52 million-year-old bat skeleton is the oldest ever found and belongs to a never-before-seen species

The oldest bat skeletons yet discovered were discovered in southwest Wyoming, and their discovery caused the bat family tree to be rearranged.

Researchers have discovered the oldest bat bones yet discovered—two exquisitely preserved specimens discovered in Wyoming—which belonged to a previously unknown species. They are 52 million years old.

The unusual fossils were found in the state's southwest in the Green River Formation.

The newly discovered species, which could easily fit into a human palm with its wings folded against its body, was somewhat smaller than the closest known related bat species, Icaronycteris index.

Lead author Tim Rietbergen, a bat paleontologist and collection manager at the Naturalis Biodiversity Center in Leiden, the Netherlands, told Live Science in an email, "When I first saw the first skeleton, I instantly felt it was strange. These are the oldest skeletons because they are found further down in the stratigraphy (sediment layers) than other fossilized bats.

The Eocene era saw the origin of bats (56 million to 36 million years ago). The more than 50 million-year-old fossil bones of I. index and another extinct species known as Onychonycteris finneyi, which paleontologists also described from Green River Formation deposits, were the oldest bat skeletons known up to this point. The best-preserved skeletons may be found, according to Rietbergen, in the Green River Formation.

The new discoveries have led to a revision in the categorization of early bats to include the newly discovered species in the family tree. The new discoveries are detailed in a paper that was published on Wednesday, April 12 in the journal PLOS One.

The researchers compared the new fossils with whole skeletons from six Eocene bat species, as well as with single teeth from two additional extinct species and with skeletons of extant bats, in order to ascertain the evolutionary history, or phylogeny, of the bats. They dubbed the new species of Icaronycteris I. gunnelli in honor of the late bat researcher Greg Gunnell after their findings, which showed that the recently discovered bat skeletons belong to a previously undiscovered species .

"It obviously stood out as being a new species after comparing the measurements with other bats," Rietbergen added. "I became really thrilled and thought maybe the bat variety from the early Eocene was far more than we realized," the author said.

The only other Icaronycteris species known to exist in North America is I. index, and the researchers also found that these two species have a sister connection, making them each other's closest living relatives.

I. index was presumably significantly bigger than I. gunnelli, although detailed scans of the fossils indicate that the latter weighed less than an ounce (22.5 to 28.9 grams), which is around the same body mass. According to the study, the mismatch between the reconstructed weight and wingspan may be the result of bone distortion during fossilization.

Emma Teeling, a professor of zoology at University College Dublin in Ireland who was not involved in the study, told Live Science in an email: "This newly described species is considered to be one of the oldest known articulated bat skeletons, providing novel insight into the phylogeny of our earliest bat fossils. However there are still phylogenetic issues that can only be answered by finding more complete and well-defined fossils of bats.

The researchers' study leads them to believe that Green River bats diverged from other Eocene bats. We still don't know a lot, according to Rietbergen. If we have a clear picture of the variety of bats, we may look at evolutionary adaptations and potentially uncover clues that will help us determine the origin of bats.