This 52 million-year-old bat skeleton is the oldest bat species ever discovered

The origin of bats has been shrouded in mystery until now. A new finding revealed a skeleton that can be traced to the oldest bat species.
Rupendra Brahambhatt
Skeleton of Icaronycteris gunnelli.
Skeleton of Icaronycteris gunnelli.

ietbergen et al., 2023, PLOS ONE, CC0

Scientists have confirmed the discovery of a new bat species at the Green River Formation in Wyoming, in a study published Wednesday. The bat species lived about 52 million years ago during the Eocene epoch (55.8 to 33.9 million years ago) and is referred to as Icaronycteris gunnelli.

The remains of I. gunnelli were first discovered in 1994 and a nearly complete single specimen (holotype) was recovered in August 2017. It is the oldest bat fossil ever discovered on Earth.

Tim Rietbergen, the first study author and a paleontologist at Naturalis Biodiversity Center in the Netherlands, told IE, “The ancestor of bats is still unknown. During the Early Eocene, bats seem to have radiated really fast. These early bat fossils are little treasure chests, holding information about the life of these bats 52 million years ago.” 

He further added, “You can learn their story if you look closely at their remains.” 

The origin of bats is unknown

The Green River Formation is an Eocene fossil hotspot. Rietbergen claims that he alone knows of the existence of about 30 specimens. Complete skeletons of previously discovered oldest bats belonging to the genera Icaronycteris and Onychonycteris were also found at the Green River Formation. 

He suggests that during the time when these bats flew around, it was a humid and warm environment, and the whole area was covered by a giant lake, also known as 'Fossil Lake'. The bats (and other fossils) are well preserved here because they ended up in the lake, which is ideal for fossilization. 

The researchers performed a phylogenetic analysis and found out that the newly discovered bat species also belong to the Icaronycteris genus. It is named I. gunnelli after Gregg Gunnell, a legendary paleontologist who spent 40 years of his life studying animal fossils and contributed immensely to increasing human understanding of bat fossils and chiropteran evolution.

The study authors further reveal that in terms of size, I. gunnelli is the smallest bat out of three bat species known from the Green River Formation and weighed between 22.5–28.9 g. Interestingly, it looked almost similar to present-day bats, but had relatively short, broad wings and thus might represent a less agile and more fluttering flight style. 

However, these physical trails made it look very different from other Eocene bats. “This new research is a step forward in understanding what happened in terms of evolution and diversity back in the early days of bats. It supports the idea that bats from this location evolved separately from other Eocene bats around the world,” said Rietbergen. 

Although humans know of more than 1,400 bat species that exist today, the researchers argue that there’s still a lot that we don’t know about the early evolution of bats because of the lack of fossils. Therefore, more early Eocene fossils are required to be found and examined, in order to get a proper view of the bat diversity at that time. 

“Only when the diversity is known, we can start looking for answers to their origin,” Rietbergen told IE

The study is published in the journal PLOS ONE.

Study Abstract:

The Fossil Lake deposits of the Green River Formation of Wyoming, a remarkable early Eocene Lagersta¨tte (51.98 ±0.35 Ma), have produced nearly 30 bat fossils over the last 50 years. However, diversity has thus far been limited to only two bat species. Here, we describe a new species of Icaronycteris based on two articulated skeletons discovered in the American Fossil Quarry northwest of Kemmerer, Wyoming. The relative stratigraphic position of these fossils indicates that they are the oldest bat skeletons recovered to date anywhere in the world. Phylogenetic analysis of Eocene fossil bats and living taxa places the new species within the family Icaronycteridae as sister to Icaronycteris index, and additionally indicates that the two Green River archaic bat families (Icaronycteridae and Onychonycteridae) form a clade distinct from known Old World lineages of archaic bats. Our analyses found no evidence that Icaronycteris? menui (France) nor I. sigei (India) belong to this clade; accordingly, we therefore remove them from Icaronycteridae. Taken in sum, our results indicate that Green River bats represent a separate chiropteran radiation of basal bats, and provide additional support for the hypothesis of a rapid radiation of bats on multiple continents during the early Eocene.

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