Oldest bat skeletons ever found described from Wyoming fossils

Scientists have described a new species of bat based on the oldest bat skeletons ever recovered. The study on the extinct bat, which lived in Wyoming about 52 million years ago, supports the idea that bats diversified rapidly on multiple continents during this time. Led by researchers at the American Museum of Natural History and Naturalis Biodiversity Center in the Netherlands, the study is published today in the journal PLOS ONE.

There are more than 1,460 living species of bats found in nearly every part of the world, with the exception 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 over 30 bat fossils in the last 60 years, but until now they were all thought represent the same two species.

"Eocene bats have been known from the Green River Formation since the 1960s. But interestingly, most specimens that have come out of that formation were identified as representing a single species, Icaronycteris index, up until about 20 years ago, when a second bat species belonging to another genus was discovered," said study co-author Nancy Simmons, curator-in-charge of the Museum's Department of Mammalogy, who helped describe that second species in 2008. "I always suspected that there must be even more species there."

In recent years, scientists from the Naturalis Biodiversity Center started looking closely at Icaronycteris index by collecting measurements and other data from museum specimens.

"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," said Tim Rietbergen, an evolutionary biologist at Naturalis. "Then we learned about a new skeleton that diverted our attention."

The exceptionally well-preserved skeleton was collected by a private collector in 2017 and purchased by the Museum. When researchers compared the fossil to Rietbergen's expansive dataset, it clearly stood out as a new species. A second fossil skeleton discovered in the same quarry in 1994 and in the collections of the Royal Ontario Museum was also identified as this new species. The researchers gave these fossils the species name Icaronycteris gunnelli in honor of Gregg Gunnell, a Duke University paleontologist who died in 2017 and made extensive contributions to the understanding of fossil bats and evolution.

Although there are fossil bat teeth from Asia that are slightly older, the two I. gunnelli fossils represent the oldest bat skeletons ever found.

"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 curator at the Fossil Butte National Monument, in Wyoming. "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."

While the I. gunnelli skeletons are the oldest bat fossils from this site, they are not the most primitive, supporting the idea that Green River bats evolved separately from other Eocene bats around the world.

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DNA reveals the evolutionary history of museum specimens

Museum specimens held in natural history collections around the world represent a wealth of underutilized genetic information due to the poor state of preservation of the DNA, which often makes it difficult to sequence. An international team, led by researchers from the University of Geneva (UNIGE) and the Museum of Natural History of the City of Geneva (MHN), has optimized a method developed for analyzing ancient DNA to identify the relationships between species on a deep evolutionary scale. This work is published in the journal Genome Biology and Evolution.

By combining and comparing the sequences of a large number of genes or complete genomes, it is possible to establish the links between related species and to trace the main steps in the evolution of organisms from a common ancestor. These phylogenomic studies are based on the amplification and sequencing of DNA fragments, followed by bioinformatics analyses to compare the sequences. They therefore typically require carefully sampled DNA in good preservation condition.

Deciphering degraded DNA

For this reason, most of the specimens preserved in natural history museums have not yet revealed all their secrets, since in most cases the DNA is often highly degraded and difficult to sequence. An international team led by Emmanuel Toussaint, researcher at the MHN, and Nadir Alvarez, researcher at the Department of Genetics and Evolution of the Faculty of Science of the UNIGE and chief curator at the MHN, has perfected a method already used for well-preserved samples in order to apply it to DNA that is highly fragmented due to partial degradation. The HyRAD-X technique consists of fishing out pieces of the genome to be analyzed with DNA probes from closely related species, and then sequencing them to detect the differences between the genomes. However, these DNA probes are only effective hooks for closely related genomes and this technique had so far only allowed to follow the evolution of a single species over time.

In this work, the scientists used HyRAD-X RNA probes instead of DNA probes to find fragments of interest in the genome. RNAs, copies of DNA molecules in charge of transferring the information encoded by the genome, have a very strong affinity for DNA and RNA-DNA pairings occur more easily than DNA-DNA pairings. RNA probes are therefore more efficient hooks, especially when the genomes to be analyzed demonstrate large divergence levels. "Thanks to this new method, we were able to trace the evolutionary history, not within a single species over a million years, but within several species and over tens of millions of years!" explains Emmanuel Toussaint, first author of the study.

The genealogy of the carabid beetle better known

The researchers were notably interested in the specimens of an emblematic carabid from the island of Saint Helena in the middle of the Atlantic Ocean, collected in the 1960s and preserved at the MHN in Geneva. The analysis of the DNA of these beetles revealed that this species, now extinct and until now classified in the genus Aplothorax, actually belongs to the genus Calosoma. It also allowed to locate its biogeographic origin probably in Africa and to generate the chronology of the evolution of the subfamily Carabinae whose origin goes back to the Lower Cretaceous. "Our study opens many perspectives to establish the evolutionary history of millions of specimens in museum collections around the world," concludes Nadir Alvarez.

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