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Flying Velociraptors

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Deinonychosauria is one of the most apreciated dinosaur clades, but more often than not for quite erroneous reasons. The classical perception of predatory, super-intelligent giant "lizards" has not died yet, depicting creatures that are less real and more twisted fantasies of delluded minds. Instead of yet another breed of mythological dragon, dromaeosaurs and troodontids were normal animals, but yet quite unique in that they represent the avian prototype, the type of animal from which modern birds emerged. And like their surviving cousins, they too had an advantage over other dinosaurs: the air.

The close relationship between deinonychosaurs and Aves/Avialae has led since very early to the speculation that troodontids, dromaeosaurs and birds share a common, flying ancestor. Indeed, the famous generic basal avian, Archeopteryx, is very similar to basal troodontids, and the earliest troondontids and one entire linage of dromaeosaurids, Microraptorinae, as well as Rahonavis, show adaptations for gliding and even at the very least rudimentary powered flight. Some recently discovered basal dromaeosaurids might have rendered this notion inaccurate, as neither Mahakala nor Tianyuraptor, considered to be the most basal dromaeosaurs, have any sort of adaptations for gliding or flying, although they could be considered anomalies, as bird groups like ratites have produced flightless members since very early in their evolutionary history.

Thus, it is often thought that deinonychosaurs were simply an early version of things like penguins and ratites, creatures with winged ancestors (and, in the case of the latter, still volant members, the tinamous, although they fly so little that they might as well be called flightless), but that gave up flight to specialise in terrestrial niches, presumably being forced to specialise this way due to competition with early birds as well as pterosaurs (although, since the latter were dominant through all of the Mesozoic, since way before dinosaurs developed feathers, it might be safe to say that they didn't account for much in terms of competition for deinonychosaur). However, the truth might more different than this simplification; non-avian dinosaurs were different from modern birds after all, even their closest relatives.

First, lets note that, while modern birds fixate on a single niche, their Mesozoic relatives did not do so. Parental care amongst early paravians, even in early avialans such as Enantiornithes, was more loose, if present at all; even in today's birds the most basal ones like Galliformes, Anseriformes and Paleognathae still have precocial young, and in the case of megapodes the chicks are superprecocial, being able to fly since their day of birth. Naturally, it is thought that megapodes are a good model for basal paravians; troodontid juveniles are known to have been superprecocial, having a well developed brain since birth like modern megapodes do. Therefore, like megapodes, deinonychosaurs most likely had very little parental care, that was most likely invested for a very short period of time, like in crocodylians.

With no parents to fetch food, the young raptors most certainly relied on themselves to find food such as insects and later larger animals. Because most energy was invested in surviving day to day rather than growing, deinonychosaurs, like most sauropsids, most likely had a very slow growth rate. This meant that they could occupy many ecological niches across their life time, ranging from small insect eaters as babies to medium and large sized predators as adults. A similar process occured in pterosaurs, sauropods and tyrannosaurs, all of them evidence that Mesozoic ecosystems had a very unusual niche distribution among terrestrial vertebrates.

As such, because raptors occupied more than one ecological niche across their lifetime, the juveniles could lead radically different lifestyles from the adults. This means that their anatomy was also different, and as such many of the limitations the adults suffered could probably have been avoided by the younger animals. Therefore, if an adult Deinonychus was too heavy to fly, there is the possibility, however remote, that a young one could do so.

This might seem extremely ridiculous, but there is some evidence that at least dromaeosaurs were capable of aerial locomotion as at least juveniles. First, there's the fact that flying raptors lasted as recently as the late Cretaceous; Rahonavis is the most famous example, occuring in Madagascar at the very end of the age of the dinosaurs. Second, there's very unusual distribution patterns that could be associated with flight capacity; velociraptorine dromaeosaurs have occured in South America and Antartica, while at least two waves of dromaeosaur invasions occured in the archipelagos that formed Eastern Europe in the Mesozoic. This phenomena could be associated with extremely brief land bridges (not likely for the first, which lived before the speculated land bridges betwen the Americas during the late Cretaceous) or with rafting, which is also not very likely because, unlike mammals or squamates, birds and their relatives have an extreme tendency to not be successful in this method of travelling (whereas rodents and lagomorphs can be observed in most islands, their avian analogues, the Galliformes, are entirely absent except for the occasional patridge or megapode, both better flyers than most fowl), the only times in which rafting being successful being when the flightless New Zealand wrens conquered Stephen's island, and maybe when kiwis arrived to the main islands.

