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Re: Avian flight stroke origin



Jaime Headden <qi_leong@hotmail.com> wrote:

> So close! From the title: "Assessing Arboreal Adaptations of Bird 
> Antecedents."
>
> AVIAN antecedents. It would have been so beautiful!


It's not just the alliteration that falls short.  I also found myself
disagreeing with quite a few of the conclusions in the paper.
Nevertheless, it's pleasing to see Dececchi & Larsson (2011) refute
the hypothesis that the ancestors of birds were specialized for
arboreality.  That's always good to see, because this is the central
plank of the BANDit's scenario-driven approach to the evolution of
avian flight.  Dececchi & Larsso single out the splayed, quadrupedal
posture of "four-winged" gliding theropods for special criticism.


However, the Introduction points out that "The apparent dichotomy of
the ecological setting for the origin has been blurred, and perhaps
made moot [22] with recent discoveries that many extant birds have a
peculiar behaviour called wing assisted incline running (WAIR)
[11,23]."  This is not completely true.  Although the
arboreal-cursorial dichotomy is certainly obsolescent, this was
recognized long before the WAIR hypothesis was proposed.


Also, a little later on: "An arboreal context for the origin of the
flight stroke requires avian antecedents to have been tree-dwelling."
and "The trees-down hypothesis is predicated on the existence a
lineage of highly arboreal theropods preceding the evolution of the
aerofoil and gliding locomotion [8,12,14,24]"  Although this is true
of the kind of "trees-down" scenarios advocated by the BANDits, it
does not have to apply to ALL "trees-down" scenarios.  It is possible
to reconcile the cursorial/terrestrial morphology of paravians with a
"trees-down" origin of flight.


In fact, a possible answer to this paradox is provided by Dececchi &
Larsson (2011) themselves:


   "In general, nonavian theropods clustered with scansorial and
    grip-based climbing taxa, though the compsognathids and
    tyrannosaurs grouped with the lizards. This clustering is
    likely due to anatomical similarities between the predatory
    function of non-avian theropod forelimbs with grip-based
    climbers,"


But what if the ancestors of birds were *not* predators?  The
"scansorial and grip-based climbing" features might then actually have
been real scansorial and grip-based climbing features.  The ancestral
predatory adaptations would therefore be exapted by paravians for
grip-based climbing.  However, Dececchi & Larsson (2011) reject
trunk-climbing for paravians, claiming "Paravian taxa also had long
manual feathers that would not have permitted trunk hugging [96]," and
citing Sullivan et al. (2010) in support of this conclusion.  But my
reading of Sullivan et al. (2010) says nothing about the long manual
feathers precluding trunk hugging, because the feathers would have
been oriented away from the trunk.  Paravians could have gripped a
trunk in a two-handed fashion the same way that _Velociraptor_ would
have gripped large prey, and in either situation the feathers would
not have gotten in the way (Gishlick, 2001).


Dececchi & Larsson's emphasis on grasping branches and negotiating
tree crowns to assess "arboreality" seems slightly misplaced for
paravians, given the dominance of large cycad (and cycad-like) plants
throughout most of the Mesozoic, including the time that the ancestors
of birds were getting their wings.  A reversed and enlarged hallux is
needed only for perching.  If paravians were scaling trunks, grabbing
food items, then returning to the ground, a perching pes would not
have been necessary at all.


So although Dececchi & Larsson are right in saying there is no
evidence that any non-avialan paravian (or basal avialan) really fits
the definition of "arboreal" (‘‘rarely on the ground, forages and
shelters in the trees’), it does not necessarily follow that the
origin of avian flight could not have involved parachuting or gliding
down from trees.  Once again we see just how arbitrary and restrictive
the traditional arboreal-cursorial dichotomy is when it comes to
reconstructing the ecologies of basal avialans and non-avialan
paravians.


Finally, in today's Nature there is an article on the position of the
putative lopopodian _Diania_ in arthropod phylogeny.  Mounce and Wills
(2011) make the following comment:


   "Many closely allied groups exploited successfully some
    but not all of the characters that typify the arthropod crown
    group. Only in retrospect do we discern a single, ladder-like
    trajectory through what was really a much more
    eccentrically branching bush".


If one were to replace "arthropod" with "avian", I think we end up
with an excellent summary of avialan evolution.




Cheers

Tim