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Re: 11th specimen of Archaeopteryx



David Marjanovic <david.marjanovic@gmx.at> wrote:


> I forgot if Kevin Middleton's thesis has been published.

Only the part relating to modern birds:

Middleton, K.M. (2001) The morphological basis of hallucal orientation
in extant birds.  J. Morphol. 1: 51-60.

DOI: 10.1002/jmor.1058

The thesis covers a lot more ground, and describes the hallucal
morphologies of many Mesozoic birds.  Although _Archaeopteryx_
specimens may show a hallux preserved in a posterior position, this is
a preservational artifact.


> It shows that the
> orientation of the 1st toe depends on whether the 1st metatarsal is twisted,
> and shows that it's not twisted in *Archaeopteryx*.


Yes, when it comes to the notion that non-avian theropods and
_Archaeopteryx_ had a reversed hallux, Middleton put the final nail in
the coffin.

As David says, modern birds have a twisted first metatarsal that
re-orients the hallux posteriorly, so it can effectively oppose the
other three digits.  Hallucal reversion in confuciusornithids and
enantiornitheans was achieved in a slightly different manner (although
there is some twisting), with the emphasis on distal bending and
curvature of this element, making it more J-shaped.  This is most
obvious in avisaurids and other enantiornitheans that appeared to have
been specialist perchers, with the metatarsal orienting the hallux
posteriorly.  In confuciusornithids and sapeornithids, the posterior
extension of the J-shaped first metatarsal is not as pronounced, so
the hallux was oriented more medially than posteriorly.  But in
_Archaeopteryx_ (and all non-avian theropods) the first metatarsal is
dead straight.  No sign of torsion or bending at all.




Cheers

Tim