Icaronycteris index (Jepsen and McPhee 1966) Eocene (~52mya), ~14 cm long, is the 3rd most primitive known bat. Half as long and derived from a sister to Onychonycteris, Icaronycteris phylogenetically preceded Myotis. Bats arise from a Jurassic sister to Microcebus.

Distinct from Onychonycteris, the skull of Icaronycteris had an enlarged bulla surrounding the inner ear bones, a strong indicator that this bat could echolocate. The cranium was larger and taller posteriorly. The orbit was smaller and lower without as distinct postorbital process. A distinct chin was present. The anterior mandible was raised, raising the incisors. The teeth were smaller and more vertically oriented.

The cervicals were more gracile. The dorsal ribs were more robust. Fewer dorsal ribs were present creating more lumbar vertebrae. The sacrals were fused.

The anterior scapula was embayed. The clavicles were more gracile. All the elements of the forelimb were more gracile. The hands were larger and the fingers were smaller. Icaronycteris had vestigial unguals (claws) on digits III-V, retaining full size claws on I and II.

The hind limb was shorter and more gracile. The toes were slightly longer. The calcar was absent.