Therefore, at least some level of flight capacity might had been present amongst dromaeosaurs. To fully understand if this is possible, however, one must take note of the flight capacities of non-avian maniraptors.

Both Deinonychosaurs and early avialans lack two fundamental aspects modern birds have: the presence of a deep sternum keel, and a primitive shoulder that does not allow the arm to be raised much higher than the shoulder itself. In some cases, the feathers might even be inapropriate; in at least Anchiornis the feathers are symmetrical, and in Archeopteryx and Confuciusornis the feathers are thinner than those of modern birds. However, in all of these cases the animals are still so otherwise well adapted to aerial locomotion that inferring that they were flightless seems absurd; in addition, many of these animals were found in areas that were once the bottom of deep lakes and lagoons, which are out of reach for flightless creatures. Most certainly they were volant, but were inferior to modern birds in terms of flight capacities. Or at least used a different style of aerial locomotion...

In both Microraptor and in Confuciusornis, the wings, very unusual in shape for modern bird standards, are nonetheless closer in shape to those of fast flyers like swifts and swallows (in the latter's case the wings somewhat resemble those of small seabirds like terns and tropicbirds). While it is perhaps absurd to suggest microraptorines and confuciusornithids were fast, energetic flyers like swallows and swifts, the way they flew was presumably based on the same basic principles. Their wings, long, thin and sharp, were a small variant of the wings of the typical dynamic soarer, gliding with speed and reducing the need to flap the wings by making a superior use of the sorroundings. Wing flapping was of course a necessity, and still possible because while short the keep still offered room for strong wing muscles. Note also that in modern birds the keel is deep because both the muscles that pull the wing up and those that pull it down are attached both to the sternum, while bats, primates and pterosaurs had the muscles that pull the wing up attached to the back, thus eliminating the need for a very deep keel. With that in mind, we still don't know if the wing muscle arrangement was different for basal paravians.

Dynamic soaring appears indeed to have been a method of aerial locomotion exploited by volant deinonychosaurs. Rahonavis has absurdly long arms, suggesting it was a soarer, although the wing bones were robust enough to suggest that it flapped the wings more efficiently than earlier paravians such as Archeopteryx itself. Microraptor itself appears to be a very aerial creature, lacking adaptations to climb and being incapable of standing on the ground without damaging the flight feathers, so it is possible to infer that, even if at disadvantage against birds, deinonychosaurs managed to become very specialised in terms of aerial niches. However, more traditional flyers still existed, and most deinonychosaurs probably still made use of short, broad wings like those of forest birds. Such were the wings of Anchiornis, Archeopteryx and others. In the case of the first, the wing feathers were symmetrical, and not asymmetrical like those of flying birds and dromaeosaurs. However, they were arranged in an unique way; while the flight feathers attached to the end of the wing are the longest in paravians with asymmetrical feathers, in Anchiornis the feathers attached to the middle were the longest, with the feathers attached more distally being much shorter. This seems to imply that before the evolution of asymmetrical feathers, this type of wing arrangement was a viable alternative for flapping flight, having a similar aerodynamic effect.

Thus, it is perfectly possible that in several "flightless" deinonychosaurs, the juveniles were capable of powered flight. In many of the more "typical" dromaeosaurs like Deinonychus and Velociraptor, the forelimbs were robust enough to suggest that the juveniles of such species could provide shallow flight strokes, not powerful enough for the adult animal to have enough lift but about powerful enough to allow a still growing dromeosaur decent aerial locomotion. In many other deinonychosaurs, though, like unenlagiines and troodontids, the arms became considerably shorter and/or thinner, suggesting that, unless the limb degenerated across the lifetime, the animal probably was merely limited to glide or parachute as a youngster. It might also explain why unenlagiines and troodontids had a shorter worldwide distribution than velociraptoriines for example.

In terms of ecological niches, this also brings interesting implications. For example, there's no known bird of prey analogues in the late Cretaceous; the closest we know of are avisaurids, which have long talons, but they might be just a sign of an arboreal lifestyle. Before this niche was occupied by "rhamphorhynchoid" pterosaurs in the Jurassic and early Cretaceous, and indeed there is evidence of a late Cretaceous "rhamphorhynchoid" from Argentina. However, it is also likely that small dromaeosaurs, at least as juveniles, might had played a part, if they were indeed volant.
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ElSqiubbonator's avatar
Holy crap1 That just makes them even MORE awesome!