[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Feather barbs show history of flight

I also like your "trees to ground to air" model, Mike.  What I
especially like is that it decouples the evolution of the incipient
wing from the evolution of the flight stroke.  But as a general rule,
I'm uneasy about any model that invokes an arboreal (or semi-arboreal)
quadruped as a transitional stage in the evolution of avian flight...

Theropods were primitively terrestrial bipeds.  Crown birds were
primitively arboreal bipeds, with highly refined perching abilities.
Having an arboreal quadrupedal stage in between the two (terrestrial
biped / arboreal biped) is highly non-parsimonious, IMHO.  I innately
prefer models in which an arboreal biped evolved directly from a
terrestrial biped.

More importantly, no paravian shows adaptations consistent with being
an arboreal quadruped.  Not _Microraptor_, not _Archaeopteryx_, not
any other paravian.  Theropod limbs are maladapted for quadrupedal
arboreality (or even scansoriality), especially the semilunate wrist
(which compelled the hand to move along an arcuate path) and the
mesotarsal ankle (essentially a hinge - suitable for a parasagittal
gait, but awful for clambering through branches).  Contrast this with
the highly mobile joints of arboreal mammals.  I won't dwell on this
because the lack of arboreal features in basal paravians is covered
well in the literature (especially by Dececchi and Larsson).  But my
scansorial model, by which incipient wings facilitated terrestrial
progress across uneven and obstacle-ridden terrains, is an attempt to
explain how small bipedal theropods became volant - without requiring
a quadrupedal intermediate. Under this model, arboreality came much
later, in forms already capable of flapping flight (and ground-to-air
launches, making trunk-climbing unnecessary).

I also think we might be reading too much into the descended (= more
distally positioned) hallux of stem avians.  This includes inferring
perching abilities in taxa like _Sapeornis_ and confuciusornithids
(especially _Changchengornis_), which have a low and partially
reversed hallux.  Throughout the course of theropod evolution the
first metatarsal slid down the metatarsus, ever since its initial
detachment from the tarsus - for reasons unknown (at least, I've not
yet seen an adaptive explanation).  The condition in crown birds (a
fully descended hallux) is the culmination of a long-term trend as old
as the theropods.  Sapeornithids and confuciusornithids show a
partially reversed hallux, which is directed somewhat posteromedially
- an incipient condition compared to the fully reversed (posteriorly
directed) hallux of crown birds.  The condition in these stem birds
might not be related to perching (or therefore arboreality) at all.  A
functionally tetradactyl foot with a more posteromedially directed
hallux might have improved contact with the substrate - for whatever

Whatever's one favorite model, the old "trees-down"-versus-"ground-up"
debate is dead and buried.


On Wed, Feb 18, 2015 at 5:27 PM, Mike Habib <biologyinmotion@gmail.com> wrote:
> Great thoughts, Tim. I am pleased to finally see a vertebrate paleontology 
> paper out that treats primary feather asymmetry as a continuous character! 
> Now, when I give my “asymmetry isn’t dichotomous” talks I can at least cite 
> one good study on fossil birds that does it right.
> My “trees to ground to air” model, presented at SVP in the fall, was my 
> personal attempt to set up a transition phase that matched the phylogenies 
> and known morphological patterns. The idea is that some semi-arboreal phases 
> might have pushed control and weight support, but secondarily less aerial 
> lineages might have been where flight stroke evolution mostly took place, 
> under a regime that essentially selected for strong burst performance. I 
> don’t necessarily think it’s better than the other models out there, but it 
> was received pretty well. The general sentiment I got after the talk was 
> there is still a bit of a “slow build” versus “fast build” split in the 
> community on the flight stroke (that’s anecdotal, though).
> Cheers,
> —Mike