“This egg position is not seen in living birds, with the exception of the basal galliform megapodes who place their eggs within mounds of vegetation or burrows. This accumulation reveals a novel nesting behaviour in Mesozoic Aves that was perhaps shared with the non-avian and phylogenetically more basal troodontid theropods.”
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. Agnolín 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.
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.
Previously, I’ve already discussed possible flight adaptations in Anchiornis (I do not need to post said links because you can seem them in the discussion). Namely, I argued that the unusual wing feather arrangement might be indicative of an early attempt at lift-generating wings before the evolution of asymmetrical feathers, and that the animal seems far too unsuited to both climbing and running to be anything other than a flyer/glider.
Since then, however, there has been some changes. Most significant is Longrich’s paper on primitive avian wings, which seems to suggest that Anchiornis and Archaeopteryx were less efficient at aerial movement than derived birds and Microraptor.
These are my two-cents on the matter.
On Longrich’s paper
This is not the first time it has been suggested that early birds weren’t as good flyers as ornithurines. For instance, see Dareth Dyke’s paper stating that the vanes of Confuciusornis‘ flight feathers were too weak to support the bird in the air (spoilers: it’s now considered to be crap)., and the many papers describing how Archaeopteryx supposedly could not fly because of the lack of certain adaptations now thought to simply not be present due to different flight muscle configurations. It’s like these workers have a fetish for stating early birds could not fly in spite of the overwhalming about of morphological evidence against that.
The Longrich paper, I’m afraid, is another one to be debunked. If you want a less biased source on the matter, see the following:
The basic gist is that it fails to adress post-mortem preservation issues, that it’s simply too incoherent, that modern birds have similar arrangements, and that it may suffer from the “David Peters syndrome”, i.e. basically imagining things.
Thus, while it’s almost certain that early paravian wings weren’t as suited for powered flight, the paper fails to depict a sincere anatomical analysis. If you’re going to take it seriously, you’re an hypocrite for not seeing the genius that David Peters is.
I almost vomited while typing that.
Eosinopteryx and actual terrestriality
So, with the Longrich silliness out of the way, we get back to stage one. Anchiornis is basically a quail-sized theropod that is appearently poorly adapted to climbing, cannot run very well due to the ginormous hindwings, and unlike Archaeopteryx it lived in an environment filled with terrestrial predators and mammalian competitors. Basically, if it could not fly out of danger and competition, it was doomed, unless it was extremely toxic or something.
Anchiornis also co-existed with another troodontid, the newly discovered Eosinopteryx. This animal differs from Anchiornis in that it has virtually no hindwings or tail feathers, and that it’s forelimb feathers are reduced somewhat. This animal was thus clearly terrestrial, either too primitive for aerial locomotion or having secondarily lost those traits as it became terrestrial. It was more suited than Anchiornis for a fully grounded life, and unless Anchiornis had some bizarre advantage, logic dictates that animals like Eosinopteryx would replaced critters like Anchiornis.
That said, conclusions like these are very hasty. If Anchiornis was volant, it should have features indicative of that.
First aeronaut pioneer?
Compared to Microraptor and Archaeopteryx, Anchiornis seems defenitely less adapted for flight. It has symmetrical feathers and it lacks tail rods, both adaptations closely correlated with flight in non-avian dinosaurs.
That said, we do now what it did had, and that would be the long hindlimbs. Both Anchiornis and Microraptor share long metatarsals and lower leg bones, both adaptations seen in cursorial animals. It’s well established that such hindlimbs seem paradoxical, as the long hindwing feathers would almost certainly get in the way of running. However, such long hindlimbs would almost certainly facilitate hopping, and thus, much like modern small terrestrial birds, these dinosaurs could possibly launch themselves quickly into the air. Furthermore, unlike ground birds, Anchiornis had hindwings, which would increase the life generating area and render the heavy hindlimbs less detrimental to flight.
Previously, terrestrial launching had been shut down in non-avian maniraptors due to the “fact” that they could not raise their forelimbs above the shoulder. Thankfully, these assertions are now considered false, and they furthermore also solve the other problem, that non-ornithurine paravians had inefficient wing musculature. Little has been described about Anchiornis‘ musculature, but Microraptor has well preserved deltoideus complexes, which means bat-like wing strokes were possible.
With symmetrical wing feathers and a tail without tail rods, Anchiornis was defenitely nowhere as aerial as Microraptor. However, it seems perfectly capable for quick take offs and parachuting, which is certainly far more than utter flightlessness. Galliforme style quick launchings followed by extensive parachuting seem to be the best model for it’s aerial locomotion
It seems therefore that Anchiornis was at least capable of quick launching and subsequent gliding, perhaps like a terrestrial version of the quick vaulting displayed by Pantodon and gasteropelecid hatchetfish.