Zuniceratops and the early acquisition and alleged dimorphism of ceratopsian brow horns


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.

More like this

Is it possible to link the evolution of cranial ornamentations, larger size and (apparently) faster speciation rate of ceratopsids (among ceratopsians)?
I think yes: go here.

Excellent post!

I wonder if anybody looked at pelvises and frills/horns of ceratopsians or, for that matter, other ornithiscians. Maybe ones with long ornaments had narrow pelvises and ones with small ornaments had broad pelvises? Similar approach was tried sometime to sex T-rex.

Second question: how ceratopsian feet looked like? Were they broad, like marsh-dwelling ungulates, or narrow, adapted to the dry ground? This could be relatively easy way to learn paleoecology of different ceratopsian genera.

What I've wondered is if Albertaceratops nesmoi is actually an sister genus or even an adult specimen of Avaceratops lammersi. The two fossils correlate to about the same time (Judith River Formation in the U.S. and the Oldman Formation in Canada). Avaceratops too posesses brow horns, and if Scott Hartman's restoration of the skeleton is correct, then the supposed "nose horn" seen in Hartman's restoration was never uncovered. And as we know from the discovery of Albertaceratops, nose horns are not exactly a given on brow-horn ceratopsians. Not to mention I have seen Avaceratops put in as a basal centrosaurine numerous times.

Since most of the basal ceratopsians have horns, it seems likely that the brow horns originally evolved for defense, like many of the horns of modern day animals. If they were used in species identification, as has been put forward by many scientists, that purpose appears to be secondarily derived. Not to mention while the idea stands up well for the centrosaurines, it doesn't work well with the chasmosaurines, many of which all have very similar horn arrangements on the skull. As for the frill, it was probably always used for display or intimidation.

By Metalraptor (not verified) on 21 Apr 2009 #permalink

For shame, Todd! You put hooves on all five fingers instead of just the first three!


So we know now where Ceratops goes?

By David MarjanoviÄ (not verified) on 21 Apr 2009 #permalink

Putting hooves/claws on digits IV and V is still incredibly common in dino-art. I used to tell everyone to stop doing it, but no-one else seems to care, so I've given up. However... claw/hoof-shaped unguals are supposedly present in heterodontosaurids (digit IV), some basal ornithopods (digit IV), hadrosaurids (digit V), and Avaceratops (digit IV). Incidentally, digit IV claws are present in a few sphenosuchians and crocodyliforms too. It does still seem that digits IV and V were blunt stumps across the majority of archosaurs.

Would subtle sexual dimorphism suggest something of herd dynamics? I remember reading that in species with horns but small gender differences, the visible similarity allows juvenile males to reach close to full adult size before being ousted from a herd by the dominant male (without olfactory differences, male / potential rival and female / potential partner young couldn't be easily told apart). What this implies for some adult gender differences or similarities might be interesting.

There we go with Avaceratops again.

By Metalraptor (not verified) on 21 Apr 2009 #permalink

Metalraptor, I have a paper describing what's thought to be an adult specimen of Avaceratops and, if the authors are right, then it's quite a unique ceratopsian. Still has a solid frill, but the postorbital horns are small and slightly procurved. It almost looks like a pygmy Triceratops if memory serves...

As for Albertaceratops, that's a very distinct genus. Not just because of the bannana-shaped nasal boss and long postorbital horns, but also because it has very odd epoccipitals.

Thanks Zach, just wondering.

Of course, one could always go the Hornerian route and say that any differences in the anatomy could be chalked up to ontology.

By Metalraptor (not verified) on 21 Apr 2009 #permalink

Interesting, because Camptosaurus and Iguanodon lacked unguals on the 4th finger, as goofy as it looks.

any differences in the anatomy could be chalked up to ontology.

Ontogeny, you mean? Development?

By David MarjanoviÄ (not verified) on 21 Apr 2009 #permalink
any differences in the anatomy could be chalked up to ontology.

Ontogeny, you mean? Development?

Well, I suppose ontology has to account for ornithopod manual anatomy (along with everything else)...

Funny thing is, I actually saw "ontogeny" when I read that the first time. Guess sometimes you see what you expect to see...

By William Miller (not verified) on 22 Apr 2009 #permalink