First bats (Onychonycteris)

In 2008 Nancy Simmons of the American Museum of Natural History and colleagues described Onychonycteris finneyi, a bat from the Green River Formation in Wyoming, dated to about 52.5 million years ago. It was a small, long-limbed bat that could fly well, but its skeleton differs from later bats in telling ways. It kept claws on all five fingers of the hand, had longer hind legs and shorter forelimbs, and lacked the enlarged inner ear structure used for echolocation.¹³

Bats appeared in the early Eocene, and by about 52 million years ago they already had fully developed wings, so flight evolved earlier still. Their ancestors were probably small insect-eating placental mammals that glided or leapt among trees, then flapped. Flight was an open niche: pterosaurs and many early birds had vanished, and flying insects were abundant at night. The claws suggest that Onychonycteris climbed well and perhaps hung differently from living bats.¹²

Its main significance is that flight appears to have come before echolocation, at least in this lineage. Another early bat, Icaronycteris from the same beds, has a more modern ear and was probably already a sonar user. Bats have since become the second largest group of mammals, with more than 1,400 species, and are found on every continent except Antarctica, where they pollinate plants and eat vast numbers of insects.¹²