These “Gorgeous” Skeletons Reveal a New Species of Ancient Bat

Sky puppies have graced the planet for more than 50 million years.

A bat skeleton etched into brown stone.
Rietbergen et al., 2023, PLOS ONE, CC0 (

In 2008, bat ecologist Nancy Simmons published a paper describing a 50-million-year-old bat fossil found in southwestern Wyoming, Onychonycteris finneyi. This discovery showed “features that are more primitive than seen in any previously known bat.” It also became only the second bat fossil unearthed in this locale, following the Icaronycteris index described in a 1966 paper.

Fifteen years later, now a curator-in-charge of the division of vertebrate zoology at the American Museum of Natural History, Simmons is still analyzing novel bats. This time, her new finding supplants her 2008 bat as the oldest bat skeleton on record.

Published on April 12 in the journal PLoS ONE, the study details two bat skeletons of a newfound species that tell us a little bit more about bat morphology and evolution.

Bats typically don’t preserve well as fossils, but the silt in what’s now the Fossil Lake deposit protected these from the elements for 50 million years.

Rietbergen et al., 2023, PLOS ONE, CC0

Welcoming a new species

In 2017, two near-perfectly preserved bat skeletons were discovered in the Fossil Lake deposit, part of Wyoming’s Green River Formation. These articulated bones, it turns out, comprise a new species of bat called Icaronycteris gunnelli, named for Duke University paleontologist Gregg Gunnell. These fossils, scientists surmise, are the oldest bat skeletons on the fossil record at about 52 million years old, dating back to the Eocene epoch, which was 55.8 to 33.9 million years ago.

“It's always a thrill to see a complete set instead of just bits and pieces,” Simmons tells Inverse. “Fossils of bats and most other animals often come to us in fragments: a few teeth here, a jaw there, a limb bone over there.” Such was the case with two other Eocene-era bat species identified only by isolated teeth and jaw parts, Icaronycteris menui in France and Icaronycteris sigei in India.

Other researchers echo this enthusiasm for the find. “The fossils are absolutely gorgeous,” writes Gerald Carter, an animal behavior professor studying bats at the Ohio State University who was not involved in this study, tells Inverse. “I cannot imagine how exciting it must have been to dig these up.”

Simmons and her colleagues estimate this new species weighed about 25 grams, about as much as one AA battery.

Researchers can tell from this bat’s arm bones in its wings that it’s more primitive.

Rietbergen et al., 2023, PLOS ONE

Why everyone’s batty about them

Though bats number more than 1,400 species — making them the second-most diverse group of mammals behind rodents — we still know surprisingly little about their evolutionary origins. Even though we have fossils from over 70 taxa from the Eocene, there’s still a lot we don’t know about bats from this period.

Simmons suspected there was more to the story than what had been found so far. “I've always thought that there was probably more bat species diversity back in the Eocene Era in North America than we've managed to find previously,” she says. “So I wasn't surprised, but I was definitely delighted.”

These fossils also clue us into bat evolution. “These few, valuable, and painstakingly-discovered fossils tell us directly [about] the ancestors,” Carter writes.

Study co-author Matthew Jones, who completed this study as part of his Master’s at the University of Kansas, and is now a postdoctoral paleontology fellow at the University of Arizona, clarifies that while this is now the oldest known bat skeleton, it doesn’t represent the world’s oldest bat. The Green River Formation has also yielded tooth and jaw fragments from bats about three million years older than this recent find. Jones says that the oldest bats hail from Silveirinha, Portugal, the Junggar Basin in China, and Australia.

“North America actually is one of the last places they seem to show up, but when they do show up in North America, we get full skeletons,” Jones tells Inverse.

Dentition, or tooth features, are a great way to identify fossils because enamel takes the longest to decompose.

Kristen Tietjen at the Division of Vertebrate Paleontology, Biodiversity Institute, University of Kansas

The dirt on fossils

Bats, Jones tells Inverse, don’t make for great fossils. Since they fly, they’re less likely to perish in a place where they’ll be preserved for millions of years, like moles or other underground dwellers. He also says the highest diversity of bats is found in humid tropical and subtropical areas where scavengers and decomposers abound. But since these particular bats likely died mid-flight or in the lake, they fortuitously sank beneath the silt where the elements couldn’t get at their remains.

The authors analyzed 699 characteristics in these fossils. Jones says that these fossils have more “primitive” characteristics compared to their modern counterparts. For example, their arm bones differ in length much more than those of contemporary bats. Today’s sky puppies have an arm-bone ratio more optimal for flying.

Mammals are a little easier to identify because their enamel preserves well over time, serving as a clear marker for which species they might belong to. Tooth features, a.k.a. dentition, become the defining feature. Jones says that if he looks at a tooth fossil, “I can recognize it off the bat — pun not intended — that it is a bat, and with a little bit of research can pin it down usually to family if not genus or species.” This is why paleobiologists can identify new species from teeth alone, as with the Eocene-era bats from France and India.

Batter on deck

A better understanding of early Eocene bats will illuminate ancient bat phylogeny to paleobiologists. Even with dozens of different fossils, there’s still not a clear consensus on what defines the extinct family Icaronitiedae. Thus, an examination of relationships between ancient bats is in order.

“The next step well in the works is to conduct a big analysis of these bat relationships,” Jones tells Inverse. He says Icaronitiedae is only identified by their “primitive” features, like the ones in their arm bones and teeth, which all bat families likely had at one point. The physical traits that define this long-gone family could still be further refined.

As for Simmons, perhaps there’s even another gorgeous, full-bodied skeleton in her future.