When Bats Took Flight

A bat flitted in the sky over what is now Wyoming about 52 million years ago. But it wasn't like the

bats

you and I know. It was small, like most modern

bats

, but it had claws on the tips of the five fingers that supported each wing. And its wings were a little shorter, while its hind legs were a little longer. With those claws and long hind limbs, it climbed better than most modern bats. And its teeth suggest it ate insects, but it probably didn't use echolocation to find and catch them. This small flying mammal was Onychonycteris and was definitely a bat: the most primitive bat for which we have good fossil evidence, and also one of the oldest.

And in the mammalian family tree, it was on the branch closest to all other bats, living and extinct. But

when

you trace that branch... well, it's a mystery. Bats appear virtually in the fossil record as recognizable flying bats. And they appear on all continents except Antarctica at approximately the same time, in the early Eocene. However, the oldest fossils consist only of teeth and limb bones, which doesn't give us many clues about what the ancestors of today's bats were like. So where do bats come from? And which of the many strange characteristics that bats have appeared first?

Did these mammals learn to fly first? Or echolocate first? Or both? And how do they fit into the mammalian family tree? We used to think that, based on their skeletal structure, bats were closely related to primates: like... us! Which is a little strange, if you think about it. But once we were able to study its genetic history, its DNA revealed something much stranger. Instead of being closely related to the mammals they resemble, bats turn out to be much more closely related to the ones they... don't have. The fossil record of bats is great and terrible, which is why we've waited so long to make this episode.

It's fantastic because several of the earliest fossil bats we have are exquisitely preserved. They are basically complete, because they were buried in an ancient lake deposit. The best bat fossils come from this type of lagerstatten, or exceptionally preserved fossil sites. On the other hand, the fossil record of bats is also terrible, because it's that or nothing. Most bats are small and have thin, brittle bones. This keeps them light, which makes it easier to fly, but also means they don't store well. But we are lucky to have some very old bats. Flying at the same time as Onychonycteris, and probably sharing the skies with it, was the Icaronycteris index.

This species also comes from Wyoming and long held the title of oldest known definitive bat, having been first described in 1966. Like Onychonycteris, published in 2008, Icaronycteris dates back to about 52.5 million years ago. . And since these two were already distinct from each other, we know that bats must have evolved at some point before this. Both bats were clearly capable of powered

flight

and also closely resembled most modern insect-eating bats. But modern bats only have claws on one or two of their fingers, and Onychonycteris had claws on all five fingers, while Icaronycteris had a well-developed claw on its second finger and bony tips on three others.

And this ancient bat was probably capable of echolocation, because it had specialized features in one of the inner ear bones, called the hammer, and at the base of its skull that are related to echolocation in living bats. Furthermore, the first bats were not limited to North America. There are at least four more spectacularly preserved taxa of bats from a little later, about 47 million years ago, from the Messel pit in Germany, a site we've talked about before. These are also complete or almost complete skeletons, and in some of them you can even see the outlines of their soft tissues!

And, like Icaronycteris, they have all the characteristics of modern bats, and several of them also appear to have been able to echolocate. But all of these fossil bats already mastered powered

flight

: so how did their ancestors manage to fly through the air? The answer to that question is linked to the evolution of another of the defining characteristics of most bats: echolocation. And among experts, there are three competing hypotheses for how bats evolved: either echolocation came first, or flight came first, or the two evolved together. The three hypotheses are based on the same basic assumptions based on the most common traits in current bats.

These are that the ancestor of bats was arboreal or lived in trees; It was insectivorous or ate insects; and it was probably nocturnal. In the echolocation-first hypothesis, the bat ancestor would have already had ultrasonic capabilities to begin with. This seems to be a possibility, because there are modern insectivores, such as some species of shrews, that use ultrasound to communicate or navigate. So the idea here is that this ancient bat might have reached out from tree branches to catch passing insects. Over time, the super-high-pitched calls would have evolved into sonar that it could use to track its prey.

And, being arboreal, its toes would have been selected to be longer, with a stretched membrane between them, to capture food more effectively. Those long, webbed hands would have been used to glide,

when

the animal began to jump to reach the insects that were flying further and further from its position. And finally, it acquired adaptations for powered flight. But the main problem with this hypothesis is that this type of hunting behavior (catching insects that simply fly as far as you can reach) has not been observed in nature. Also, oddly enough, it turns out that it takes a lot of energy to echolocate, especially when you're stationary, so this whole foraging strategy seems pretty inefficient.

