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Re: BADD BCF



GO:

>. Before the advent of the retro-hallux, arboreal archosaurs had to use
>all four limbs for clambering about in trees; this limited the development of
>the forelimb into a wing, because it also had to serve in the more mundane
>grade of locomotion. But the advent of the retro-hallux allowed the foot to
>assume more of the climbing and grasping burden and freed the forelimb to
>become more winglike.

OR: (assuming for argument's sake that we are dealing with an arboreal form)
- the retroverted hallux would be a useful climbing and perching adaptation
for an arboreal biped, freeing the forearms for use in grasping prey.  In
fact this argument would provide an immediate benefit for such an adaptation
- to me a more likely scenario than some promise of future flight.  If this
is correct (and I submit it is at least plausible) then the retroverted
hallux would be a benefit to nonvolant arboreal bipeds that could have
included dromaeosaur ancestors.  As pointed out earlier, though, the
retroverted hallux may also have beeen evolved in terrestrial predators to
allow the foot to be used to grasp prey.

 This was a key stage in the evolution of avian
>flight--the decoupling of the forelimb from portal locomotion--and it was
>made possible (in this instance) by the retro-hallux. Of course, the
>proximate cause behind the retro-hallux was simply to improve the grasping
>ability of the hind foot, an arboreal adaptation.

Assuming that the ancestors of these forms were bipeds, the forearm was
ALREADY decoupled from locomotion.
>
>What possible use could a retro-hallux have for a strictly cursorial biped?
>None. 

Prey capture and manipulation (as in, to repeat myself, secretarybirds,
harrier-hawks etc today).  This would have been most important for predators
taking small prey (insects, lizards etc); hence it is not surprising that it
became less important in larger forms.

>Pneumatization in non-avian dinosaur lineages may in all instances be traced
>back to the arboreal ancestral forms. At least, this accords with the
>phylogeny of dinosaurian pneumatization recently presented by Brooks Britt.

Let's just say, back to ancestral foms  which may (or may not) have been
arboreal, thus indicating that pneumatization may (or may not) have been
associated with such habits.   Certainly it developed long before the origin
of flight, so its persistence in theropods need not have anything to do with
flight.

>The question is, which of the splendid evolutionary modifications in
>_Velociraptor_ occurred to overcome the limitations inherent in its arboreal
>origin.

This only becomes a question if you can prove the arboreal origin.  Again, I
am not saying its ancestor wasn't arboreal - I'm not even saying its
ancestor wasn't volant, though I confess I am far from convinced of this.  A
better question would be: are there character states in V. that appear to be
reversals from adaptations in ancestral forms that could be indicative of an
arboreal (or volant) lifestyle, and what is the evidence for this?

>>As I noted (much) earlier, even if these adaptations were evolved in an
>>arboreal ancestor that later became terrestrial this does not conflict with
>>BADD.
>
>And to that extent, BCF is in full agreement with BADD!

This raises a very important point - as I see it the ONLY difference between
the two views that really matters is the idea that some maniraptors are
secondarily flightless - and therefore BCF does not really say that birds
did not evolve from theropods sensu lato.  All the other arguments seem to
be around whether the ancestry of the bird/maniraptor line (or perhaps the
whole theropod line) went through an arboreal stage - and I don't see anyone
ruling out that possibility.  But "arboreal" does not necessarily mean
"ancestral to birds"; it could mean "ancestral to birds", "ancestral to
theropods including birds", or "ancestral to theropds not including birds"
depending on which taxon, or which point in the phylogenetic tree, you are
talking about.

>Again, if this is how you interpret BADD, then you are also in substantial
>agreement with BCF. The difference lies mainly in the presumed lifestyle of
>that common ancestor: in BADD, it would have been a bipedal cursor; in BCF,
>an arboreal climber or glider.

Well, if you are talking about phylogeny rather than lifestyle (and they are
two different things) then your question relates to the position of
branching points in a tree, not to what the taxa occupying those branching
points were doing.  In that department it doesn't matter whether the
ancestor was arboreal or terrestrial.

If you are talking about lifestyle, then you must, I think, to have BCF say
anything new, separate adaptations to arboreality from adaptations to flight
and show that the LATTER are present in forms usually regarded as derived
from non-volant ancestors.  Which brings me back to the point above - are
you really talking about anything other than the evolution of a few
terrestrial taxa you see as secondarily flightless?  If so, then except for
these few taxa (which, if I am not mistaken, all postdate Archaeopteryx so
cannot be, in themselves, bird  ancestors), BCF and BADD are not just in
agreement, they are phylogenetically identical - and therefore identical
with respect to the origin of birds.  The question then becomes not what did
birds evolve from, but what did post-Archaeopteryx terrestial maniraptors
evolve from.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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