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Description
Some 57 million years ago, in the late Paleocene, somewhere around what's today's Europe, the first bat emerges. This is a type of flying mammal, and the common ancestor of all living species of bats. It coexists with more primitive stem-bats in this early Cenozoic forest, including a more primitive flying form, and an even more basal gliding form of the Apo-Chiroptera clade. These would be the only times, in history, where mammals managed to become true powered fliers.

Among the three groups of known flying vertebrates, bats probably have among the most mysterious of origins. While avialans at least have several transitional basal forms leading to the known flying forms, and pterosaurs at least have well agreed flightless relatives in the fossil record, the evolutionary history of flight in the bat lineage is largely shrouded in assumptions, rather than tangible fossil evidence. Bats are the only living members of the mammalian order Chiroptera, and is the second most speciose order of mammals today (with rodents taking the prize home). Before molecular and genetic analysis were used to infer phylogenetic relationships, bats were believed to be somewhat related to the euarchontans, which include treeshrews, primates and the gliding colugos. It would make logical sense to assume that bats would be nested well within a group of tropical arboreal placental mammals. However modern genetic evidence overwhelmingly confirms that bats are actually part of the Scrotifera clade, a group that also includes ungulates (hoofed mammals) and the ferans (carnivorans and pangolins). Which of those is exactly the bats closest living relatives is unclear, though the most parsimonious trees tend to place bats as equally related to both ungulates and ferans. This detail matters little, because either way, we get a poor understanding on what was the appearance of the bat ancestors. Bats would have split off from the feruungulates (ungulates + ferans) in the Late Cretaceous, some 79 million years ago, or so. And we only start identifying fossils within the bat lineage in the late Paleocene to early Eocene. So we clearly have a large gap of evolutionary history.

The most basal representatives of the Apo-Chiroptera group are the tiny mammals Eosoricodon and "Wyonycteris" microtis, which are only known from fragments of their jaws and teeth. It doesn't tell us anything about their general morphology, other than their teeth were probably pretty well adapted for the consumption of small insect and invertebrate prey (consistent as these taxa were once believed to be related to the eulipotyphlans, which includes modern shrews, hedgehogs and moles). Soon in the early Eocene we start to see well preserved full bodies of stem-bats but these are already fully capable of flight. One of the most basal ones is Onychonycteris. This species of stem-bat had claws on all its five wing fingers, and its wings were overall shorter and more round, with longer back limbs, and its flight style likely alternated between fluttering and gliding during aerial locomotion, implying this was just coming out from a gliding ancestor. However, by the early Eocene we also see full fledged bat-like apo-chiropterans, if not true bats all over (including as far back as Australia, implying they achieved global distribution incredibly early). All of this points out that the evolution of flight occured very shortly after the K-Pg extinction event that ended the age of the dinosaurs, 66 million years ago, with the eccological vacuum and perhaps a boom in insect diversity, triggered the evolution of a unique lineage of arboreal mammals, first evolving large webbed hands for gliding between branches, and then transitioning to well developed flight, with the first true bats eventually evolving by the end of the Paleocene, merely 9-10 million years after the end Cretaceous extinction event.

