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>Has anyone worked out the systematics for ceratopsians?
You could say that... Try Sereno 1986 (to an extent), Dodson and Currie
1990, Lehman 1990, Dodson 1990, Forster's dissertation, Sampson's new
pachyrhinosaur material, etc. (plus the old Lull and Marsh stuff, plus some
people I must have missed).
>>Some relationships to consider:
Despite Ostrom & Wellnhofer's tentative suggestion, most evidence suggests
that this is not so. A phylogenetic (but not sexual) relationship between
Torosaurus and Arrhinoceratops is favored by many authors.
A lot of authors belive that these two, with Brachyceratops in between, are
simply growth stages for any centrosaurine (nose-horned, short-snouted)
ceratopsid, such as the two below.
>Styracosaurus/Centrosaurus (or is it Eucentrosaurus)
These two seem to be related, but male and female morphs are known for both
(Dodson 1990: the various morphs were previously referred to various
species of Styracosaurus, Centrosaurus, and Monoclonius). Also, Creisler
has found enough evidence to show that Centrosaurus is not preoccupied (the
old 1800's Centrosaurus is not valid by any stretch of the imagination), so
it is Centrosaurus.
>>How does the new pachyrhinosaurs fit in?
Sampson has shown (at SVP '94), that Einiosaurus is the sister group to the
Aceulousaurus-Pachyrhinosaurus clade, and that all these "pachyrhinosaurus"
are the sister-group to the Centrosaurus-Styracosaurus clade. Works for
Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile Phone: 703-648-5280
U.S. Geological Survey FAX: 703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092