[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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.