Here I reconstruct three different stages of apo-chiropteran evolution, coexisting at one same time, in Europe. It should be noted, however, that the first flying apo-chiropterans may not have originated in Europe, necessarily, with some evidence pointing to an asian origin, though regardless, it took apo-chiropterans little time to expand across the northern hemisphere, with plenty of fossils being found in the early Eocene of Europe, Asia and North America, with their arrival to the southern continents also occurying briefly after the start of the Eocene. On the bottom left we see a very basal apo-chiropteran inspired by Eosoricodon. This is a completely speculative reconstruction, with a longer snout similar to that of a typical basal shrew-like mammal, and large webbed hands used for gliding. This primitive form of stem-bat would have used gliding as a means to move from tree to tree in the super-vegetated environments of the warm Paleocene epoch, expanding its range of motion and helping also evade arboreal predators. Its primary diet would likely be insects with the occasional fruit. Although we have plenty of gliding mammals today and in the fossil record, it seems that, more often than not, gliding never specializes into powered flight. Gliding often stems from a need to cover large distances in the air without wasting much energy, while flight is very energy-expensive. The transitional forms towards powered flight are, however, very rare in the evolution of vertebrates, and have to come with a good incentive. In the heavily productive forests of the Paleocene, however, the ancestors of bats would find an incentive to keep themselves airborne to expand even further in the environments they lived in. On the top left, we see a reddish colored primitive flying form, based after the Eocene Onychonycteris. This would have claws present in all its five fingered wings, and would have alternated between fluttering wing claps and gliding while moving in the air. This would have given these mammals a wider range of expansion, permitting them to explore new lifestyles. Eventually, however, by this time the first bat would have evolved. This is the last common ancestor of all living bat species. This reconstruction is loosely based on the recently discovered Eocene stem-bat Vielasia, already very close to the crown group of bats. What we can gather from Vielasia is that the first bats would have already been competent fliers capable of producing echolocation, and only two claws are left in its wings. Already in the late Paleocene, the product of this novel mammalian achievement is a functional type of flying mammal that is capable of successfully hunting flying insects and orienting itself in the dark corners of the forests. By this time, bats were not the only flying vertebrates evolving, as birds were also going through explosive radiation events after their narrow survival in the K-Pg extinction event, and by this time, predatory aerial birds were a thing that could pose a threat to bats, and even insectivorous birds could pose as competition. Because of that, many bats, over the course of their history, remained nocturnal to evade competition with mostly diurnal birds. Of course, over millions of years, both birds and bats became extraordinarily successful, and even some bats eventually lost their abbility to echolocate in favour of pursuing diurnal lifestyles, while some birds specialized to become nocturnal hunters of bats, like owls (which, quite curiously, probably already evolved by this point in time, perhaps to appropriate themselves with this new type of aerial prey).

Description
Some 57 million years ago, in the late Paleocene, somewhere around what's today's Europe, the first bat emerges. This is a type of flying mammal, and the common ancestor of all living species of bats. It coexists with more primitive stem-bats in this early Cenozoic forest, including a more primitive flying form, and an even more basal gliding form of the Apo-Chiroptera clade. These would be the only times, in history, where mammals managed to become true powered fliers.

Among the three groups of known flying vertebrates, bats probably have among the most mysterious of origins. While avialans at least have several transitional basal forms leading to the known flying forms, and pterosaurs at least have well agreed flightless relatives in the fossil record, the evolutionary history of flight in the bat lineage is largely shrouded in assumptions, rather than tangible fossil evidence. Bats are the only living members of the mammalian order Chiroptera, and is the second most speciose order of mammals today (with rodents taking the prize home). Before molecular and genetic analysis were used to infer phylogenetic relationships, bats were believed to be somewhat related to the euarchontans, which include treeshrews, primates and the gliding colugos. It would make logical sense to assume that bats would be nested well within a group of tropical arboreal placental mammals. However modern genetic evidence overwhelmingly confirms that bats are actually part of the Scrotifera clade, a group that also includes ungulates (hoofed mammals) and the ferans (carnivorans and pangolins). Which of those is exactly the bats closest living relatives is unclear, though the most parsimonious trees tend to place bats as equally related to both ungulates and ferans. This detail matters little, because either way, we get a poor understanding on what was the appearance of the bat ancestors. Bats would have split off from the feruungulates (ungulates + ferans) in the Late Cretaceous, some 79 million years ago, or so. And we only start identifying fossils within the bat lineage in the late Paleocene to early Eocene. So we clearly have a large gap of evolutionary history.

