re: Pterosaurs - prolacerts or ornithos?

[David Peters <davidrpeters@earthlink.net>]

Tim ~

Sometimes what you _don't_ see is more important than what you do see.
What I see here is classic misdirection, as in a card trick or
creationist arguments.

Kellner ignored the opposition, as a number of others have also done
recently  ?  and he took one ankle and made a case, rather taking a
suite of characters to avoid being hoodwinked by convergence. Here the
ankles are wonderfully convergent. And hard to tell, because the parts
don't come labeled, but the medial distal tarsal is the centrale (Peters
2000).

I'm shaking my head in disappointment at Kellner's pre-cladistic
thinking.

David Peters
St. Louis



This study suggests that pterosaurs are closer to dinosauromorphs than
to basal archosauromorphs (prolacertids, tanystropheids, etc). I doubt
if
it's the last word...

Kellner, A.W. (2004). The ankle structure of two pterodactyloid
pterosaurs from the Santana Formation (Lower Cretaceous), Brazil.
Bulletin AMNH
285: 25-35.

ABSTRACT: The extremely well-preserved tarsus of the tapejarid
_Tapejara_ sp. and the anhanguerid _Anhanguera piscator_ (Pterosauria,
Pterodactyloidea) are described and regarded as representative of the
ankle structure of Pterosauria. The pterosaur ankle joint (PAJ) shows
the
following features: astragalus mediolaterally elongated forming a
hemicylinder; proximal part of the astragalocalcaneal contact
characterized by a
ridge bordered on each side by a depression on the astragalus that has a
perfect counterpart in the calcaneum, and distal part that is
concavo-convex, with the concavity present in the astragalus; calcaneum
extremely reduced not reaching the posterior portion of the tarsus;
absence of an astragalar posterior groove, perforating foramen,
calcaneal tuber, and astragalar ascending process; proximal tarsals
fusing very
early in ontogeny, forming a tibiotarsus. The main movement between the
crus and foot in the PAJ occurs between the proximal and distal tarsals
as in the advanced mesotarsal-reversed joint (AM-R). The main
differences from the latter are the lack of an ascending process and the
extreme
reduction of the calcaneum that make the PAJ unique. The absence of an
astragalar groove and the reduction of the calcaneum reinforce the
hypothesis that pterosaurs are basal ornithodirans and closely related
to the Dinosauromorpha. As has been demonstrated by this and other
studies, the ankle structure (a complex of characters) is
phylogenetically informative and, in light of characters from other
parts of the animal's
body, can contribute to a better understanding of archosaur
relationships.