Onychonycteris finneyi (Simmons, Seymour, Habersetze and Gunnell 2008) Eocene (~52mya), ~27 cm in length, is the most primitive known bat. It retained unguals (claws) on all five digits, a primitive trait not shared with other bats. Derived from a sister to Protictis, Onychonycteris phylogenetically preceded Icaronycteris, Myotis and Pteropus.
Distinct from Protictis, the skull of Onychonycteris closely resembed those of Old World fruit bats, like Pteropus and likewise did not have the enlarged cochlea characteristic of microbats, which echolocate. The rostrum was reduced and lower. Upper incisor 3 was absent. The upper premolars were sharper and closer together. Both upper and lower molars 1 were smaller, molars 2 and 3 were larger, creating three similar-sized molars that had longer cusps. The lower canine was smaller. Premolars 2-4 were closer together. The dentary was shallower and the coronoid process was smaller. The angular process was raised.
Distinct from Chriacus, the cervicals were shorter. There were fewer and longer caudals.
The scapula was larger and longer. The humerus was longer and longer than the femur. The radius was elongated and much longer than then tibia. The ulna was reduced and fused distally to the radius. Except for the thumb, the hand was much larger and the elements were more gracile. As in Chriacus, metatacarpals III and IV were subequal and the longest with metatarsal V barely shorter.
The femur and tibia/fibula were more gracile. The foot was smaller. The metatarsals were relatively shorter and subequal. Digits I and II were longer. The unguals were larger. A new bone, the calcar, developed from the calcaneum (heel bone). It formed the rear margin of the uropatagium or tail membrane.