The most basal representatives of the Apo-Chiroptera group are the tiny mammals Eosoricodon and "Wyonycteris" microtis, which are only known from fragments of their jaws and teeth. It doesn't tell us anything about their general morphology, other than their teeth were probably pretty well adapted for the consumption of small insect and invertebrate prey (consistent as these taxa were once believed to be related to the eulipotyphlans, which includes modern shrews, hedgehogs and moles). Soon in the early Eocene we start to see well preserved full bodies of stem-bats but these are already fully capable of flight. One of the most basal ones is Onychonycteris. This species of stem-bat had claws on all its five wing fingers, and its wings were overall shorter and more round, with longer back limbs, and its flight style likely alternated between fluttering and gliding during aerial locomotion, implying this was just coming out from a gliding ancestor. However, by the early Eocene we also see full fledged bat-like apo-chiropterans, if not true bats all over (including as far back as Australia, implying they achieved global distribution incredibly early). All of this points out that the evolution of flight occured very shortly after the K-Pg extinction event that ended the age of the dinosaurs, 66 million years ago, with the eccological vacuum and perhaps a boom in insect diversity, triggered the evolution of a unique lineage of arboreal mammals, first evolving large webbed hands for gliding between branches, and then transitioning to well developed flight, with the first true bats eventually evolving by the end of the Paleocene, merely 9-10 million years after the end Cretaceous extinction event.

Here I reconstruct three different stages of apo-chiropteran evolution, coexisting at one same time, in Europe. It should be noted, however, that the first flying apo-chiropterans may not have originated in Europe, necessarily, with some evidence pointing to an asian origin, though regardless, it took apo-chiropterans little time to expand across the northern hemisphere, with plenty of fossils being found in the early Eocene of Europe, Asia and North America, with their arrival to the southern continents also occurying briefly after the start of the Eocene. On the bottom left we see a very basal apo-chiropteran inspired by Eosoricodon. This is a completely speculative reconstruction, with a longer snout similar to that of a typical basal shrew-like mammal, and large webbed hands used for gliding. This primitive form of stem-bat would have used gliding as a means to move from tree to tree in the super-vegetated environments of the warm Paleocene epoch, expanding its range of motion and helping also evade arboreal predators. Its primary diet would likely be insects with the occasional fruit. Although we have plenty of gliding mammals today and in the fossil record, it seems that, more often than not, gliding never specializes into powered flight. Gliding often stems from a need to cover large distances in the air without wasting much energy, while flight is very energy-expensive. The transitional forms towards powered flight are, however, very rare in the evolution of vertebrates, and have to come with a good incentive. In the heavily productive forests of the Paleocene, however, the ancestors of bats would find an incentive to keep themselves airborne to expand even further in the environments they lived in. On the top left, we see a reddish colored primitive flying form, based after the Eocene Onychonycteris. This would have claws present in all its five fingered wings, and would have alternated between fluttering wing claps and gliding while moving in the air. This would have given these mammals a wider range of expansion, permitting them to explore new lifestyles. Eventually, however, by this time the first bat would have evolved. This is the last common ancestor of all living bat species. This reconstruction is loosely based on the recently discovered Eocene stem-bat Vielasia, already very close to the crown group of bats. What we can gather from Vielasia is that the first bats would have already been competent fliers capable of producing echolocation, and only two claws are left in its wings. Already in the late Paleocene, the product of this novel mammalian achievement is a functional type of flying mammal that is capable of successfully hunting flying insects and orienting itself in the dark corners of the forests. By this time, bats were not the only flying vertebrates evolving, as birds were also going through explosive radiation events after their narrow survival in the K-Pg extinction event, and by this time, predatory aerial birds were a thing that could pose a threat to bats, and even insectivorous birds could pose as competition. Because of that, many bats, over the course of their history, remained nocturnal to evade competition with mostly diurnal birds. Of course, over millions of years, both birds and bats became extraordinarily successful, and even some bats eventually lost their abbility to echolocate in favour of pursuing diurnal lifestyles, while some birds specialized to become nocturnal hunters of bats, like owls (which, quite curiously, probably already evolved by this point in time, perhaps to appropriate themselves with this new type of aerial prey).