The Rise of the Bats
After the fall of the reptiles, during the death of the dinosaurs around sixty-five million years ago, the rise of the mammals soon began. Eventually, a new mammal even took to the skies. They were a kind of diurnal insectivore that fed on flying bugs like beetles and moths. The archaic extant bats were medium sized compared to modern extant forms that are more or less larger or smaller. On top of that, they had limb proportions unlike any living mammal because they were still a transitional species.
All of this resulted from the fact that the very first bats really only glided between trees as perch hunters. They even used their forelimb claws, rather than their hind limb claws, to hang from. However, as bats have evolved over time their arms have gotten longer and their legs have gotten shorter, to better facilitate flying and roosting, in better ways. They have also been losing their claws overtime, as well. Originally they had five, one on each finger. Now they only have a couple remaining.
Regardless, as bats have specialized more and more in their dietary needs, the shape of their wings has adapted accordingly. This has radically altered the ratio between the hand-wing and the arm-wing sections of their membranous appendages. The higher or faster that bats needed to go, the longer the tip of the wing became through the generational or mutational lengthening of the fingers. In contrast to this, bats that needed greater maneuverability evolved into animals with shorter and broader wings. Naturally, this was all driven by where they lived and what they ate.
Around fifty million years ago, archaic bats still had rather stubby wings. This is because they used a more rudimentary approach to flight, flapping and gliding as they went. Ultimately, this had emerged from a common ancestor which only glided. However, bats quickly diversified so some hunted above the canopy, while others foraged within it. The former relied on speed and the latter agility. Thus, in time there were long narrow wings and short wide ones. Some even began to hunt earlier and later in the day, so needing to get around in the dark better meant they had to develop echolocation. As a result, their cochleas and hyoid bones changed to accommodate a more nocturnal lifestyle.
Then, less than forty million years ago, the megabats went down a path that caused them to lose the ability to hear images. This happened because their ancestors ate insects among fruits and flowers and began to consume more and more of the former and less and less of the latter. In the transition to a richer source of nutrients, they grew larger and they stopped relying on echolocation to hunt. This allowed them to conserve far more energy in the process. So, in finding a new niche to exploit they went back to being diurnal, only now they were frugivores.
So in summary, having learned to fly the archaic bats brought forth progeny both small and large. As a result, the microbats took to the night and then the megabats took to the day. In this way, the former discovered the best way to look for bugs with its ears and the latter discovered how to look for fruit with its eyes. Such is the elegant and majestic story of the seemingly enigmatic rise of the flying mammals.