Re: Triassic archosaur bipedality and cursoriality

[Tim Williams <tijawi@gmail.com>]

Dan Chure <danchure@easilink.com> wrote:

> Fair enough, what you wrote was "don't appear to have been terribly useful"
> so my restating of that was inaccurate.


No Dan, your quote was accurate.  :-)  To clarify what I meant a, I
opined that in "many non-avian theropods the forelimbs don't appear to
have been terribly useful".  Given that many theropods had forelimbs
that were short and/or had limited anterior reach and/or were clothed
in large feathers, this limited the ability of the forelimbs to seize
prey.  "Forelimb-first" predation appears to have been quite rare
among theropods, although possible for certain long-armed
deinonychosaurs (where the feet also played a major role).


> Nevertheless, I suspect they were
> still of more use than that. As for carnotaurines, Carnotaurus has, as I
> recall, a rather large ungual on the manus, which to me looks like it might
> well have had some function.


This is the conical element tentatively identified by Ruiz et al.
(2011) as the ungual of phalanx of manual digit III of _Carnotaurus_.
Even if _Carnotaurus_ had an ungual phalanx (or more than one), I
wouldn't read too much into this in terms of function.  Kiwis
(_Apteryx_) can also retain a manual phalanx, although apparently the
presence of this particular phalanx is variable between specimens.


> Has anyone looked at how forelimbs become
> reduced and lost in vertebrates, i.e. are there patterns of reductions (such
> as reduction and loss of unguals before reduction and  loss of rest of
> digit, etc.) that might be of predictive value when looking at things such
> as the odd forelimbs of carnotaurines or alvarezsaurids?  If we can discount
> that the limbs are being reduced, rather  than dramatically specialized,
> that would be a good starting point for discussions and investigation.


The fact that wing-claws are present in some ratites, but not all, and
that this phenotype seems to be variable in kiwis, suggests that
loss/retention of unguals is unconnected with forelimb reduction or
loss of function.


Similarly, _Carnotaurus_ may retain at least one ungual (depending
upon interpretation), but the articulated manus of the closely related
_Aucasaurus_ preserves no unguals.






Cheers

Tim