Pterosaurs - prolacertiforms or ornithodirans?

[Tim Williams <twilliams_alpha@hotmail.com>]

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.

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