All things considered, vertebrate flight is the ultimate triumph of animal bioengineering, producing active locomotion on the thinnest of availiable mediums, releasing the individual from the ground and opening it to an entirely new realm to navigate upon. Flight has led to massive adaptative radiations and ecological niches completly impossible for terrestrial or aquatic animals to explore, and it leads to exaptations for other credible feats, like extremely effective lungs and large brains. Flight is freedom, and there are ten thousand reasons as to why we pitiful land primates have envied birds, bats and insects, and most surely pterosaurs if they were still alive.
Yet, when one looks at the groups of flying animals that have existed, one thing becomes apparent: it’s very, very rare for an organism to develop true, powered flight. Birds, bats, insects and pterosaurs are and were incredibly speciose and morphologically diverse, but immense adaptative radiations were all resulted from a mere four successful attempts at being an aeronaut. All the +10,000 living bird species, and their possibly millions of extinct relatives? Descendents from a single dinosaurian aeronaut back in the Jurassic, whose aerial prowess quickly kickstarted an adaptative radiation more or less at the same time pterosaurs were during their golden age. Pterosaurs themselves, from the miniscule anurognathids to the ginormous azhdarchids and ornithocheiroids, all descended from a currently unidentified form of sauropsid that took to the skies in the Triassic, a single species possibly akin to Scleromochlus. Going hundreds of million years further in time, we have the single common ancestor of over 60% of all animal life on the planet, the winged arthropods, and forward in time to the earliest Cenozoic we have a small, shrew like mammal who has made ghastly wings out of it’s hands.
Four species. That’s not even 0.01% of all animal life, no matter how mind crushingly diverse their descendents would be.
The sheer minority of successful aeronauts becomes even more puzzling when one witnesses the incredible diversity of gliding and parachuting animals, both from the past and presence. Among synapsids, extensive gliding has evolved at least 11 times, with the earliest known gliding mammal known from the Jurassic/Cretaceous boundary, the bizarre Volaticotherium, while squamates have produced gliders among all major lineages, to say nothing of the sheer amount of Triassic gliding reptiles. Some frogs developed long toes and webs that seem like smaller mockeries of bat wings, while countless fish species have turned their pectoral fins with airfoils and taken to the air; in fact, one lineage of said fish, Thoracopterigids, are quite possibly the oldest vertebrate group to have taken into the air. To say nothing of all the spiders and other arhtropods that glide and parachute, with webs or sheer chitinous extentions, as well as of the pelagic flying squid.
Yet, once again, all this incredibly diverse menagerie has achieved nothing akin to the powered flight of birds, insects, bats and pterosaurs, with the possible exception of a few fish, which propell themselves into the air with their pectoral fins. Indeed, many of these groups have become extinct, with no known close relatives and certainly no radiation of aerial critters derived from them.
It has been suggested that the presence of aerial vertebrates would have limited the presence of availiable niches for experimental flyers, something that falls extremely falt when A) birds evolved at the peak of pterosaurian diversity and bats when birds were already very well established (and posssibly even further back, when pterosaurs were still around), B) the idea that any of these groups have ever outcompeted or competitively excluded each other is at best severel uncircumstancial, and C) there’s plenty of ecological niches each individual group has occupied that the others haven’t (i.e. birds haven’t ever produced robust, boar-like terrestrial molluscivores like dsungaripterids, bats have never produced freshwater filter feeders, et cetera). Gliding animals also don’t seem to competitively exclude each other, so there’s little reason why animals in between gliding and full volancy wouldn’t have enough ecological space to exist.
An alternative option is what is reflected by an examination of gliding animal groups: gliding simply does not translate into flight, simply never leads to powered volancy, and that the flying vertebrates we know and love, as well as insects, have simply evolved under extremely anomalous sets of behaviour completly unrelated to gliding.
This is what I’m going to explore today.
What Living Gliders Tell Us
Living animals and their extreme speciation compared to their long gone cousins can be exceptionally deceiving when it comes to studying the past, but they nonetheless offer a few clues about how flight may have aroused within the known flying vertebrates. As mentioned previously, there is a conspicuous absence of gliding vertebrates in a state of tentative flight, while the few fish that jump using their pectoral fins are not related to any gliding species. This alone can be considered pretty damning for the hypothesis that flight evolved from gliding, though it’s obviously not enough to establish a certain correlation.
