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Re: 11th specimen of Archaeopteryx



Jason Brougham <jaseb@amnh.org> wrote:

> OK, but Li and Zhang (2008) looked at the taphonomy and anatomy of several
> specimens just to answer this question. They found the ungual curvature
> was 113.1 degrees, and that metatarsal I has undergone some morphological
> changes that result in a partially reversed hallux.


This abstract is very interesting.  Nonetheless, it mentions
morphological changes of phalanx I-1, not metatarsal I.  In Li and
Zhang's words: "... the first phalanx of pedal digit I had undergone
some morphological changes to enable the initial reversal of the
hallux, prior to the medial rotation of metatarsal I as seen in most
modern birds. The orientation of the phalanx I-1 is assumed to have
contributed to the reversal of the hallux."


This is tantalizing, but not especially informative when it comes to
said "morphological changes".  The hallux of _Jeholornis_ (and
_Archaeopteryx_) is quite low on the foot, and elongated, compared to
the basal theropod condition.  The ball-like articulation of the
phalanx in _Archaeopteryx_ and _Confuciusornis_ could have allowed the
first toe to be swung medially - but certainly not posteriorly
(plantarly).  However, if this condition was present in _Jeholornis_ ,
and if the articulation or the shaft of the phalanx was modified, then
it could have allowed a more posteromedial orientation for this digit.
 But in the absence of hard anatomical data (not provided in the
abstract) this is just pure speculation on my part.



> Li, Z. & Zhang, Y. 2008. Reconstructing the habits of Jeholornis prima.
> In: Abstracts of the 7th International Meeting of the Society of Avian
> Paleontology and Evolution, Sydney, 18-22 August 2008, p. 11
>
> The habits of Jeholornis are reconstructed based on measurements of pedal
> claw arcs, reexamination of the
> reversed hallux and statistic analysis of the major hindlimb bone
> proportion. The average curvature of the
> pedal claw of digit III in five specimens of Jeholornis is 113.1°, similar
> to that of extant perching birds.
> Since early avians were usually two dimensionally preserved, the
> recognition of the reversed hallux in the
> most basal birds, e.g., Archaeopteryx and Jeholornis, is often
> controversial. Our observation shows that
> during the evolution of a reversed hallux, the first phalanx of pedal
> digit I had undergone some
> morphological changes to enable the initial reversal of the hallux, prior
> to the medial rotation of metatarsal I
> as seen in most modern birds. The orientation of the phalanx I-1 is
> assumed to have contributed to the
> reversal of the hallux. Three-dimensional reconstruction method is also
> applied in the identification and
> reconstruction of the disputed orientation of the hallux in Jeholornis. By
> comparing the feet in various
> individuals of Jeholornis, we notice that the observed direction of the
> hallux seems to be related to the
> preservational process of the pedal digits to some extent. The hallux
> tends to be preserved as opposed in
> ventral view, but not opposed in dorsal view. We conclude that Jeholornis
> generally possessed a reversed
> hallux, though not well developed as in more advanced birds. Ternary
> diagrams are used to analyze the
> relative contribution of the three main segments (femur, tibiotarsus,
> tarsometatarsus) to the total hindlimb
> length in Jeholornis and other basal birds, indicating a pattern of
> progressively increased arboreality and
> change in locomotion style in early avian evolution. In conclusion, the
> above-mentioned evidence suggests
> that Jeholornis was arboreal or mainly arboreal in habit.
>
>