KEMMERER — Two of the oldest known bat fossils in the world were found in the Green River Formation near Kemmerer. The complete skeletons were found deep in what is called Fossil Lake in Fossil Basin. The formation’s site spans Wyoming, Utah and Colorado, in what was once an ancient lake and swamp during the era called the Eocene, around 50 million years ago.

John Collins, educational technician at Fossil Butte National Monument, said, “A third bat skeleton like the other two was found in 2022, which we purchased, and it will be on display at the Fossil Butte Museum before Memorial Day weekend.”

These newly discovered completely articulated bat skeletons have come from mid-shore deposits of the Green River Formation in southwest Wyoming. They were all discovered in the Fossil Lake sediments deposited by the smallest lake of the Green River Lake system.

The lowlands of the Fossil Lake Basin during the Eocene period were warm and humid — similar to a subtropical environment; surrounded by highlands and mountains, where a more temperate highland flora could be found.

Based on the living bat environments of today, it is plausible that the Green River formation hosted more bat species than previously discovered.  This new species represents a 50% increase in known bat diversity, according to an article published in Smithsonian Magazine on April 12.

“The Fossil Lake deposits of the Green River Formation are simply amazing, because the conditions that created the paper-thin limestone layers also preserved nearly everything that settled to the lake’s bottom,” said Arvid Aase, park manager and museum curator at the Fossil Butte National Monument. “One of these bat specimens was found lower in the section than all other bats, making this species older than any of the other bat species recovered from this deposit.”

There are more than 1,460 living species of bats found in nearly every part of the world, with the exceptions of the polar regions and a few remote islands. In the Green River Formation of Wyoming — a remarkable fossil deposit from the early Eocene — scientists have uncovered more than 30 bat fossils in the last 60 years, but until the new skeletons were found, they were all thought to represent the same two species. Most of the bats coming from the Green River Formation were identified as representing a single species, Icaronycteris index.

Researchers at the American Museum of Natural History and Naturalis Biodiversity Center in the Netherlands published their study of the new bat fossils in the journal PLOS ONE. Scientists describe the complete bat skeletons as a new species based on a comparison with the oldest bat skeletons previously discovered.

Evolutionary biologist at Naturalis Biodiversity Center Tim Rietbergen was quoted in PLOS ONE saying, “Paleontologists have collected so many bats that have been identified as Icaronycteris index, and we wondered if there were actually multiple species among these specimens. Then we learned about a new skeleton that diverted our attention.”

According to the Smithsonian Magazine story, Rietbergen discovered the new bat skeleton when it was advertised online by a commercial dealer who had collected it in 2017. When he saw the photo of the fossil, he immediately recognized it as a new species due to its smaller and stouter bones.

Rietbergen contacted the American Museum of Natural History and encouraged them to purchase the fossil, which they did. He then researched other museums and found another matched one at the Royal Ontario Museum.

Rietbergen theorized that the bats were so well preserved because they either fell into the lake and sank to the bottom or had been washed from shore into the lake and preserved in the calm waters at the bottom. He said “… every bat fossil is a fossil that formed against the odds of preservation.”

Early bat evolution remains relatively poorly understood. Complete bat fossils are rare and the majority of bat-based records consist of isolated teeth. More complete remains of bats occur at several Eocene sites in North America, Europe and Africa.

Paleontologists compared the newly found fossils with other known species and have concluded that they belonged to a group of extinct bats known as Icaronycteris.

The new fossils have claws on the wing’s first and second digits and have relatively short forearms and broad wings that differentiate them from other Eocene bats, so researchers have named them a new species.

This previously unknown species of bats have been named Icaronycteris gunnelli after the late Duke University paleontologist Gregg Gunnell who was a major contributor to the study of bat evolution.

“About the third specimen,” Rietbergen said in an email to the Gazette last week, “this new specimen was presented to us … [after] we had already submitted our manuscript. Based on the pictures and some measurements, it seemed to be a third specimen of this new species that was described in our paper, but the ID still needs to be confirmed by one of us. With these bats, the differences are not very clear, so we need to look at the dentition of this specimen before I want to say I am sure about the ID. But for now, it looks more like I. gunnelli...”