Differences between *Vancleavea* and thalattosaurs

[David Marjanovic <david.marjanovic@gmx.at>]

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.