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Differences between *Vancleavea* and thalattosaurs
Judging *Vancleavea* from the illustrations here:
- 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
- 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
- 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
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.