Re: Eocursor, new basal ornithischian

["heby@libero.it" <heby@libero.it>]

> heby@libero.it wrote:
> 
> <<Butler Richard J.,Smith Roger M. H., and Norman David B., in press. A
> primitive ornithischian dinosaur from the <<Late Triassic of South Africa,
> and the early evolution and diversification of Ornithischia, Proc. R. Soc.
> B, 1-6
> 
> <<Nice specimen, look somewhat like a "fabrosaurid" with large hands, there
> is a cladogram of Ornithischian with <<basalmost Heterodontosaurids (except
> Pisanosaurus), but no mention of putative basal ornithischian like
> <<Sacisaurus, Silesaurus and possible related form (Technosaurus,
> Pseudolagosuchus, Eucoelophysis, ?Bromsgroveia, <<Spondylosoma,
> ?Caseosaurus).
> 
> Sacisaurus + Silesaurus are dinosauriforms and often referred to as
> "putative basal ornithischians"; however this has never been fully tested
> in a phylogenetic analysis featuring a wide diversity of taxa (including
> crocodile-line archosaurs as well). The characters in these taxa that
> supposedly support a relationship with the ornithischia are based on dental
> characters which can be homoplastic. 

I agree, however, some possible sinapomorphies (of the hindlimbs) listed on 
Ferigolo et al. (2006) and Dzik (2003) are not used in the recent-most 
cladogram of dinosauriforms, that of Langer and Benton (2006). Moreover, this 
analysis suffer of a problem: too few basal taxa as you stated(I confess to 
dislike the use of a single OTUs Outgroup instead of sequential taxa (like 
Euparkeria, Proterochampsids, Lagerpeton, etc.), and the use of 'Ornithischia' 
instead of various taxa like Stormbergia, Lesothosaurus, Agilisaurus, 
Heterodontosaurus, Scutellosaurus, etc. Until now no comprehensive study has 
been made to compare the above listed taxa to 'Silesaurids'.
Although I cannot say much, a forthcoming paper will show an intriguing 
position for Silesaurus...

Revueltosaurus is a good example of
> this and I'll never forget what Richard Butler told me when he saw the
> material, that the teeth of Revueltosaurus are more "ornithischian-like"
> than most ornithischians. Are there any other characters that support a
> Sacisaurus/Silesaurus + Ornithischia relationship to the exclusion of other
> dinosauriforms?

Ok, but you're telling just that teeth are homoplastic. Other characters, 
acetabulum perforation, vertebral pneumaticity, sacral vertebral count, loss of 
osteoderm, and many other are present in both crurotarsal and dinosauromoph 
archosaurian, but not for this reason we overlook these characters. My 
suggestion to end this problem would be a 'super-matrix' study of 
Archosauriformes, integrating even poorly known taxa (Hallopus, Trialestes, 
Agnostyphis, Teyuwasu....).
I think that the most convincing (to me) sinapomorphy is the presence of an 
anterior beak on the dentary, exactly like the presence of an Os rostrale is 
sinapomorphic for Ceratopsia. Even if Silesaurus don't show any evidence for a 
separate ossification, in Sacisaurus two paired element are where we would 
expect to find an unpaired predentary. I found very little parsimonious 
consider the 'proto-predentary' of Sacisaurus only a convergence with 
Ornithischian, especially when we are looking for VERY basal and Triassic taxa 
with ornithischian affinities. However, I must admit that the anatomy of 
Silesaurus is not only very basal, but also aberrant, so at present I don't 
favor any hypothesis instead of another. Although plesiomorphic in a number of 
traits, Eocursor is already a true ornithischian; its pelvic anatomy is 
radically modified and derived respect to Eoraptor, Marasuchus, Saturnalia, 
Herrerasaurus, Silesaurus etc., so compare this taxon (or Lesothosaurus) to 
basal Saurischian
 or Dinosauromorphs will be more complicated than we think.
> 
> Technosaurus is possibly closely related to Sacisaurus and Silesaurus and
> not an ornithischian.
> 
> Pseudolagosuchus and Eucoelophysis are dinosauriforms.

Both Ezcurra (2006), and Nesbitt et al. (2007), have found that femoral anatomy 
in these taxa and Silesaurus is very similar, and almost unique among 
dinosauromophs, so a reappraisal must be done to comprehend if these features 
are sinapomorphic for a 'silesaurid' group or not.

> 
> Bromsgrovea is a ctenosauriscid poposaur closely related to Arizonasaurus.
> 
> Spondylosoma is non-dinosaurian and most likely a "rauisuchid".


Mh...for both Spondylosoma and Bromsgroveia there is no reported tarsal 
anatomy, so any relationship must be deeply investigated. The paper of Galton 
reviewing Spondylosoma and Bromsgroveia were written before the pubblication of 
Sacisaurus and Silesaurus, so his conclusion on pelvic an cervical vertebrae 
anatomy must be taken with an open mind. Both taxa show similar iliac and 
vertebral anatomy (together with Caseosaurus) to 'rauisuchid' and Silesaurus 
and Sacisaurus, so...I confess to really wonder which are their true phyletic 
relationship. At present I could only suggest that only further work will give 
us a comprehensive view of archosaurian relationship. Many characters used to 
link Bromsgroveia are present also in dinosauromorphs, and as noted above, the 
peculiar iliac anatomy suffer of convergence between advanced 'rauisuchians' 
and 'silesaurids'. 

> Caseosaurus is based on an isolated ilium that shares character affinities
> with basal saurischians and sauropodomorphs.

But also with Silesaurus: the supra-acetabular crest, which is buttressed by 
the anterior iliac process, and forms a vertical ridge coming close to the 
dorsal margin of the acetabulum are features present also in Silesaurus and 
Sacisaurus, besides Plateosaurus, Efraasia (although poorly developed in these 
sauropodorphs) and 'rauisuchians'. 
I wonder if this specimen could belong to Eucoelophysis....

Best regards,

Lukas Panzarin


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