The new study, which appears tomorrow in the journal Nature, may put to rest the idea that echolocation evolved first.
That theory suggests that small tree-dwelling mammals developed echolocation to snatch their insect prey in midair.
This hunting method could have gradually led to evolutionary selection for longer arms and digits that enabled the animals to leap—and later glide—after their prey, until they eventually developed full-flapping flight.
"We don't know for sure, but what makes the most sense in terms of the evolution of flight is that bats evolved from a gliding ancestor, something similar to a gliding squirrel," Simmons said.
Gliding has evolved several times in tree-dwelling mammals as they gradually acquired membranes to enable gliding through canopies.
"The hypothesis for bat flight is that it evolved as a means to expand their range and maneuverability to find food and escape predators," Simmons added.
Onychonycteris also has morphological features that suggest it was an expert climber. Its limb proportions and claw-tipped fingers resemble characteristics of both modern bats and non-flying animals, such as sloths, that hang under branches.
"Probably what it was doing was flying to get from place to place, landing in some vegetation, and then climbing around looking for food," Simmons explained.
"As a bat that can't echolocate, it probably wasn't chasing food in the air but listening for prey-generated sounds [like a beetle hitting a leaf].
"That's the way that some living bats hunt, even using echolocation for navigation."
(See a picture of a bizarre new bat.)
While Onychonycteris makes it clear that ancient bats could fly without the ability to sense prey, scientists are still wondering if the mammals flew in the dark.
John Speakman, the chair of zoology at the University of Aberdeen in the United Kingdom who wrote a commentary on the study, has a theory.
"The possibility is there that bats were originally [awake during the day] and were forced into the nocturnal niche by the appearance of avian predator species some 50 to 60 million years ago," Speakman said.
Both birds and mammals increased dramatically during this time period, which followed the extinction of the dinosaurs at the end of the Cretaceous, 65 million years ago.
New nocturnal lifestyles might have led some bats to develop echolocation, while others may have relied on increased night vision to get around.
Unfortunately existing Onychonycteris fossils will shed no light on this question.
"The eye sockets were crushed, so it can't be determined if they were enlarged as in other nocturnal, non-echolocating animals," Speakman explained.
Despite this setback, however, the fossils represent a breakthrough in the understanding of bat evolution.
"This kind of puts the icing on the cake," Speakman said.
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