Among gliding vertebrates, we witness a very wide range of adaptations for aerial locomotion, from the simple membranes of Holaspis to the complex airfoils of gliding squirrels. Some of these gliders have gone into extra lengths to acquire aerodynamical prowess, developing elaborate tail rudders (Ptychozoon), unique structural support for their wing membranes (such as the cartilage spurs of several flying squirrels and the ribbed wings of several squamates), proportionally massive wings (colugos, Draco lizards, et cetera), and even airfoil adaptations convergent with the aerodynamic design of flying animals (Exocoetidae). Some of these critters can be considered true aerial animals of their own right, travelling long distances on air and spending a good portion of their time gliding, though of course they are far more limited than true flyers.
Within the massive variety of living gliders, it’s evident that some will never develop powered flight: the rib wings of Draco and similar squamates, for example, cannot develop the necessary musculature and articulation to form anything other than a simple parachuting airfoil, and indeed extinct sauropsids with similar adaptations didn’t seem to have gone anywhere beyond a few “one shots”. Likewise, as suggested by Mark Witton 2013, flight is presumed to be unlikely to develop in ectothermic animals, as a warm blooded metabolism is likely a necessary exaptation in order to provide for the energetic demands of true volancy. This leaves pretty much every single living non-mammalian glider alive today as invalid candidates for powered flight, and therefore ill-suited models for how flight developed in bats, birds and pterosaurs. Thankfully, the menagerie of living gliding mammals is large enough to supply enough samples as to determine whereas flight can develop from gliding or not. And within these, two groups in particular can be held in particular regard.
The first, erroneously named “flying lemurs”, are often described as the most aerially adapted of all gliding mammals and just a step beneath the full volancy of chiropterans, having a well developed protopatagium and uropatagium, very long limbs that produce a massive brachiopatagium, and even a significant cheiropatagium, the fingers being quite long and displaying well developed webbing; in total, a colugo’s patagium is as massive as geometrically possible (Kathy MacKinnon 1984), and all the animal needs to develop functional wings is to elongate the hand more. They are capable of gliding for as much as 70 meters without losing altitude significantly, and their wing membranes indeed often an effective aerodynamic profile, the propatagium angled in the same as as that of birds and bats (and, presumably, as in pterosaur), the fingers serve as bastard wings and the uropatagium as an effective rudder (see image above). Such is their aerial competence that they were for the longest time assumed to be stem-bats, a relictual lineage wrestled out of the skies by more derived chiropterans, but genetic and morphological studies firmly place them as sister-taxa to primates, meaning that they became aeronauts independently (fringe “megabats are primates” pseudo-science nonwithstanding).
Thus, instead of being “living fossils”, colugos represent a lineage of gliding mammals that came very close to volancy… and stopped right there. The fact that the oldest members of this clade may date as far back as the Paleocene, when the first recognisable bats flapped their wings, means that they presumably had enough time to go go an extra mile and become true flyers, though in the interest of fairness we do not know if any extinct dermopterans were capable of gliding and if the aerial adaptations seen in modern forms are extremely recent. Colugos are specialised folivores, so it has been argued that powered flight is simply out of reach for these mammals, as folivory provides relatively little energy input, and gliding is a supposedly cheap way to travel around, while flight is obviously very energy demanding.
However, recent studies showcase that gliding is actually very energy taxing for colugos (Byrnes, G., Libby, T., Lim, N. T.-L. and Spence, A. J. (2011)), wasting as much as 1.5 times more energy by travelling through the air than by clambering amidst the branches. Part of the reason this is so energy demanding is due to the need to climb in order to achieve enough altitude for the next glide, often needing to climb as much as 8 meters for a non-problematic 30-50 meter “flight”, trouble flying animals do not have to deal with. Indeed, based on the studies’ conclusions, flight would have been a relatively cheaper mode of locomotion, and likewise the one flying animal with the closest lifestyle, the hoatzin, manages to be a specialised folivore and waste relatively little energy by engaging in short bursts of flight, less impressive than the colugo’s majestic “soaring” but certainly without the need to climb 8 meters in order to move around.
Thus, flight in colugos would have been relatively advantageous, and yet they have not made any further leaps into volancy throught the Cenozoic.
