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Re: Bid tarsus



LN Jeff wrote:

>I just finished Larry Martin et al's 1980 paper:
>
>Martin, L.D., J.D. Stewart, and K.N. Whetstone. 1980. The origin of birds:
>     structure of the tarsus and teeth. Auk 97: 86-93.
>
>     The first part of the paper deals mainly with the morphology of the
>astragalus and calcaneum.  According to the authors, the ascending process
>of the bird tarsus ossifies with the calcaneum rather then the astragalus,
>and therefore can't be homologous with the ascending process of the
>theropod astragalous.  They cite not only ontogenetic studies
>of modern birds, but Cretaceous birds (specifically Hesperornis and
>Baptornis) as having this set up.  Moreover, they restore the Archaeopteryx
>tarsus as having the same design, with the ascending process coming off of
>the calcaneum.  In thier figures, the process they show is
>definitely coming off of the calcaneum, but in Archaeoptery
>it looks more iffy to me.  Even the way they have drawn it,
>it still looks firmly attached to the astragalus.  Interestingly, although
>the paper is supposed to ally birds and crocodilians, they don't say a
>word about the crocodilian tarsus.
>    The second part of the paper deals with tooth morphology.  They
>claim that the teeth of Archaeopteryx, Cretaceus birds, and
>crocodilians have a constricted area around the base of the crown, and
>expanded roots, while theropods lack the constriction and have straight
>roots. They also note that crocodilian and bird teeth are
>unserrated (probably not terribly relevant- loosing serrations is pretty
>easy).  Also, they suggest that straight roots and no constriction is a
>basal psuedosuchian characteristic retained by theropods, and that
>constricted bases and expanded roots are a synapomorphy uniting birds and
>crocodilians.
>     In the summary, they suggest that birds and crocodilians share a
>common pseudosuchian ancestor.
>
>     I would appreciate any clarification or information on the nature of
>the ascending process of the bird tarsus and tooth morphology.  Are these
>errors, covergences or what?

Both Thulborn and Hamley (1982) and Cracraft (1986)
commented that the analyses of Martin et al. (amongst others) were either
insufficiently comparative, or that the arguments presented were not
supported by rigorous phylogenetic methods.  More specific criticism comes
from Howgate (1984, p. 173) who, in reply to the claims made for the teeth
of _Archaeopteryx_, states that, "no matter how similar are the teeth of
Cretaceous birds and of crocodiles, there is little similarity of either to
the teeth of _Archaeopteryx_.  Of the characters supposedly indicative of
close relationships only one is present, namely the lack of serrations."

Thulborn and Hamley (1982, p. 623) comment that, "Martin et al. (1980, p. 89)
have made a similar suggestion, maintaining that the ascending process of
the _Archaeopteryx_ ankle is 'primarily associated with the calcaneum'.
However, these authors present a decidedly ambiguous reconstruction of the
_Archaeopteryx_ ankle; it shows the ascending process associated equally
with the astragalus and calcaneum.  The _Archaeopteryx_ ankle depicted by
Martin et al. (1990, fig. 1G) seems, in fact, to be structurally
intermediate between the theropod ankle and the neornithiform bird ankle."

Cracraft, J. 1986. The origin and early diversification of birds.
Paleobiology, 12: 383-399.

Howgate, M. E. 1984. The teeth of _Archaeopteryx_ and a reinterpretation of
the Eichstatt specimen. Zoological Journal of the Linnean Society, 82:
159-175.

Thulborn, R. A. & Hamley, T. L. 1982. The reptilian relationships of
_Archaeopteryx_. Australian Journal of Zoology, 30: 611-634.


Chris


cnedin@geology.adelaide.edu.au                  nedin@ediacara.org
-------------------------------------------------------------------
Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.