What if, instead, they flew first? In this model, powered flight evolved from an ancestor of gliding, which had originally begun by jumping between trees or branches. This arboreal creature would have developed longer fingers and a membrane between them as part of its gliding phase, and would have eventually transitioned to powered flight. Once this protobat was flying, it probably encountered insects, possibly picking them up with its wings or catching them in the air. And from there, an energy-efficient form of echolocation was developed, in which bats exhaled (and chirped) in time with their wing beats. Some researchers have argued against this idea, saying that a nocturnal animal that jumps or glides without specialized senses (either vision or echolocation) would not be able to see where it intends to land.

But at least here the fossil record can tell us something: the skeleton of Onychonycteris shows that it was definitely capable of powered flight, but it did not have the cranial features related to echolocation. Because the skull was partially crushed, we can't tell if it had large eyes like many gliding and jumping nocturnal creatures like flying squirrels have, but it's still good evidence that flight probably came first. But there is still the third option to consider: that perhaps echolocation and flight evolved together. In this hypothesis, the bat ancestor originally used ultrasound to communicate and was able to begin using it as basic sonar to help it plot its nocturnal jumps between branches.

As their echolocation ability evolved in power, so did their ability to make longer jumps, which eventually developed into powered gliding and flight. And those two adaptations—stronger echolocation and powered flight—turned bats into the stealthy aerial predators of insects that many still are today. The problem with this model is that Onychonycteris did not echolocate, but it did fly. Okay, so it looks like the fossil evidence we have is in favor of the flight-first hypothesis. So that's one piece of the bat origins puzzle that we can fit into place. But it still doesn't tell us where the bats came from!

To find out where bats really fit on the mammalian family tree, paleontologists have teamed up with geneticists to study the DNA of living bats. For a long time, bats were thought to be part of the superorder Archonta, the group that includes shrews, colugos and primates, because those are the mammals they most resemble. Now, superorders are, by their very nature, incredibly diverse. But members of Archonta share many of the same skeletal characteristics, from the presence of a small bone in the inner ear to the particular way the ankle bones fit together. And some studies even suggested that bats and colugos were more closely related to each other than to the rest of the group, based on some characteristics of their hands, elbows and feet.

Colugos are nocturnal, arboreal, and glide using a membrane of stretched skin between their limbs, as the transitional pre-bat is supposed to have done. So it's easy to see why scientists thought they were closely related. And in the 1980s and 1990s, an Australian neuroscientist even suggested that fruit bats evolved from primates, based on similarities in the pattern of connections between the retina and the brain. But in more than two dozen molecular studies conducted since the early 1990s, bats have never been grouped with Archonta. Instead, all of these studies placed bats in an entirely different superorder, one known as Laurasiatheria.

This includes several placental mammals thought to have originated on the supercontinent Laurasia during the Late Cretaceous period. And this group is also very diverse, including orders containing moles, camels, horses, whales, pangolins and bears, most of which look nothing like bats. So that's right. It turns out that bats are more closely related to whales than to us. Within this group, analyzes typically place bats with a clade containing pangolins, carnivores, and ungulates, or as the sister group to shrews, moles, and perhaps hedgehogs. But it is still unclear to whom Laurasiatheria bats are most closely related and how. They appear to have come from some very primitive mammal close to the base of Laurasiatheria that also gave rise to one or more of the other groups of the superorder.

Another benefit of all this genetic data is that it can give us an idea of ​​when bats emerged. According to studies based on that model known as the molecular clock, bats appear to have originated about 65 million years ago, just after the extinction of the non-avian dinosaurs. So where does that leave us in understanding the origins of bats? Well, it looks like we're getting closer to discovering the order in which bats developed their most distinctive traits. It seems that flight came first, closely followed by echolocation. And we're still unearthing beautifully preserved fossils of early bats. As genomics continues to grow as a field, hopefully we can hone in on exactly which group bats are closest to.

And this could tell us what kind of traits to look for in a bat ancestor. It may end up looking totally different than what we expected. After all, it's happened before. But for now, the lack of sufficient evidence, both in the soil and in their DNA, keeps the true origins of bats in the dark. So what do you think? Did bats evolve flight or echolocation first, or did they evolve together? Let us know in the comments which hypothesis you support and why! Also thanks to this month's eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve.

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