Already around 52 million years ago, bats showed an astonishing variety of species. This is proven by a well-preserved fossil from Wyoming - the oldest complete skeleton of a bat found so far. According to a study, the fossil cannot be assigned to either of the two species previously discovered at the site. Instead, it is a new species that the researchers named Icaronycteris gunnelli. The find suggests that bats in America evolved separately from relatives in other parts of the world.
With around 1460 living species, bats make up around a fifth of all mammal species. With the exception of the polar regions, they are distributed worldwide and colonize a variety of habitats and ecological niches. The earliest records of bats are from the early Eocene, 56 to 47.6 million years ago. During this time they spread across North America, Europe, Africa, Asia and Australasia. However, most fossils consist only of individual teeth – complete skeletons are rare.
New species from known locality
A team led by Tim Rietbergen from the Naturalis Biodiversity Center in Leiden in the Netherlands has now described what is probably the oldest complete skeleton. The specimen was discovered in 2017 by a private collector who later sold it to the American Museum of Natural History in New York. "The fossil comes from the Green River Formation in Wyoming and differs from other Eocene bat species by a combination of characteristics," report Rietbergen and his team.
The Green River Formation has been a known site for bat fossils since the 1960s - more than 30 specimens have already been discovered there. "But interestingly, most specimens from this formation were identified as belonging to a single species, Icaronycteris index, until about 20 years ago, when a second bat species belonging to a different genus was discovered," explains co-author Nancy Simmons of the American Museum of Natural History, which itself was involved in describing the then newly discovered species in 2008. "I always suspected there must be more species out there," she says.
Unique combination of features
To find out if other species were represented in the region, Rietbergen and his team collected data from numerous museum specimens and cataloged their characteristics. "Paleontologists have collected many bats that have been identified as Icaronycteris index, and we wondered if there might be multiple species among these specimens," Rietbergen said. "Then we learned of a new skeleton that caught our attention." In fact, the research team found that the new skeleton was smaller than any specimen previously found in the Green River Formation. It also had claws on the first two phalanges and a tiny ossified third phalanx on winglets three through five, combined with broad wings and short, sturdy hind legs—a hitherto unique combination of traits. The teeth also differed from those of previously known species.
The researchers therefore concluded that it was a new species, which they named Icaronycteris gunnelli in honor of paleontologist Gregg Gunnell, who did much research on bat evolution. Another fossil that was discovered in 1994 could also be assigned to the newly described species. The exact age of the fossils is unclear. The previously oldest specimens from the Green River Formation have been dated to about 52.5 million years. "One of the specimens of I. gunnelli was found in a deeper layer of the section than any other bat, meaning that this species is older than any other bat species recovered from this deposit," explains co-author Arvid Aase of Fossil Butte National Monument in Wyoming.
Old but not primitive
At the site there was once a lake, on the bottom of which volcanic ash and wafer-thin layers of limestone have gradually been deposited, in which many fossils are preserved. “Deposition near the coast is faster than in the middle of the lake, making it difficult to estimate deposition rates. Until reliable deposition rates have been determined, there is no reliable way to determine the exact age of the specimens," say the researchers. "However, the relative stratigraphic position of these fossils suggests that they are the oldest bat skeletons yet found anywhere in the world."
Nonetheless, the fossils of I. gunnelli show less primitive features than other specimens. The study also concludes that all three bat species found in the Green River Formation form a distinct clade distinct from known archaic Old World bat lineages. The researchers conclude that bats spread rapidly around the world during the early Eocene, evolving independently on multiple continents. "This is a step forward in understanding what happened in the early days of bats in terms of evolution and diversity," Simmons said.
Source: Tim Rietbergen (Naturalis Biodiversity Center, Leiden, The Netherlands) et al., PLoS ONE, doi: 10.1371/journal.pone.0283505