the skies between 54 million and 50 million years ago—could echolocate, says Nancy B. Simmons, a vertebrate paleontologist at the American Museum of Natural History in New York City. In fact, of the six bat species previously known from that era and with enough remains to analyze, all apparently were sonar capable, she notes. Evidence includes a large cochlea, or inner ear, that enabled the bats to detect the echoes of their high-pitched squeaks.

Paleontologists have long debated whether bats’ ability to fly preceded, followed, or evolved in tandem with their ability to echolocate. Now, in the Feb. 14 Nature, Simmons and her colleagues describe the almost complete fossils of a creature that suggests the “flight-first” hypothesis is correct.

The ancient bat, dubbed Onychonycteris finneyi, had a 30-centimeter-wingspan and lived in what is now western Wyoming about 52.5 million years ago, says Simmons. Onychonycteris, which means “clawed bat” in Greek, refers to the creature’s most distinctive feature: It has claws on all five digits of its forelimbs, whereas all living bats and previously studied fossil bats have claws on no more than two digits. The name finneyi honors the fossil collector who excavated the specimens, says Simmons.

O. finneyi’s wings were relatively short and broad compared with those of other bats, and the outermost portion of the wing membrane—the part stretched between the bat’s fingers—was relatively small. Modern species that are built similarly have an unusual flying style, alternately gliding and fluttering their wings, says Simmons. This undulating mode of flight saves energy at most flight speeds and may replicate the evolutionary bridge between gliding and full-fledged flapping, she and her colleagues speculate.

Also, the researchers note, O. finneyi’s legs are relatively longer than those of any known bats, and the ratio of its forelimb length to its hindlimb length approaches that of nonflying arboreal mammals such as sloths and lemurs.

Together, these traits—numerous claws, unbatlike limb proportions, and a presumably clumsy flying style—suggest that O. finneyi represents the most primitive known lineage of bats. Although O. finneyi clearly could fly, the bat’s cochlea was small, a sign that the creature couldn’t echolocate. That inability suggests that the evolutionary predecessors of bats also could not echolocate, the researchers note.

“This is a really big find, a huge piece of the [evolutionary] puzzle,” says Emma C. Teeling, a paleontologist at University College Dublin. Not only do these fossils answer the age-old question of whether flight and echolocation developed separately, but they also provide hints about what protobats may have looked like, she says.

“To find something in the fossil record that isn’t a full-fledged bat is great,” says Suzanne Hand, a vertebrate paleontologist at the University of New South Wales in Sydney, Australia. However, she adds, O. finneyi may simply represent an evolutionary experiment, one newfound bat lineage among the many that were flapping about at the time.