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RE: Differences between *Vancleavea* and thalattosaurs

  In addition to all this, following Parker and Barton (the OTHER paper on 
*Vancleavea campi*), *V. campi* also possesses thecodont dentition, separate 
and distinct interdental plates (also contingent on but elaborate to the 
presence of thecodonty), and (P&B, p.13):

  "Unambiguous synapomorphies supporting the inclusion of *Vancleavea campi* 
into Archosauriformes include the lack of post-axial intercentra, a femur with 
a medially inflected head, a sigmoidal femoral shaft, the lack of a distinct 
intertrochanteric fossa on the proximoventral surface of the femur, and the 
presence of osteoderms."

  Despite *V. campi* being _another_ apparently aquatically derived taxon (such 
that it SHOULD resemble things like pachypleurosaurs, mesosaurs, nothosaurs, 
metriorhynchoid crocs, etc.) *V. campi* is distinguihsed from them by a host of 
features that ensures that it is not, in fact, a nonarchosauriform diapsid. 
That is at least based so far on the two cladistic analyses to included it with 
the new data (Parker and Barton, 20009 and Nesbitt et al., 2009).

Parker, W. G. and B. J. Barton. 2009. New information on the Upper Triassic 
archosauriform *Vancleavea campi* based on new material from the Chinle 
Formation of Arizona. _Paleontologica Electronica_ 11(3)[14A]:20p. 


Jaime A. Headden

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> Date: Sat, 5 Dec 2009 14:48:12 +0100
> From: david.marjanovic@gmx.at
> To: dinosaur@usc.edu
> Subject: Differences between *Vancleavea* and thalattosaurs
> Judging *Vancleavea* from the illustrations here:
> http://scienceblogs.com/tetrapodzoology/2009/11/vancleavea_2009.php
> - Thalattosaurs retain the supratemporal. *Vancleavea* lacks it.
> - Thalattosaurs have an open lower temporal arch (the caudal process of
> the jugal doesn't reach very far back, so the lower temporal fenestra is
> open ventrally); I'm not even sure if they have a quadratojugal at all
> (none is in any case preserved in *Miodentosaurus*). It's closed in
> *Vancleavea*, the qj is large, and the caudal process of the j is very
> broad dorsoventrally, much unlike the very delicate T-shaped jugals of
> thalattosaurs.
> - The maxilla contacts the naris in thalattosaurs. In *V.*, they are
> separated by a huge caudodorsal process of the premaxilla.
> - Thalattosaurs have a very large pineal foramen. *V.* lacks any trace
> of it.
> - Thalattosaurs retain caudal ribs -- real free mobile ribs on the first
> few tail vertebrae. Can't see such a thing in the low-resolution
> reconstruction of *V.*.
> *V.* shares all these character states with all other archosauriforms,
> except for the thin jugal of... maniraptoriforms or something.
> Both, however, have a single bone called the "postorbital" in *V.* and
> the "postfrontal" in thalattosaurs... The quadrate looks similar, too,
> differing from the pillar dinosaurs have, but I'm not familiar with
> enough diapsid quadrates to tell what that means.
> - In thalattosaurs the rostral margin of the orbit is formed by the
> maxilla. In *Vancleavea*, the prefrontal does that, and (perhaps because
> the orbit is so remarkably small) it even contacts the jugal, excluding
> the maxilla from the orbit altogether. Or is the "prefrontal" of *V.*
> actually the lacrimal, which is altogether missing in thalattosaurs (or
> at least *Miodentosaurus*)?
> - *V.* has very long hemal spines. Thalattosaurs never.
> - *V.* has considerably smaller limbs, especially hands & feet, than
> thalattosaurs.
> Finally, *V.* is much smaller than *Miodentosaurus*. This means that
> things like the relatively much smaller orbit of *V.* (complete with
> taller jugal and maxilla) or its relatively much shorter extension of
> the jaw joints caudal to the occiput cannot be size-related -- the
> opposite relationship would be expected.
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