​52-million-year-old bat skeletons are the oldest ever—and tell a ‘really weird’ tale

The oldest bat skeletons in the world have been identified as a new species, helping scientists fill in the spotty fossil record of these flying mammals and providing new clues about how they evolved. Both skeletons were recovered from an ancient lakebed in southwestern Wyoming, a site that preserves an entire subtropical lake ecosystem and surrounding forest from about 52 million years ago.

The newly discovered bat, Icaronycteris gunnelli, weighed only about 25 grams, roughly as much as five marbles. It had already evolved the ability to fly and likely had developed the capacity to echolocate. The small bat probably lived in the trees surrounding the lake, flying over the water to hunt insects, says Tim Rietbergen, an evolutionary biologist at the Naturalis Biodiversity Center in the Netherlands and lead author of the study describing the species in the journal PLOS ONE.

Today bats are among the most successful animals on the planet, with more than 1,400 species accounting for one-fifth of all mammal species. They live on every continent except Antarctica, and they are often critical to ecological stability, providing key functions such as pollination, seed dispersal, and insect population management.

Despite the ubiquitousness of bats, scientists know very little about their origins. The skeletons from Fossil Lake, the name of the preserved lakebed in Wyoming, date to the early Eocene epoch. At that time, global temperatures were on the rise and mammals, insects, and flowering plants were rapidly spreading and diversifying. These bats look remarkably similar to modern bats, with elongated fingers to hold wing membranes.

“The thought is that … bats originated from some sort of small, insectivorous mammals that were probably arboreal,” says Matthew Jones, a paleontologist at Arizona State University and one of the authors of the study. “But there’s a lot of those,” he adds, pointing out that we don’t know which ones may be related to bats. “Most of them are only known from isolated teeth and jaw fragments.”

After bats appear in the fossil record, they quickly spread around the world. The most ancient bat teeth and jaw bones found so far are roughly 55 million years old. Incomplete specimens from Portugal and China predate the newly described skeletons by a few million years. Scientists don’t know where bats first appeared, though it was likely in Europe, Asia, or North America before the animals spread to the Southern Hemisphere.

“It’s kind of a mystery,” says Alexa Sadier, an evolutionary biologist at the University of California, Los Angeles, who was not involved in the new study. “We don’t have any transitional forms.”

Small clues

Rietbergen first saw one of the skeletons of Icaronycteris gunnelli in 2017 when he was scrolling through Facebook. “I was like, hmm, this looks a little bit different,” he says.

After asking for some measurements of the fossil, which had been found at a private quarry and was listed for sale, he reached out to Nancy Simmons, a bat expert at the American Museum of Natural History. She agreed with him that it looked like a new species, and AMNH bought the fossil for its collections.

In addition to analyzing the new fossil, the study team reexamined bat skeletons that were already in museum collections. They found another fossil of I. gunnelli that had been acquired by the Royal Ontario Museum in 2002 and was originally classified as the related species I. index. 

The two skeletons look similar to modern bats, but there are subtle differences. “One thing that stood out for me in the first place,” Rietbergen says, “was the robustness of the bones, especially the hind limbs.”

Most bats today have thin, light bones that make them better suited for flight. The thicker limbs of I. gunnelli may indicate that the species retained some traits of its evolutionary predecessors, such as stronger legs for climbing trees.

The bat also had a claw on its index finger as well as its thumb, while most modern bats have only thumb claws to dangle from as they sleep—another hint that bats from this time may represent the last phases of a transition from climbers to specialized fliers.

The picture gets more complicated when considering a larger bat species from a different genus that also lived at Fossil Lake around the same time, Onychonycteris finneyi. This bat had a claw on each finger and relatively short wings, suggesting it got around by climbing and a fluttering method of flight. Based on the size and shape of its inner ear, O. finneyi was probably not capable of echolocation, unlike I. gunnelli and I. index. Scientists originally considered O. finneyi to be evidence that flight had evolved in bats before echolocation.

But an analysis of the evolutionary relationships between these three bat species from Fossil Lake, as well as other fossil and living bats, found that I. gunnelli and I. index were most closely related to O. finneyi rather than to other echolocating bats. That “is really unexpected and really weird,” Jones says.

“We have in the fossil record a non-echolocating bat that’s most closely related to a group of echolocating bats,” he says. But he noted that this is also true for present-day flying foxes, a group of large fruit-eating bats that cannot echolocate but are most closely related to a group of bats that can. “There’s possibly multiple origins of echolocation or there’s multiple losses of echolocation among even these earliest bats,” Jones says, “which is really, really bizarre.”

Untangling the past

Complete bat skeletons from the Eocene are rare, and Fossil Lake in Wyoming is one of the only places they have been found. The warm, wet forests that bats lived in were poor environments for preserving their small bones. Only when the bats were quickly buried, such as after sinking to the bottom of a deep lake, have their entire skeletons been preserved.

A few million years after the appearance of bats in southwestern Wyoming, several species lived near a similar lake ecosystem in Germany—and these later animals looked even more like modern bats.

Scientists believe that advantages such as flight and echolocation would have helped bats spread and diversify rapidly. The mammals may have adapted to nocturnal lives to avoid competition with or predation by birds—but when this transition occurred is unclear.

Genetic research has shed little light on the direct ancestors to bats. Instead, studies of DNA have revealed that bats belong to a superorder of mammals called Laurasiatheria, which includes other insectivores such as shews and moles. But it also includes animals that outwardly seem to have no relation to bats. Among them are whales; ungulates like horses, rhinos, and hippos; and the order carnivora, which includes cats, dogs, and bears. Surprisingly, all these animals are more closely related to bats than rodents.

“If you look at the DNA, we get, like, hoofed animals as the closest relatives, which is crazy,” Rietbergen says. “There’s something going on there.”

To fill in the gaps, and possibly even discover the immediate ancestor to bats, scientists are hoping to find more skeletons.

“The more we will find, the more we’ll better understand about how many species of bats were there at that time, how different they were, how diverse they were,” UCLA’s Sadier says. And maybe, searching deeper in time, we will even find the “transitional forms that we are all hoping to find one day.”