The other group, the iconic flying squirrels, come right behind colugos in terms of aerial speciation. While not having such extensive patagia, flying squirrels have well defined propatagia, brachiopatagia and uropatagia, and most importantly they are the only gliding mammals that have elongated the forelimb – the first step into the formation of a true wing -, developing unique cartilaginous wrist spurs that support the dystalmost part of the brachiopatagium. True to form, this development makes them even better gliders than colugos, capable of “flights” of 90 meters and more, and are seemingly better at steering in the air (Flaherty, E.A.; M. Ben-David, W.P. Smith (2010)).
No similar studies have been performed on whereas they spend more energy gliding, but if the studies on colugos are of any indication then flying squirrels should logically be also subjected to the same energetic pressures. Being omnivores, they’re also in a better position to develop powered flight, and in fact they presumably would benefit more from flight, as it would reduce energy espenses and aid them massively when foraging. Flying squirrels are thought to be a monophyletic lineage dating to the late Oligocene, which would have granted these rodents enough time to develop powered flight. Yet, as obvious, they remain exclusive, if particularly specialised, gliders. Once again, competition appears to not be an issue: no bats, and indeed no birds or even other sciurids, forage in the same manner as flying squirrels.
A third, ancient lineage of living gliding mammals is represented by the anomalures. Relatively basal rodents, these animals have been around since the Eocene, and not only haven’t become true aeronauts, they are also relatively cumbersome gliders, having a small propatagium and a bizarre brachiopatagium supported by an elbow spur, that helps stabilise the membrane but reduces any significant expansion of it. Flight, in this case, might had been prevented by the “flaw in design” rather than lack of opportunity.
Gliding marsupials are recent arrivals, having evolved around the latest Miocene; with relatively simple wing membranes, gliding is still on it’s infant steps for most of these animals, let alone powered flight. Outside of these lineages, there are a vast variety of mammals that have been reported to parachute, but of these only the sifakas and their possible rudimentary patagia show any possible adaptations for aerial locomotion in living mammals.
With at least two long lived lineages of specialised gliding mammals with no significant ecological competition in the form of bats or birds, and that are subjected to ecological pressures that favour powered flight, as well as almost certainly several long gone and unreccorded gliding mammal clades, it appears that gliding in mammals is not conductive to powered flight. It is possible that the necessary step for true volancy, the wing stroke, simply does not occur to animals “hardwired” to keep the wing stable during gliding, making the very speciation for gliding ironically the very reason why powered aerial locomotion is out of reach.
Of course, to strike out living gliders is not enough to determine whereas gliding is inconsistent with flight. To this end, examining true flyers is of extreme importance.
What Bats Tell Us
The idea that bats are not descendents of gliders goes as far back in time as Glenn L. Jepson’s Bats: Origins and Evolution (1970), but it was more recently explored in detail in Kevin Padian et al. 2011. Here, the observance of fluttering in juvenile bats, as well as the very early evolution of echolocation in these animals, has provoked a hypothesis that, rather than descending from gliders, bats evolved from cave-dwelling mammals that specialised in stalking aerial insects, using echolocation to guise themselves in the darkness and fluttering in order to control their descent. While without doubt a rather fantastical idea, it did correlate to Weatherbee SD et al. 2006, in which the development of the bat patagium has been noted as rather atypical, developing first the specialised cheiropatagium and latter the rest of the patagia, inconsistent with the development in other gliding mammals but certainly on par with fluttering as means to develop the wing.
Later, Padian et al. 2013 dismissed the idea of cave-dwellers, but the emphasis on the hypothesis that early bats fluttered instead of gliding remained. In this scenario, creatures like Onychonycteris see themselves in a more plausible scenario: as arboreal mammals that weren’t that different from modern gliding mammals, but that flapped their arms to control their fall rather than simply letting themselves fall with style. Combined with the impetus to capture aerial prey, the drive to develop large, webbed hands occured, and subsequently it would only be a matter of time until true flight emerged.
This hypothesis, while so far only barely supported, certainly explains why bats achieved what no other mammal managed in a such a short amount of time. If it is a correct accessment, it defenitely parts mammalian flight from mammalian gliding, relying on the presence of arm movement since it’s very earliest stages, something that by itself is very rare among parachuting mammals. This also explains why bats managed to achieve profecient aerial hawking early on when birds and pterosaurs seemingly only developed it much later, having evolved from day on to snare aerial prey.
What Birds Tell Us
How avian flight developed has been one of the most debated topics in paleontology across it’s existence as a discipline. You’ve all heard about gliding, about running and beating the wings like a manic, WAIR, et cetera.
