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Icaronycteris index
Fossil Bat
Green River Formation
Kemmerer, Wyoming

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 known definitive bat. Four exceptionally preserved specimens, among the best preserved bat fossils, are known from the Green River Formation of North America. The best known species is Icronyceris. index.

Icaronycteris 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. Note the small flower just to the right of the tail in the lower right corner.