Newfound bat skeletons are the oldest on record

Two fossilized bat skeletons unearthed in western Wyoming represent a new species and are the oldest set of bat bones yet discovered, researchers say.

The incredibly complete fossils of Icaronycteris gunnelli, which show all the animals’ bones in lifelike positions, are from limestone rocks that accumulated as lake sediments about 52.5 million years ago, vertebrate paleontologist Tim Rietbergen and colleagues report April 12 in PLOS ONE. Skeletons of the few other bat species, including another one from the Icaronycteris genus, found in the same limestones, called the Green River Formation, were preserved at least 40 centimeters above the new fossils — and are thus younger.

But it’s difficult to estimate just how much younger those fossils are because researchers don’t know how rapidly sediments accumulated over time, says Rietbergen, of the Naturalis Biodiversity Center in Leiden, Netherlands.

An analysis of the newfound skeletons indicates I. gunnelli is the runt of its genus. The species has a wingspan, typically estimated from the length of forearm bones, that is almost 7 percent smaller than that of the bat’s closest known cousin, Rietbergen and colleagues say. During its lifetime, I. gunnelli weighed somewhere between 22.5 and 28.9 grams — roughly half as heavy as a tennis ball, the team calculates.

These skeletons “are a great discovery,” says Zhe-Xi Luo, a vertebrate paleontologist at the University of Chicago who was not involved in the study. The arrangement of bones in I. gunnelli’s feet suggest that it, like many modern bats, hung upside down when roosting, he says.

Because the skeletons are so much like those of some modern bats, “we’re not any closer to knowing what type of creatures bats evolved from,” says Brock Fenton, a vertebrate paleontologist at the University of Western Ontario in Canada, who also was not involved with the work. The world’s oldest known bats lived about 56 million years ago, but the fossils of those species are mostly isolated teeth, not entire skeletons.