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

Re: Vancleavea campi - what is it? - not an archosauriform.

 David [P.], I am not sure what you mean about thalattosaurians, but
 thalattosaurus (from *Askeptosaurus* to *Dakosaurus*) possess both
 supratemporal fenestrae as well as antorbital fossa and fenestrae.

Hang on a second, Jaime. You're confusing Thalattos_aur_ia, a clade of Triassic marine diapsids of rather uncertain affinities (though they're probably somewhere close to the ichthyosaurs, somewhere around the base of the diapsid crown-group), and Thalattos_uch_ia, a clade of Jurassic marine crocodyliforms. *Askeptosaurus* belongs to the former (as do *Thalattosaurus*, *Clarazia*, *Hescheleria*, *Xinpusaurus*, *Miodentosaurus*), *Dakosaurus* to the latter (like *Steneosaurus*, *Teleosaurus*, *Machimosaurus*, *Metriorhynchus*, *Geosaurus*).

The supratemporal fenestra is always small in thalattos_aur_s when it's present; in some, the upper temporal bar (formed by postorbital and squamosal) approaches the parietal so closely as to reduce the fenestra to a slit, and in some it even touches the parietal, eliminating the fenestra altogether.

Thalattos_uch_ians retain the antorbital fenestra of their archosaurian ancestors. Thalattos_aur_s retain the _lack_ of an antorbital fenestra from their non-archosaurian ancestors. (I'm not aware of any antorbital fenestrae outside of Archosauriformes and the non-amniote Chroniosuchia, unless you count the naris extension of the trematopid temnospondyls or the orbit extension of the baphetids.)

In lacking both the antorbital fenestra and the supratemporal fenestra, *Vancleavea* does indeed resemble the thalattos_aur_s. In particular, where the supratemporal fenestra would be, we find a straight rostrocaudal contact between the parietal and a very narrow upper temporal bar, just like in those thalattos_aur_s that have completely lost the fenestra.

Now, I still haven't read the *Vancleavea* paper, so I don't know if there's more than these two characters, though of course I suspect there are when the paper has 51 pages!

 If, as previously argued, you think the diapsid condition
 cannot reverse to the anapsid condition,

That would not be defensible -- the loss of the supratemporal fenestra happens gradually _within_ Thalattos_aur_ia (or perhaps rather Thalattosauriformes... the nomenclature is a bit strange in that part of the tree).