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Buckaroobwana@aol.com wrote:
<<What is the precise relationship between Stegosaurs and Ankylosaurs? Is it an ancestor/decendant one? Or are they both derived from a common ancestor? it's something I've been wondering about lately.>>

Well, they're ornithischians outside of Cerapoda.  Most analyses, my own included, come up with a monophyletic group containing Ankylosauria and Stegosauria, exclusive of all other ornithischians.  There are more characters supporting this arrangement than the presence of dermal armor, such as seven premaxillary teeth in basal taxa of both groups (premaxillary teeth are lost in later members of both groups independantly), and the presence of three ossified palpebral elements.

Scelidosaurus is generally regarded as being outside of Eurypoda ={Stegosaurus + Ankylosaurus} but Norman has recently suggested it's really a basal ankylosaurian.  Scuttelosaurus is also considered as being outside of Eurypoda, but within Thyreophora.

Tracy and George support a paraphyletic Thyreophora with stegosaurs (I think) closer to Cerapods than either are to ankylosaurs.  A similar topology was supported by Maryanska and Osmolska 1984, 1985 based on the fact that ankylosaurs had closed acetabula and all other ornithischians had open acetabula.

<<Besides, all this dino-bird talk gets tiresome after a while. This is the Dinosaur Mailing List, not the Dinosaur/Bird only list.>>

Darn right!

Steve Brusatte wrote:
<<I'm currently trying to use the literature to discern any possible characters that might be able to divide _Psittacosaurus_ into two or more genera, as Jaime Headden and Pete Buchholz have both discussed onlist before.>>

What's a genus?

<<First, there seems to be some confusion among different authors (okay, basically among the exact same author: Paul Sereno) about the presence of antorbital fossae in _Psittacosaurus_.>>

You are correct.  Sereno illustrates the AOF as being present and also not being present in P mongoliensis, and no photos I've seen show the area with enough clarity to really discern if it's there or not.  What is interesting is that no other Psittacosaurus species have an AOF present, and that basal neoceratopians retain it.

Psittacosaurus mongoliensis seems to share one character with P meileyengensis, namely the deep flange on the dentary.  This is apparently also present in Liaceratops, so it may in fact be plesiomorphic for the clade.

<<I had once read (maybe onlist) that seven species were recognized.  Sereno sees these as valid: _P. mongoliensis_, _P. sinensis_, _P. neimongoliensis_, _P. ordosensis_, _P. xinjiangensis_, and _P. meileyingensis_.  Am I missing any valid species?  Of coures, some of the others (_P. osborni_, _P. tingi_, and _P. guyangensis_ ) might be valid, but likely not.>>

P youngi is probably valid.  Sereno's referal of this species to P sinensis is a little surprising as P youngi doesn't have the extremely large jugal horns that point horizontally and are perpendicular to the sagital plane of the skull, the two characters that make P sinensis so unique.  P youngi also has a very deep rostral dentary and a square-shaped lower jaw tip, instead of the gradually tapering rostral dentary and triangular lower jaw tip seen in P sinensis and almost all other Psittacosaurus specimens.

The only species that shows a similar mandibular morphology is P neimongoliensisP youngi may in fact be a senior synonym of P neimongoliensis, but more work needs to be done to discern this for sure.

Pete Buchholz