There's a simpler explanation for the rareness of bat fossils: "Bat fossils are extremely rare. Ancient bats, like their modern counterparts, were small and fr

There's a simpler explanation for the rareness of bat fossils: "Bat fossils are extremely rare. Ancient bats, like their modern counterparts, were small and fragile, and they tended to live in tropical habitats, where decay occurs very rapidly. Just about the only way a bat can become fossilized is if it dies in a place where it is swiftly covered with sediment that protects it from scavengers and microorganisms alike." However a transitional bat fossil was discovered around 2000, so probably after your school book was written: "Onychonycteris, however, has a combination of archaic and modern traits that make it exactly the sort of transitional creature evolutionary biologists have longed for. "We chose the genus name Onychonycteris (“clawed bat”) because the fossil displays claws on all five fingers, just as its terrestrial predecessors did. The presence of these claws is not the only feature of Onychonycteris that recalls nonflying mammals. Most bats have very long forearms and tiny hind limbs. Onychonycteris, however, has proportionately shorter forearms and proportionately longer hind limbs than those of other bats. Compared with other mammals, the limb proportions of Onychonycteris are intermediate between those of all previously known bats (including Icaronycteris) and those of arboreal mammals that rely heavily on their forelimbs for locomotion, such as sloths and gibbons. These animals hang from branches much of the time as they climb around in the trees. Perhaps bats evolved from arboreal ancestors that used a similar form of locomotion. "Despite these primitive limb features, other aspects of the anatomy of Onychonycteris indicate that it was capable of powered flight. Its long fingers would have supported wing membranes, and robust clavicles (collarbones) would have helped anchor the forelimbs to the body. Meanwhile a wide rib cage and a keeled sternum would have supported large flight muscles, and a faceted scapula would have secured other specialized, flight-related muscles." https://www.scientificamerican.com/article/uncovering-bat-evolutionary-origins/ https://www.scientificamerican.com/article/uncovering-bat-ev...