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Re: [dinosaur] Enantiornithine Concornis and Eoalulavis flight reconstruction: bounding flight in Early Cretaceous birds.



Mike Habib <biologyinmotion@gmail.com> wrote:

> Thanks Ben for sharing/updating, as always!

I'll second that.


> I just read through this paper, and itâs a solid piece of work. I 
> particularly appreciated that the authors accounted for the drag estimation 
> problems in the popular Flight 1.24 software when doing their power analysis. 
> In terms of
> predicting bounding gaits for Cretaceous birds, this is definitely the most 
> solid paper that Iâm aware of to date. Intermittent gaits, especially 
> bounding, have been implicated variably as a solution to gearing constraints,
> anatomical constraints, and/or power limitations. This particular paper 
> recovers some support for the gearing and/or anatomy arguments (since they 
> donât find power to be limiting), but this paper also doesnât really test 
> for
> potential bounding gait power advantages (it just tests if available power 
> likely exceeded required power). Iâm curious what others think about the 
> implications for origins of bounding flight.


I'd suggest (provisionally) that a preference for bounding flight is
consistent with the anatomical constraints of most enantiornithine
birds.  This includes the underdeveloped sternal keel (compared to
ornithurines) and the lack of a triosseal canal.  One hypothesis is
that extreme expansion of the pectoral musculature (especially the
supracoracoideus) as seen in ornithurine birds was at odds with
'primitive' (plesiomorphic) theropod terrestrial locomotion (e.g.,
Mayr et al., 2017).  In other words, stem avians walked and ran on the
ground with a theropod-style hip-driven gait, and this would be
impeded by pectoral musculature that was too hypertrophied (i.e., the
biped would become too 'front-heavy' and unstable during terrestrial
locomotion).  Derived ornithuromorphs resolved this problem by the
development of full avian-style knee-driven locomotion which allowed a
more anteriorly (cranially) shifted center of mass that was a
byproduct of further enlargement of the pectoral musculature (attached
to a larger/deeper keel) (e.g., Allen et al., 2013; Xu et al. 2014).
For ornithurines, commitment to knee-driven stride generation and
fully 'crouched' hindlimbs might be correlated with the development of
even larger flight muscles (and an enormous keel).

So it's possible that the flight style of most (but not all*)
enantiornithines (including _Concornis_ and _Eoalulavis_) was less
energetically efficient for prolonged flapping flight, which favored
bounding flight.  For this kind of flight in modern birds, I like
Rayner's (1985) analogy that Luftwaffe pilots used intermittent thrust
to improve fuel economy, and used it as a way of increasing range in
emergencies.


*Certain Late Cretaceous enantiornithines, such as _Neuquenornis_,
developed larger and deeper keels, so their flight abilities might
have been comparable to ornithurines. The keel of _Concornis_ is low
and restricted to the caudal half of the sternum, whereas that of
_Eoalulavis_ is a faint midline ridge.