Velociraptors on air: Introduction
The topic of flight in non-avian dinosaurs is a largely barely explored one, due to a sense of general assumition most people have in regards to the flight capacities of animals like deinonychosaurs. On other words, people don’t bother with silly ideas like flying dromaeosaurs and troodonts because, seriously, wouldn’t you laugh your ass off if someone stated that Velociraptor was a soaring predator not unlike modern Aquila eagles, making use of thermals that formed above the ancient mongolian deserts and striking at prey from the air?
However, we do know that flight existed outside of Avialialae (I’ll use this term to reffer to all volant dinosaurs that descended from the last common ancestor between Jeholornis/Shenzhouraptor and Passer); since their discovery do we know that the dromaeosaurs Rahonavis and Microraptor were capable of powered flight, the first having quite long and robust forelimbs and the latter having clearly asymmetrical feathers and a specialised shoulder girdle (we’ll discuss the nature of the deinonychosaur shoulder girdle further on), and now we know that Archaeopteryx, for years considered the earliest true avialan, was actually a basal deinonychosaur, and it was obviously volant (interestingly, it has been claimed that both Rahonavis and Microraptor were better flyers than Archaeopteryx. This, coupled with the fact that deinonychosaurs assigned with Archaeopteryx in the tentative clade Archaeopterygidae like Xiaotingia turned out to have been even less capable flyers, provided quite the restrospective irony).
However, even then, the flight capacities of these animals have been laid to doubt, because of the following details:
– The absence of a sternal keel;
– The sideways orientation of the shoulder joint, which made it impossible to raise the arm above the shoulders;
– The fact that the rachises were thinner, and possibly weaker, than those of modern birds (this applies to Archaeopteryx; whereas it applies to Microraptor and Rahonavis is unknown)
These features also apply to early avialans like Confuciusornis, despiste their adaptations to powered flight. This is why, therefore, the nature of these features was eventually questioned.
Rule of 3: three very simple reasons why the three arguments above are wrong (two of them can be called rubish at the very least)
Because many of the maniraptors supposedly incapable of powered flight were found in large numbers in deep water areas, some people naturally began to doubt the arguments presented above. Eventually, refutal has been offered:
-The presence of a sternal keel is only genuinely an issue if the flight muscles are arranged as in modern birds; such system is the bizarre “pulley system”, seen above. This muscle arrangement, which is characterised by the fact that the muscles responsible for pulling the wing down – the supracoracoideus complexes – attach to the sternum, is only known from the clade of avialans known as Ornithothoraces, which includes modern birds and several extinct clades like the enantiornithe birds. The other flying tetrapods, the pterosaurs and the bats, don’t have this system; instead, the muscles that pull the wing up are attached to the vertebrae, like in most tetrapods. Instead of relying on the supracoracoideus complexes, they rely on the deltoideus complexes.
Thus, deinonychosaurs did not require large keels; many modern bats lack them too (pterosaurs do have sternal keels, although they’re more like deeper regular sternums than true keels), as they’re not required in this style of muscle arrangement. There’s also the fact that sauropsid muscles tend to quite naturally strong, being less dependent on aerobic respiration than mammalian ones, so even if far less capable of powered flight than birds or pterosaurs, volant deinonychosaurs were still more than capable of beating their wings.
– The paper that suggested the limitations in forelimb movement, written by Senter, seems to be currently under credibility issues, and the range of forelimb motion appearently was not as limited as suggested by him. For further information, see the following essay:
The true extent of forelimb motion in deinonychosaurs is therefore unknown, but it appears that they could indeed raise the arms higher than the shoulder. Thus, they could provide at least shallow strokes, which is enough for flight (we will eventually discuss how these animals flew).
– The paper that suggested that the wing feathers were not robust enough to support powered flight, written by Robert L. Nudds and Gareth J. Dyke, is currently heavily criticised for overestimating the weight of the animals studied, namely Archaeopteryx and Confuciusornis (Paul, G.S. (2010). “Comment on ‘Narrow Primary Feather Rachises in Confuciusornis and ArchaeopteryxSuggest Poor Flight Ability.'” Science, 330(6002): 320. (15 October 2010).). In any case, these measurements probably do not apply to neither Microraptor nor Rahonavis, the first not having been analysed by the study, and the latter clearly having robust flight feathers that left marks on the ulna; not all modern birds leave feather marks on the arm bones, but species that engage in intense powered flight or that reach large sizes do tend to have these marks, as strong feathers are required in either case. Thus, not only was Rahonavis capable of flight, but either engaged in intense powered flight, or was merely the juvenile of a much larger species.
Rule of 3: three is not enough
Since the discovery of Microraptor the idea that all of Deinonychosauria is derived from volant species has been considered. Indeed, it largely explains the wide distribution of dromaeosaur dinosaurs, which include several Gondwannan taxa – the most prevalent of those southern continent taxa, the clade Unenlagiinae, had Rahonavis nested well within it, and overall had several bird like traits that indeed indicate a volant ancestry. However, many of the basal deinonychosaurs found lacked the iconic asymmetrical feathers required for powered flight, rendering them mere gliders. This would therefore render the three known volant species unique animals that evolved flight independently from birds.
However, there are some issues in regards to these earlier species. Most, if not all of them, had the iconic “hindwings”, which would render running difficult at best, something at odds with the cursorial nature of their hindlimbs. Considering that we know that the deinonychosaur sickle claw evolved as a climbing tool, this problem could have been solved by climbing and subsequent gliding; however, these basal deinonychosaurs were less adapted to tree climbing than more derived forms. Without the ability to run or climb decently, survival would have been considerably hard.
The basal deinonychosaur Anchiornis might offer a possible explanation. It had symmetrical feathers, but they were arranged in an unique away; in species with asymmetrical feathers, the most distally attached wing feathers are the longest ones. In Anchiornis, the longest are anchored near the wrist, making the center of the wing the broadest area. This is not an unusual profile among flightless maniraptors – oviraptors like Caudipteryx have this sort of arrangement as well. Anchiornis, however, differs in that the feathers at the front (as in, anchored more distally) of the longest feather decrease rapidly in size as they are closer to the end of the supporting digit; this results in a rounded, yet slightly pointy wing shape.
It is possible that this arrangement could had been an early adaptation to the demands of powered flight, before true asymmetrical feathers evolved. If so, it is possible that Anchiornis did engage in powered flight, or even a method of escape akin to rudimentary WAIR. So far, no tests have been conducted to examine the aerodynamic capacities of it’s wings.
So far, flight has been observed or suggested in a variety of small species. However, aerial locomotion might had been far more widespread amidst known deinonychosaurs; the presence of feather marks as in Rahonavis implies that several derived species probably had robust wing feathers as well, something that flightless animals would not require. The fact that Rahonavis is a fairly derived dromaeosaur seems to indicate that flight was at the very least retained in clades thought to have become flightless quite early on.
One possible way in which flight was retained was on the juveniles of larger species.