At this moment in time, the debate is still long from settled, but it seems that a few options can be safely struck down as unlikely. The running hypothesis, for example, has never witnessed much support, and nowadays it is rarely defended outside of conjunction with WAIR; the fact that most known early paravians had hindwings also renders this possibility unlikely.
With Dyke, G., de Kat, R., Palmer, C., van der Kindere, J., Naish, D. & Ganapathisubramani, B. 2013, gliding should also be out of the picture: the most famous supposed glider, Microraptor, was appearently a really lousy aeronaut, ill suited for long distance gliding. Therefore, as with bats, avian flight was not derived from gliding, and the theropods that rose into the skies did so in methods equally as active as their chiropteran successors, by flapping their wings.
Thankfully, there is a menagerie of ideas about how birds flapped their way into true volancy:
– Wing Assisted Incline Running (W.A.I.R.), in which dinosaurs runned up trees by flapping their arms, like modern terrestrial birds;
– The Leaping Grapple, in which dinosaurs leaped at their prey, using the forelimbs to generate power to elevate themselves.
– RPR, or “ripper”, in which dinosaurs flapped their forelimbs to retain balance when eating their prey, much like modern raptors.
Of these, WAIR has gone under the most scrutiny due to the assumption that non-ornithothorace paravians could not provide enough of a vertical stroke in order to produce WAIR. Ignoring several examples in which this it has been argued that early paravians could provide vertical strokes, Holtz argues that a vertical stroke isn’t necessary to generate WAIR (https://www.facebook.com/thomas.holtz/posts/173481572740497), so WAIR is within the realm of possibilities. However, the fact that WAIR is only observed in Neognathae, with no evidence among living palaeognaths, may cast suspicion into it’s veracity.
The other two suggestions are reliant on the assumption that early paravians were predatory, an assumption progressively considered less plausible given the extensive evidence of omnivory in paravians like troodontids, oviraptorans and microraptorids. Likewise, forms suspected of using these, like eudromaeosaurs, were presumably too large and specialised to be ancestral to birds. However, it is very likely that similar processes, unrelated to predation, were what developed dinosaur arms into avian wings.
Regardless of the method, it appears that exaptations for flight occured very early in theropod evolution, as the evidence of scutes in Concavenator implicates the presence of hindwings in non-coelurosaur theropods.
What Pterosaurs Tell Us
Currently, pterosaurian origins are sorrounded in mystery. The most accepted hypothesis is that they’re sister taxa with Dinosauromorpha, and are especially closely related to the small archosaur known as Scleromochlus, though even this tells us very little about the hypothetical pterosaur ancestors, given the morphological diversity of early avemetatarsalians. If we go by animals like Scleromochlus, we find ourselves in an interesting position: while pterosaurs are quadrupedal animals with powerful forelimbs and plantigrade hindlimbs, Scleromochlus is a bipedal animal with long, digitigrade hindlimbs and small forelimbs. In the 90’s, this wouldn’t have been something hard to invision, but now that we know pterosaurs quadrupedally launched and were generally happy running on all fours, it seems very bizarre indeed.
Scleromochlus does not seem like a gliding animal, to say the least, and although basal pterosaurs suggest an arboreal ancestry, gliding is not necessarily on the equation. A suggestion posited by David Peters (I know, but this doesn’t involve squamates, so it’s okay) is that the key of the pterosaurian wing, the massive fourth finger, evolved as a display device, which would make sense in these original bipedal runners, with the forelimbs free to occupy other functions. With time, these display devices would have become larger and more elaborate, and eventually be useful in WAIR or fluttering. Eventually, once the wing was large enough, flight would have become possible, and subsequently the animals would have shifted to quadrupedality as the wing membranes connected the limbs. This hypothesis would also explain the elongated fifth toe, which would have also evolved as a display device, before becoming an effective support for the cruropatagium.
As of this writting, the fog sorrounding pterosaurian origins is still thick, and therefore one can only imagine how these animals first took to the sky.
Based on birds and bats, as well as modern gliders, gliding appears to be effectively independent from flight, gliding animals seemingly unable to produce powered flight and animals that have evolved powered flight having developed it in unusual circumstances. Exceptions might exist, including possibly pterosaurs, but as it stands vertebrate flight has only been able to evolve from behaviours that prommote forelimb movement, with aerodynamic speciation not leading at all to volancy.