Earliest bat skeleton

The earliest bat skeletons are from the newly described microbat (Icaronycteris gunnelli), known from two articulated fossil skeletons unearthed from the Fossil Lake deposits of the Green River Formation in Wyoming, USA, dating back to the Early Eocene epoch approximately 52 million years ago. The species was documented in the journal PLOS One on 12 April 2023.

According to the study: "The relative stratigraphic position of these fossils indicates that they are the oldest bat skeletons recovered to date anywhere in the world".

Prior to this species' discovery, the earliest-known bat skeletons were from two species only marginally younger that were found in this same Green River locality. Icaronycteris index, officially named and described in 1966, is known from four exceptionally well-preserved fossil skeletons. It was approximately 14 cm (5.5 in) long, and sported a wingspan of 37 cm (14.5 in). Also unearthed here, but in 2003 and first described in 2008, was Onychonycteris finneyi. This species is unusual in having claws on all five fingers (bats alive today have only one claw, or occasionally two). Onychonycteris had well-developed wings, so it is now predicted that the first bats will have evolved earlier, perhaps as much as 60 million years ago.

The oldest remains of a bat outright date to c. 55 million years ago in the earliest Eocene era: a single tooth attributed to Archaeonycteris preacursor from the Silveirinha site of Portugal, as reported in the Journal of Vertebrate Paleontology on 12 June 2009.