*Silesaurus*: the basalmost ornithischian after all?

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

In its description, *Silesaurus* is provisionally excluded from Dinosauria
on the basis of

- its very proximally located deltopectoral crest (but it is long enough for
a dinosaur)
- and its lack of epapophyses (which according to Fraser et al. [2002,
description of *Agnosphitys*] are only known in "herrerasaurs and
theropods" -- did they overlook...?).

But it shares with ornithischians, to the exclusion of (basal) saurischians,
the following features (quotes from Glut's encyclopedia, p. 62):

- 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"
- a beak on the tip of the lower jaw. There is no predentary, but because
the beak must have come before the predentary (assuming that the predentary,
like the rostral, evolved to give additional support to the beak), this
should not exclude *Silesaurus* from being the basalmost ornithischian.
- 4 sacrals (respectively 4 fused vertebrae, 2 of which contact the ilia).
Ornithischians start at 5 (all of which contact the ilia), basal
saurischians retain 2.
- the lack of the 5th toe
- a (slight) crossing of radius and ulna
- 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
- "large lateral process of premaxilla excluding maxilla from margin of
external naris". I don't know how large large is, but, judging from the
excavations on the nasal and mx, there was such an unpreserved process. Even
though ns and mx still met underneath it.
- "prefrontal with long caudal ramus overlapping frontal". Again not
quantified, but, judging from the facet on the frontal, the unpreserved prf
did overlap it for over 1/5 of the latter's length. But I wonder if that
isn't a plesiomorphy. It is a synapomorphy of Dinosauria (or something more
inclusive) that the frontal participates at all in the margin of the orbit,
right?
- "palatal process of premaxilla horizontal or broadly arched" -- not quite
clear but likely from fig. 6.
- "quadrate massive, elongate" -- yes, if I've correctly interpreted the
absent quantification. Looks pretty standard to me, like in theropods. Hm.
- "lateral swelling of ischial tuberosity of ilium"; is this the
antitrochanter, which according to Fraser et al. is a much more widespread
feature?
- absence of gastralia, assuming we can trust the fact that none are
preserved. But clavicles, sterna and many other bones aren't preserved
either, so this is absence of evidence.

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

Do *Guaibasaurus* and *Saturnalia* really have completely closed acetabula
like *Silesaurus*? :-o (Regarding *Saturnalia*, there is a paper on its
hindlimb anatomy in the latest PaleoBios. I haven't had time to copy or read
it yet, but will probably do that tomorrow.)

Oh, and since when is *Chindesaurus* a poposaurid, as stated on p. 571?
Fraser et al. say it has an ascending process on the astragalus.