The discovery of a new prehistoric bat species sheds light on the origins of these flying mammals and raises questions about how they developed echolocation capabilities.
The world's oldest bat skeleton has been identified as a new species, which helps scientists fill in the spotted fossil record of these flying mammals and provide new clues about how they evolved. Both skeletons were discovered from an ancient lakebed in southwestern Wyoming, a site that preserves the entire subtropical lake ecosystem and surrounding forests from about 52 million years ago.
The newly discovered bat, Icaronycteris gunnelli, weighs only about 25 grams, which is roughly equivalent to five marbles. It has evolved the ability to fly and most likely has developed the ability to echololocation. The small bat, which probably lives in trees around the lake and flies over the water to prey on insects, says Tim Rietbergen, an evolutionary biologist at the Naturalis Center for Biodiversity in the Netherlands, who is the lead author of the study describing the species in the journal PLOS ONE.
Today, bats are one of the most successful animals on the planet, with more than 1,400 species, accounting for one-fifth of all mammalian species. They live on every continent except Antarctica, and they are often essential for ecological stability, providing critical functions such as pollination, seed dispersal, and insect population management.
A staircase leads to the preserved rock of an ancient lakebed in Wyoming's Fossil Mountain National Monument. Bat fossils were found on private land outside the park
Although bats are ubiquitous, scientists know little about their origins. Skeleton from Wyoming's well-preserved lakebed name fossil lake dates back to the early Eocene. At that time, global temperatures were rising and mammals, insects and flowering plants were spreading and diversifying rapidly. These bats looked very similar to modern bats, holding the wing membrane with elongated fingers.
"The idea is ... Bats originated from some kind of small, arboreal insectivorous mammal," said Matthew Jones, a paleontologist at Arizona State University and one of the study's authors. "But there's a lot of them," he added, noting that we don't know which ones might be related to bats. "Most of them are known only from isolated fragments of teeth and jaws."
Once bats appeared in the fossil record, they quickly spread around the world. The oldest bat teeth and jawbones found to date are about 55 million years old. Incomplete specimens from Portugal and China predate the newly described bones by millions of years. Scientists don't know where bats first appeared, but they likely appeared in Europe, Asia or North America before the animals spread to the Southern Hemisphere.
"It's kind of mysterious," said Alexa Sadier, an evolutionary biologist at UCLA who was not involved in the new study. "We don't have any transitional forms."
Small clues
In 2017, Rietbergen first saw a skeleton of Icaronycteris gunnelli while browsing Facebook. "I was like, well, this looks a little different," he said.
After inquiring about some of the dimensions of the fossil, which had been found in a private quarry and put up for sale, he contacted 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 as a collectible.
In addition to analyzing the new fossils, the team reexamined bat bones from the museum's collection. They found another piece of I. Gunnelli's fossil, which was acquired by the Royal Ontario Museum in 2002 and was originally classified as a related species I. index。
A male stingray from Fossil Lake in Wyoming named Asterotrygon maloneyi.
Ancient palm leaves from Fossil Lake.
The two skeletons look a lot like modern bats, but there are also subtle differences. "The first thing that struck me was the robustness of the bones, especially the hind limbs," Rietbergen says.
Most bats today have thin and light bones, which makes them more suitable for flight. I. gunnelli's thicker limbs may indicate that the species retained some of the characteristics of its evolutionary predecessors, such as strong legs for climbing trees.
Bats also have a claw on their index finger and thumb, while most modern bats sleep with only their thumb claws to drape – again hinting that bats from this period may represent the final stage of the transition from climber to professional flyer.
The situation is even more complicated when considering Onychonycteris finneyi, a larger bat species that lived in different genera at about the same time in Fossil Lake. The bat has a claw and relatively short wings on each finger, indicating that it moves around by climbing and flapping its wings. Depending on the size and shape of its inner ear, O. finneyi may not be able to perform echolocation, which is different from the condition of I. finneyi. Gunnelli and I. Index is different. Scientists initially thought that O. O. Finneyi is evidence that bats evolved the ability to fly before echolocation.
However, an analysis of the evolutionary relationships between these three species of bats from fossil lakes and other fossil and living bats found that I. gunnelli and I. gunnelli were found to be the ones in the lake. index and O. Finneyi is most closely related, not to other echolocation bats. Jones said it was "really unexpected and weird."
"We found a non-echolocated bat in the fossil record that is most closely related to a group of echolocation bats," he said. But he noted that the same is true of modern foxes, a group of large fruit-eating bats that cannot echolocate but are most closely related to a group of bats that can echololocation. "Echolocation could have multiple origins, or even multiple echolocation losses in these earliest bats," Jones said. ”
Sort out the past
Complete Eocene bat skeletons are very rare, and Fossil Lake in Wyoming is one of the few places where they have been found. The warm, moist forests where bats inhabit are harsh environments that preserve their little bones. Only when bats are buried quickly, such as sinking to the bottom of a deep lake, will their entire skeleton be preserved.