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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
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?