Flying Velociraptors: A Reality

Microraptor by mangycoyote, flying to a perching spot.
Microraptor by mangycoyote, flying to a perching spot.

A paper finally comes along listening to reason. Said paper is “Avian ancestors: A review of the phylogenetic relationships of the theropods Unenlagiidae, Microraptoria, Anchiornis and Scansoriopterygidae” (Federico L. AgnolĂ­n and Fernando E. Novas 2013)., which not only expresses the now pratically omnipresent tendencies to consider maniraptor phylogeny to be in serious need for a revamp, but also points out exactly why Microraptor and kin were most likely flying animals.

The primary point of interest is that the paper highlights one thing pretty much nobody had taken into consideration previously: the microraptoran shoulder girdle is uniquely specialised for powered flight. It has a socket curvature that places the shoulder much higher than in the average theropod, allowing for a full vertical upstroke; combined with previous assessments that archosaur limb range was far more flexible than previously thought, this pretty much means that the notion that maniraptors couldn’t raise their arms significantly is pretty much dead.

In addition, the paper also highlights the unique adaptations for powered flight seen in Microraptor, like the presence of an allula, the presence of a propatagium and the already noted well developed deltoideus complexes and tail rods.

HOWEVER

The paper, as the name implies, also restructures deinonychosaur phylogeny. In particular, it renders it polyphyletic, with microraptorans and unenlagiines being closer to birds than to other deinonychosaurs, and within these some taxa are moved closer to Aves (i.e. Rahonavis).

This pretty much suggests one of two things: that powered flight was probably acquired in the last common ancestor between Passer and Microraptor, to the exclusion of several “classical deinonychosaurs”; or that flight was in fact acquired and lost multiple times among dinosaurs (an idea supported if Xiaotingia+Anchiornis is closer to Aves than the other taxa examined, as well as if Pelecanimimus‘ infamous flight-y traits still apply).

Considering that eudromaeosaurs still possess tail rods and quill knobs, that their common ancestor evolved flight independently also becomes a serious possibility.

Flying Pelecanimimus?

My distressingly poor attempt at illustrating my proposal.

With the discovery that ornithomimids had wings, a new possibility for Pelecanimimus has emerged:

Pelecanimimus does have large paired sterna with ossified ribs and possibly uncinates, so maybe basal ornithomimosaurs were more flight-y anyway.”

Mickey Mortimer.
So far, no other explanations were offered for these skeletal structures, and honestly, I don’t think there are any other possibilities other than flapping adaptations.

Since WAIR is extremely controversial, given the relatively low angle the arms could be raised at (and even then, this is not well established), flight is actually the more sensible explanation; an animal capable of powered flight needs robust bones and extensive musculature, but not as much as an animal that practises WAIR.

If Pelecanimimus was a flyer, this opens a wide range of implications. For starters, it shows that flight evolved independently in at least two lineages of dinosaurs (or more, given that microraptorines, Archaeopteryx and Rahonavis might had become volant independently from birds, after all; however, do note that, as Mortimer proposes, ornithomimosaurs as non-paravians isn’t well established, so they might be highly derived archaopterygids), that the size limit for flying non-ornithurine dinosaurs was considerably larger than previously thought (and that flight development might also not be reserved for small animals), that pterosaurs indeed co-existed with flying dinosaurs for most of their temporal range, et cetera. To say nothing about the fact that Ornithomimus and kin really were ratite mimics.

If Pelecanimimus was indeed a flyer, it was almost certainly a soarer, as even it’s unique anatomy would perhaps be insuficient for a Galliforme-like extreme flapper. Given what we know of other flying non-avian dinosaurs, hindwings would had been very likely present. Hindwings were indeed quite vital for non-avian dinosaur flyers, as the inferred gliding models for Microraptor show that these animals could gain some altitude simply by raising them at an angle, and this could explain why their anatomy is often underdeveloped when compared to modern birds.

Inferred gliding model for Microraptor. Note that, in mid-flight, the animal’s trajectory gains height, forming a U like curve, simply by raising it’s hindwings. If applied to long term flight, while the animal would gradually loose altitude, it would have remained airborne for an ostensibly immense period of time. Cobined with complimentary wing strokes, the end result would be a long, but relatively low energy demanding flight.

A terrestrial bustard/crane like lifestyle was likely, which would have been quite interesting as it lived in a time when chaoyangopterid and tapejarid pterosaurs were at their prime. This further indicates that niche partitioning between “birds” and pterosaurs ensured little direct competition, although the absence of chaoyangopterids in the area Pelecanimimus lived in and the low diversity of flying ornithomimosaurs seems extremely suspect.