Icaronycteris Temporal range: Early Eocene PreꞒ Ꞓ O S D C P T J K Pg N
Fossil of Icaronycteris index holotype at the American Museum of Natural History
Holotype specimen of I. gunnelli
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Chiroptera
Family:	†Icaronycteridae
Genus:	†Icaronycteris Jepsen 1966
Species
†Icaronycteris gunnelli Rietbergen et al. 2023 †Icaronycteris index Jepsen 1966 †Icaronycteris menui? Russell, Louis & Savage 1973 †Icaronycteris sigei? Smith et al. 2007

Icaronycteris is an extinct genus of microchiropteran (echolocating) bat that lived in the early Eocene, approximately 52.2 million years ago, making it the earliest bat genus known from complete skeletons, and the earliest known bat from North America.^[1][2] Multiple exceptionally preserved specimens, among the best preserved bat fossils, are known from the Green River Formation of North America.^[1][2] The best known species is I. index.^[3] Fragmentary material from France has also been tentatively placed within Icaronycteris as the second species I. menui.^[4] I. sigei is based on well-preserved fragments of dentaries and lower teeth found in Western India.^[5] In 2023, the species I. gunnelli also from the Green River Formation was distinguished from I. index, and I. menui and I. sigei were proposed to be removed from the genus due to them not being closely related.^[2]

Description

Icaronycteris^[6] measured about 14 centimetres (5.5 in) long and had a wingspan of 37 centimetres (15 in). It closely resembled modern bats, but had some primitive traits. The tail was much longer and not connected to the hind legs with a skin membrane, the first wing finger bore a claw and the body was more flexible. Similarly, it had a full set of relatively unspecialised teeth, similar to those of a modern shrew. Its anatomy suggests that, like modern bats, Icaronycteris slept while hanging upside down, holding onto a tree branch or stone ridge with its hind legs.^[7]

Phylogeny

According to Simmons & Geisler 1998,^[8] Icaronycteris is the first genus, followed by Archaeonycteris, Hassianycetris, and Palaeochiropteryx, in a series leading to extant microchiropteran bats.^[9]

Rietbergen et al. 2023 found Onychonycteris to be sister to the North American species of Icaronycteris.

^[10]