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Re: [dinosaur] Mesozoic Aviary (free pdf)



This succinct review emphasizes several key elements of the evolution
of flight in theropods, all interconnected:

(1) In the early days of flight evolution, there were some oddball
experiments in becoming (and staying) airborne.  Only one was
ultimately successful: the line that lead to modern birds.  (However,
to give credit where it's due, some of these other 'experiments' were
apparently quite successful during the Jurassic and Cretaceous.  They
only look 'oddball' by comparison with the birds we have now).
(2) It is entirely possible (and arguably even likely) that powered
flight evolved multiple times in paravian theropods.
(3) From the moment wings evolved, there is no overarching trend in
advancement or refinement of flight ability.
(3) Exaptation played a crucial role in the assembly the avian flight
apparatus.  As Brusatte puts it:

"This supports the hypothesis that wings developed for nonflight
reasons (such as display, egg brooding, or something else entirely)
and that their early evolution was not shaped by selection for aerial
flapping. Only much later, it seems, did some paravians evolve the
right combination of small body size, large wings, and other
anatomical features to begin powered flight. It was at that point that
selection was able to mold these animals into more effective flying
machines."

This fits with the idea of ZOIVA - zone of incipient volant adaptation
- proposed by Sullivan et al. (2016).  According to this idea, certain
paravian taxa built up a number of characters that facilitated the
transition to aerial ability.  However, the exact characters might
have differed from one lineage to another.   (In some ways,
_Microraptor_ is more 'advanced' than _Archaeopteryx_ when it comes to
flight adaptations e.g., fused & ossified sternum, alula.)  According
to Dececchi et al. (2016), both _Archaeopteryx_ and _Microraptor_ were
capable of powered flight.  Neither is a novel idea, of course; but
Dececchi et al. provides biomechanical data to back up their
hypothesis, and proposes that a stationary, hindlimb-driven launch was
aerodynamically possible.

All this suggests that birds and microraptorines likely evolved flight
independently. There may be others: the flight abilities of
_Rahonavis_ were probably comparable to those of _Archaeopteryx_.  If
it's a unenlagiid, then _Rahonavis_ potentially represents a third
path to powered flight.  The fact that _Rahonavis_ lived close to the
end of the Cretaceous (when 'advanced' birds were thriving) suggests
that these 'primitive' experiments in flight were successful in their
own right.  Alternatively, flight was primitive (symplesiomorphic) for
Paraves, but was lost numerous times - but this hypothesis seems
highly non-parsimonious.

One thing Brusatte doesn't really discuss is that paravian flight
(including by birds and microraptorines) could have evolved via a
ground-level take-off.  So the evolution of flight didn't necessarily
have to pass through a 'passive' gliding stage.  It's tempting to
regard certain paravians like _Anchiornis_ as arboreal gliders,
because they had relatively large wings (composed of symmetrical
feathers) but were incapable of powered flight.  But there is no
evidence for arboreality in these forms; so the wings were likely used
in an entirely terrestrial context (display, leaping, maneuverability,
etc - none mutually exclusive) before being co-opted for flight.



References

Brusatte SL (2017) A Mesozoic aviary. Science 355: 792-794 DOI:
10.1126/science.aal2397

Dececchi TA, Larsson HCE, Habib MB (2016) The wings before the bird:
an evaluation of flapping-based locomotory hypotheses in bird
antecedentsPeerJ 4: e2159. DOI 10.7717/peerj.2159

Sullivan C, Xu X, O'Connor JK (2016) Complexities and novelties in the
early evolution of avian flight, as seen in the Mesozoic Yanliao and
Jehol biotas of Northeast China.  Palaeoworld http:
//dx.doi.org/10.1016/j.palwor.2016.12.001

On Fri, Feb 24, 2017 at 6:16 AM, Ben Creisler <bcreisler@gmail.com> wrote:
>
> Ben Creisler
> bcreisler@gmail.com
>
> A commentary in the new Science issue with free pdf:
>
> Perspective
>
> Stephen L. Brusatte (2017)
> A Mesozoic aviary
> Science 355(6327): 792-794
> DOI: 10.1126/science.aal2397
> https://urldefense.proofpoint.com/v2/url?u=http-3A__science.sciencemag.org_content_355_6327_792&d=DwIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=NjMuNu0LCnEFfZ1vHXViEdRZzHLc3XSCKKgpBJddQgM&s=ThTLIAum4ikv1f860svXDX0mQJRpc44flHA0HIWdyQU&e=
>  
> https://urldefense.proofpoint.com/v2/url?u=http-3A__science.sciencemag.org_content_355_6327_792.full&d=DwIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=NjMuNu0LCnEFfZ1vHXViEdRZzHLc3XSCKKgpBJddQgM&s=oYvloMoaPG8UUG7su_Rpl1Bn4Xnz-XsoQThBGQRoprQ&e=
>  
>
> Free pdf:
>
> https://urldefense.proofpoint.com/v2/url?u=http-3A__science.sciencemag.org_content_sci_355_6327_792.full.pdf&d=DwIFaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=NjMuNu0LCnEFfZ1vHXViEdRZzHLc3XSCKKgpBJddQgM&s=cZ1cG1BMOj-2akx-Iv5rgOM8x5lfU8IBMGWQ7kbaXwk&e=
>  
>
>
> Summary
>
> The evolution of birds from a group of small dinosaurs between 170 million
> and 150 million years ago has emerged as a textbook example of a major
> evolutionary transformation in the fossil record (1). The attainment of
> powered flight—that is, active flapping that generates thrust—has been
> widely regarded, sometimes explicitly but often implicitly, as a long
> evolutionary march in which natural selection progressively refined one
> subgroup of dinosaurs into ever-better aerialists. However, recent fossil
> discoveries reveal a much more interesting story that is beginning to be
> corroborated by biomechanical studies. According to this story, the
> development of flight was chaotic, with different dinosaurs experimenting
> with different airborne behaviors using different airfoil and feather
> arrangements (see the figure), until ultimately only modern birds survived.