*Rugops* and the Origin of Abelisaurids

["Jaime A. Headden" <qilongia@yahoo.com>]

*Rugops primus* is represented only by a unique skull, MNN IGU1, which
preserves the snout, lachrymal, parietal and frontal, prootic, and
laterosphenoid. The remainder of the skull is missing. The skull is very
roughly textured, an aspect shared with all abelisaurids for with cranial
material is known, with the exception of *Indosuchus raptorius,*
(Chatterjee, 1976) and it is likely the jugal and dentary are equally
textured. This texture matches the maxilla described last year by Lamanna
et al. (2002), and the shapes are nearly identical, suggesting that this
maxilla may belong to *Rugops,* or that it belongs to a similar, basal
taxon. *Rugops* is diagnosed by the unique retention of a sagittal pair of
series of pits or foramina in the nasal, medial to enlarged ridges, and by
the presence of a "foramen" or lacuna between the frontal and the
lachrymal/postorbital contact. Unlike other abelisaurs known from skull
material (essentially, only abelisaurids), the nasals of *Rugops* are
triangular in dorsal view, prior to expanding around the external nares.

  A note should be made on the diagnosis. There are several tetanurans
with foramina on their nasals; this may be indicative of the extremely
cornified nature of the tissue dorsal to the snout. The snout of *Rugops*
was likely highly vascularized, as in other abelisaurids. The second
diagnostic feature given is, theoretically, plesiomorphic, as
pre-abelisaurids are inferred to lack this closure seen in abelisaurids;
one expects to find an abelisaur without this postorbital/lachrymal
contact. In the carnosaur *Sinraptor,* *Dilophosaurus,* and in some
tyrannosaurids, the lachrymal and postorbital come close to contacting one
another, or do so, forming a similar lacuna as noted by Sereno et al.,
indicating that while these features are unknown in abelisaurians as we
know them, it is unlikely that the condition is entirely stable as an
autapomorphy, though no abelisaurian possesses the same condition.

  The dorsal margin of the frontal directly anterior to the parietal
contact bears a fossa or pitting where the supretemporal fossae form
parallel ridges or slopes; such a feature is found in *Indosuchus
raptorius* (holotype), and in *Carnotaurus,* among other abelisaurids.
There is no anterior ramus of the lachrymal, a primitive feature for
abelisaurids apparently, and the antorbital fenestra occupies >85% of the
antorbital without lateral exposure of the maxillary foramen (unlike
*Abelisaurus*); there is a rostral lobe of the maxilla, but less distinct
than in some abelisaurids, such as *Rajasaurus,* and more so than in
*Abelisaurus* -- It is largely the same as in *Carnotaurus* and
*Majungatholus.* The naris is large and rounded, and the parietal bears
the distinctive raised, quadrangular or pyramidal nudal/supraoccipital
crest, and is more distinct than is known in *Rajasaurus,* which slopes
back rather than being vertical as in other abelisaurids. The skull roof
is also not thickened, a feature it shares with *Indosuchus,* but is
distinct from "carnotaurines" like *Indosaurus,* *Rajasaurus* (which may
be the same thing), *Majungatholus,* and *Carnotaurus,* as well as
*Abelisaurus.*

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


        
                
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