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Pterosaurs - prolacertiforms or ornithodirans?
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:
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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