Bats: Flapping flight
The first fossil evidence of fully developed bats were dated at fifty million years ago (Icaronycteris), so the evolution of flight started early on in the radiation of the placentalmammals. Bats are the only mammals that have mastered true, flapping flight. The bat's flying membrane stretches not just from the wrist, like the colugo, but along the extended second finger. The other two fingers form struts extending back to the trailing edge. Only the thumb remains free and small. This retains its nail and the bat uses it in its toilet and to help it clamber about its roost. A keel has developed on its chest bone which serves as an attachment for the muscles which flap the wings.