In spite of their small and delicate skeletons, bats have a long fossil record, something that was not obvious to biologists even 50 years ago. By the Eocene, there were species in
at least 10 families of bats, four now extinct. Eocene bats are known from the United States, Germany, and Australia, as well as Pakistan.
It is assumed that bats evolved from nocturnal, arboreal, insectivorous animals that lived in forests. The combination of their small size, delicate skeletons, and the forest conditions make the ancestors of bats unlikely candidates for fos-silization. There are no fossils of animals that are part bat, part something else, but it is speculated that a shrew-like animal would be a good candidate as a remote ancestor of bats.
The fact that bats appear fully formed and diverse in the Eocene means that it is not known when they first appeared. The very first bats could have shared the skies with the last of the pterosaurs, overlapping in time with the last dinosaurs. Furthermore, although the fossil record suggests a high level of variety, there are relatively few fossil bats and, for most living families, there is no fossil record.
Living species of bats are classified in two suborders: the Megachiroptera (flying foxes and their relatives, family Pteropodidae), which are the fruit- and flower-visiting bats of the Old World tropics, and the Microchiroptera, which include all of the other bats. The two groups are easy to distinguish. The Pteropodidae have dog-like faces, simple ears, and most have claws on their second fingers. Microchiroptera
(18 families) lack claws on their second fingers, do not look like dogs, and their ears (and related structures) are more complex. The teeth of megachiropterans tend to be more specialized than those of microchiropterans (excepting vampire bats). Microchiropteran specializations for flight, particularly in the shoulder girdles, are more complex than those of megachiropterans. While the flying foxes and their relatives use flight to get from one place to another, many microchi-ropteran species feed on the wing and require higher levels of agility and maneuverability.
From the late nineteenth century, some biologists have questioned the closeness of the relationship between Megachiroptera and Microchiroptera. Two theories appeared. The monophyletic theory holds that the two suborders of Chiroptera are more closely related to one another than either is to any other group of mammals. The diphyletic theory proposes that the Megachiroptera are more closely related to some other group of mammals than they are to Microchiroptera. The neural details of how their eyes connect to their brains indicated that while megachiropterans were like primates, the microchiropterans were like all other mammals. Additional evidence about morphology and genetics has been presented. By the year 2000, the monophyletic theory was more accepted than the diphyletic one. New information could reopen and extend the debate.
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