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Re: Yet more on pterosaur quad arm posture



Jason Brougham <jaseb@amnh.org> wrote:

> Tim, would you mind terribly sourcing any of your categorical statements? Or 
> supporting them factually in any
> manner?


Happy to.  BTW this hypothesis - that the hallux moved gradually from
a medial orientation to a posterior (caudal) orientation - is hardly
controversial.  Considering that this involves a major change in pedal
morphology, one would expect this re-orientation of the hallux to be
gradual, not immediate.


Fowler et al. (2011) are of the same view: "As such, the first
appearance of a fully reversed hallux is uncertain, and may not be
strictly definable since translocation was probably gradual."


> For example, you say " translocation of the hallux was a gradual process". 
> Can you tell us how this was
> revealed to you?


I'd advise checking out the literature on Mesozoic avialans:
_Archaeopteryx_, _Jeholornis_, _Sapeornis_, _Confuciusornis_,
_Changchengornis_, and various ornithuromorph and enantiornithean
taxa.  Refer specifically to the descriptions of the first metatarsal
(MTI): this is the element that has the greatest influence on the
orientation of the hallux.


Briefly, in archaeopterygids and jeholornithids the hallux is
apparently medially oriented, with MTI having a more-or-less straight
shaft (as in the primitive maniraptoran condition).  In
confuciusornithids and sapeornithids the MTI shaft is distally 'bent'
or curved, which has the net effect of orienting the hallux more
posteriorly (albeit not fully reversed).  This condition in
enantiornitheans was taken to an extreme, resulting in a fully
reversed hallux.  Note that these conditions represent a spectrum in
the relative orientation of the hallux (medial / posteromedial /
posterior) rather than discrete steps.  In ornithurans, the reversal
of the hallux is achieved mostly by torsion within the shaft of MTI.
Check out Middleton's work on this -  not just his 2001 paper in J.
Morph., but also his 2003 PhD. dissertation if you can get your hands
on it.


> Also, re-reading your text explaining the difference between the halluces of 
> various Avialae I cannot see any
> actual differences that you specify.


I'm not sure what the question is here.


> Lastly, how did you determine that Epidexipteryx has an elevated hallux when 
> its distal metatarsus has never
> been described, and is missing in the type specimen?


Although the distal portion of the metatarsus is not preserved, the
figures in the description (including Figure S1) show MTI visible.  I
had thought that MTI (i.e., the distal trochlea) terminated just prior
to where the distal metatarsus is broken off - but it may be that the
distal MTI is broken off too.  In that case, the distal extent of MTI
is not preserved.  So it is possible that _Epidexipteryx_ had a fully
descended hallux (i.e., not elevated). I'm happy to concede that
point, and have the position of the hallux in _Epidexipteryx_ as
"undetermined".





Cheers

Tim