The world’s oldest bat skeletons have been identified as a new species, helping scientists fill the patchy fossil record of these flying mammals and providing new clues to their evolution. The two skeletons were recovered from an ancient lake bed in southwestern Wyoming, a site that preserves an entire subtropical lake ecosystem and surrounding forest from around 52 million years ago.
The newly discovered bat, Icaronycteris gunnelli, weighed only about 25 grams, about as much as five marbles. He had already developed the ability to fly and had probably developed the ability to echolocate. The little bat probably lived in the trees surrounding the lake, flying above 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 review. PLOS ONE.
Today, bats are among the most successful animals on the planet, with more than 1,400 species representing one-fifth of all mammal species. They live on every continent except Antarctica and are often essential for ecological stability, performing key functions such as pollination, seed dispersal and insect population management.
Despite the ubiquity of bats, scientists know very little about their origins. The skeletons of Fossil Lake, the name of the preserved lake bed in Wyoming, date to the Early Eocene. At that time, global temperatures were on the rise, and mammals, insects, and flowering plants were spreading and diversifying rapidly. These bats look remarkably like modern bats, with elongated toes to hold the wing membranes.
“The idea is that…bats originated as a kind of small insectivorous mammal that was probably arboreal,” says Matthew Jones, a paleontologist at Arizona State University and one of the study’s authors. “But there are a lot of them,” 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 appearing in the fossil record, bats quickly spread around the world. The oldest bat teeth and jaw bones discovered to date are around 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 the 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 transition forms.”
Rietbergen saw for the first time one of the skeletons of Icaronycteris gunnelli in 2017 while browsing Facebook. “I was like, hmm, that sounds a little different,” he says.
After asking for some measurements of the fossil, which had been found in a private quarry and was up for sale, he contacted Nancy Simmons, a bat scientist at the American Museum of Natural History. She agreed with him that it looked like a new species, and the AMNH purchased the fossil for its collections.
In addition to analyzing the new fossil, the study team re-examined bat skeletons that were already in museum collections. They found another fossil of I. Gunnelli which had been acquired by the Royal Ontario Museum in 2002 and was originally listed as the related species I. index.
Both skeletons resemble modern bats, but there are subtle differences. “One thing that stood out to me the most,” says Rietbergen, “was the robustness of the bones, especially the hind limbs.”
Today, most bats have thin, light bones that make them better suited for flight. The thicker members 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 only have thumb claws to swing around in while sleeping – another clue that bats from this time could represent the last phases of a transition from climbers to specialists. flyers.
The picture gets complicated when you consider a larger bat species of 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 that it moved by climbing and floating. Depending on the size and shape of his 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 prior to echolocation.
But an analysis of the evolutionary relationships between these three Fossil Lake bat species, along with other fossil and living bats, found that I. Gunnelli And I. index were most closely related to O.finneyi rather than other echo-eating bats. It’s “really unexpected and really weird,” Jones says.
“We have a non-echolocating bat in the fossil record that is 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 bats that cannot echolocate but are more closely related to a group of bats that can. “Maybe there are multiple origins of echolocation or there are multiple losses of echolocation even among those early bats,” Jones says, “which is really, really weird.”
Untangling the past
Complete Eocene bat skeletons are rare, and Fossil Lake in Wyoming is one of the only places they have been found. The warm, humid forests in which bats lived were poor environments for the preservation of their small bones. It was only when bats were quickly buried, for example after sinking to the bottom of a deep lake, that their entire skeletons were preserved.
A few million years after bats first appeared 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 benefits such as flight and echolocation would have helped bats spread and diversify quickly. Mammals may have adapted to nocturnal life to avoid competition or predation by birds, but when this transition occurred is unclear.
Genetic research has shed little light on the direct ancestors of bats. Instead, DNA studies 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, from the outside, seem to have nothing to do with bats. Among them are whales; ungulates such as horses, rhinos and hippos; and the order Carnivora, which includes cats, dogs, and bears. Surprisingly, all of these animals are closer to bats than rodents.
“If you look at DNA, we get, for example, hoofed animals as the closest relatives, which is crazy,” says Rietbergen. “Something’s going on over there.”
To fill in the gaps, and perhaps even uncover bats’ immediate ancestor, scientists hope to find more skeletons.
“The more we find, the better we will understand how many species of bats existed at that time, how different they were, how diverse they were,” says UCLA’s Sadier. And perhaps, by searching deeper in time, we will even find the “transitional forms that we all hope to find one day”.
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