So far we’ve looked at leptoceratopsids and chasmosaurine ceratopsids. This time, it’s the turn of the basal ceratopsoid Zuniceratops. If these terms are giving you grief, a cladogram showing a few of them is below…
Zuniceratops christopheri [adjacent life restoration by Todd Marshall] was named in 1998 by Douglas Wolfe and Jim Kirkland for several specimens discovered in the Turonian Moreno Hill Formation of New Mexico. Its generic name honours the Zuni people, while its specific name is for Christopher Wolfe, co-discoverer of the first specimen. In many features Zuniceratops was intermediate between ceratopsids and non-ceratopsids. Indeed phylogenetic studies indicate that it is the sister-taxon to Ceratopsidae (e.g., Wolfe & Kirkland 1998, Xu et al. 2002, Dodson et al. 2004, Makovicky & Norell 2006, Chinnery & Horner 2007). This newly recognized Zuniceratops + ceratopsid clade was named Ceratopsomorpha, and the wording on p. 307 of Wolfe & Kirkland (1998) shows that this name applies to the branch-based clade including these taxa. However, the name Ceratopsoidea also applies to the same clade: it was defined by Sereno (1998) as a branch-based clade that includes all taxa closer to Triceratops than to Protoceratops [cladogram below from Tom Holtz’s Marginocephalia lecture: to see diagram at full size, plus tons of additional information, go here. Note that Tom uses the term Ceratopsinae for what I’ve referred to as Chasmosaurinae; the former is technically correct but, at the moment, less familiar].
While Zuniceratops lacks the characteristic features of Ceratopsidae, it did possess elongate brow horns indicating, somewhat surprisingly, that brow horns were a primitive feature for Ceratopsoidea and Ceratopsidae. Those lineages that have short or even absent brow horns must, therefore, have reduced and lost their horns over time. This hypothesis has since been supported by additional taxa, most notably the long-horned basal centrosaurine Albertaceratops nesmoi.
Some Zuniceratops individuals possessed short brow horns, and others possessed long, gently curved brow horns. Some authors – most notably Lehman (1990) – have argued that long-horned ceratopsians exhibit dimorphism in brow horn size and orientation, and have then proposed that this variation represents sexual dimorphism. This is an interesting and plausible hypothesis (and it has been accepted by at least some students of the group); the problem is that the alleged dimorphism is slight, and indeed so slight that the range of observed variation may simply be continuous, intrapopulational variation spread across both sexes. If both sexes are similarly ornamented, we have an interesting situation – but more on that much later…
The facial region of the skull in Zuniceratops was longer and lower than that of most ceratopsids and its frill was large. Unlike those of ceratopsids, its teeth only had single (rather than double) roots.
Refs – –
Chinnery, B. J. & Horner, J. R. 2007. A new neoceratopsian dinosaur linking North American and Asian taxa. Journal of Vertebrate Paleontology 27, 625-641.
Dodson, P., Forster, C. A. & Sampson, S. D. 2004. Ceratopsidae. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 494-513.
Lehman, T. 1990. The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics. In Carpenter, K. & Currie, P. J. (eds) Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press, Cambridge, pp. 211-229.
Makovicky, P. J. & Norell, M. A. 2006. Yamaceratops dorngobiensis, a new primitive ceratopsian (Dinosauria: Ornithischia) from the Cretaceous of Mongolia. American Museum Novitates 3530, 1-42.
Sereno, P. C. 1998. A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 20, 41-83.
Wolfe, D. G. & Kirkland, J. I. 1998. Zuniceratops christopheri n. gen. & n. sp., a ceratopsian dinosaur from the Moreno Hill Formation (Cretaceous, Turonian) of west-central Montana. New Mexico Museum of Natural History and Science Bulletin 14, 303-317.
Xu, X., Makovicky, P. J., Wang, X.-l., Norell, M. A. & You, H.-l. 2002. A ceratopsian dinosaur from China and the early evolution of Ceratopsia. Nature 416, 314-317.