Re: *Silesaurus*: the basalmost ornithischian after all?

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

> > - herbivory, with assorted dental characters:
> >     - "cheek teeth with low triangular crowns with well-developed
cingulum
> >        beneath"
> >     - "cheek teeth with crowns having low and bulbous base"
> >     - "maxillary and dentary teeth with overlapping adjacent crowns" -- 
to
> >        a very small extent in the mx, somewhat more in the d
> >     - "maxillary and dentary teeth not recurved" -- well, they are, but
> >        very little
> >     - "maximum tooth size near middle of maxillary and dentary tooth
rows"
>
> Are any of these present in Eoraptor, with its semi-herbivorous teeth?

Normally it's you who knows such things :-) I only have the photo in Glut's
encyclopedia, which could be better. Maximum tooth size is at the tip of the
snout, the teeth are variably recurved, the "low and bulbous base", which
ought to be the same as the "well-developed cingulum", is apparently present
in the 9th mx tooth, but not in the 3rd. I wouldn't say there are "low
triangular crowns", but that isn't quantified. Overlapping crowns are
apparently not present.

> > - reduced postcranial pneumaticity -- at least that's how the "chonoi"
of
> > *Silesaurus*, which fail to enter the vertebrae, and the absence of
> > pneumatic features in undoubted ornithischians can be interpreted
>
> But is this a plesiomorphy instead?  Haven't there been studies suggesting
> the supposedly pneumatic structures of erythrosuchids, crurotarsans and
such
> are actually vascular in nature?

All of them?
The "chonoi" look pneumatic, in any case.

> While only pterosaurs, sauropods and
> theropods have true pneumatic axial skeletons?

And perhaps *Thecodontosaurus*.

> > - "large lateral process of premaxilla excluding maxilla from margin of
> > external naris". [...]
>
> Eoraptor has a long process too, while Herrerasaurus' is very similar to
> ornithischians' (long and broad).  Could be a dinosaurian plesiomorphy.

*Saltoposuchus*, *Postosuchus*, *Riojasuchus* and *Euparkeria* have it, too
(fig. 10.1 and 10.2 of DA). It ought to be a plesiomorphy.

> > - "lateral swelling of ischial tuberosity of ilium"; is this the
> > antitrochanter, which according to Fraser et al. is a much more
widespread
> > feature?
>
> Yup, that's an antitrochanter.

That feature occurs in lots of archosaurs.

> > Makes at least 6 and at most 16 characters, versus 0 to 2 that argue
> > otherwise. I need not invite comments :-)
>
> Very interesting.

OK. New count... at least 6, at most 14. :o)

> > Do *Guaibasaurus* and *Saturnalia* really have completely closed
acetabula
> > like *Silesaurus*? :-o
>
> Guaibasaurus has a slightly open acetabulum, as the ilial portion is
> slightly concave ventrally.  Saturnalia's is "not fully open".

OK. Then the fully closed one of *Silesaurus* argues against its placement
in Dinosauria. Makes at least 1 and at most 3 characters :o)

Therefore I suggest that Saurischia and Ornithischia opened their acetabula
independently. A reversal in *Silesaurus*, with its unspecialized locomotion
(unlike ankylosaurs), seems unlikely to me.