Guest Post: presenting Diabloceratops eatoni

Dr. James Kirkland (L), and Paleontologist Don Bileux (R) who discovered the skull, answer questions about the Ceratopsian skull from children from the Cosgriff Catholic School. Scott Sommerdorf / The Salt Lake Tribune

This newly named ceratopsian dinosaur, based on a superbly preserved and amazing looking skull has already been doing the rounds in the media and on various blogs. Lead author Jim Kirkland has been good enough to pen this guest post on the discovery of the specimen and its importance in terms of  the evolution of the group and the characters of the skull within the clade.

Well, on May 28^th Diabloceratops eatoni became a part of the ceratopsian pantheon with the publication of “New Perspectives on Horned Dinosaurs: The Royal Tyrell Museum Ceratopsian Symposium” only two and a half years after the symposium. The road was long; Don DeBileux found the skull in 2002 as part of a paleontological inventory we (Utah Geological Survey) were conducting of the middle Campanian Wahweap Formation for the Grand Staircase – Escalante National Monument (GSENM) in southern Utah. Don and many volunteers from Utah Friends of Paleontology spent many long hours rock sawing the skull out of the hard sandstone. It sat for several years until GSENM paleontologist Alan Titus secured free helicopter transportation to get the skull block to the road. Don then spent over 800 hours freeing the beautiful skull from the sandstone. The skull was beautiful and undistorted, but much of the right side was missing, although all aspects of the skull were represented. This was followed by Rob Gaston of Gastondesign doing his usual skilled job molding and reconstructing the missing elements to construct a full reconstruction of the skull. Given opportunities to study all the other North American ceratopsian skulls and thanks to my friend Hailu You made several trips to China, I was able to fully appreciate the distribution of character states in this clade and place it in its proper phylogenetic context and there lies the importance of this specimen.

Diabloceratops. Image courtesy Jim Kirkland.

Sure, for the moment, it is the oldest known ceratopsid, sure it is spectacularly ornamented, sure it is the first centrosaurine described south of Montana, and sure it is clearly a basal centrosaurine based on the presence of the derived stepped-squamosal and nasal-premaxillary process and in having long brow-horns as in the ceratopsid sister taxon Zuniceratops. But Diabloceratops also has a well-developed character, whose significance had not been realized before this discovery. It possessed a well-developed opening at the contact of the nasal, premaxilla, and maxilla forward of the antorbital fenestra that we call the accessory antorbital fenestra (AAF). Following examination of the distribution of this distinct character much was revealed regarding the origin and radiation of the ceratopsids.

Diabloceratops head on. Image courtesy Jim Kirkland.

Diabloceratops skull, rear view. Image courtesy Jim Kirkland.

First, among more basal neoceratopsians (i.e. “protoceratopsids”), only Magnirostris and Bagaceratops share this distinct character. Protoceratops itself has a well-developed fossa in this position, but there is no opening. The sole specimen of Hailu You and Dong Zhiming’s (2003) Magnirostris dodsoni at the IVPP has a well-preserved AAF of virtually the exact morphology as that in Diabloceratops. Additionally, Magnirostris possesses tiny orbital horns. Thus, the distribution of this character in the Asian “protoceratopsian grade” neoceratopsians eliminates the last hope of preserving a distinct clade for Protoceratops and its closest sister taxa. The resulting phylogenetic scenario indicates that Protoceratops, Bagaceratops, Magnirostris, represent a progression of taxa with increasing affinities to the ceratopsids.

Our own Zuniceratops, with its distinct mosaic of primitive and derived characters, is well accepted as a more derived sister taxon to the ceratopsids, but as it is represented by disarticulated skulls; initial reconstructions suggested that it possessed a distinctly enlarged antorbital fenestra. Reexamination indicates that it possessed an AAF like that of Diabloceratops.

Variaiton in ceratopsian skull characters. Diabloceratops is listed as the 'Last Chance' specimen. Image courtesy Jim Kirkland.

Also of some significance is the distribution of the AAF in the more derived ceratopsids. It is not known in more derived centrosaurines, but a reduced AAF between the maxilla and nasal is present in specimens of Chasmosaurus with large brow horns, suggesting a reappraisal of the species of Chasmosaurus is needed, and in the southern chasmosaurines Pentaceratops and Aguaceratops. Therefore, given the various evolutionary scenarios floating around for ceratopsids, it would appear that the AAF was lost in at least 3 times in the Ceratopsia. Rigorous studies phylogenetic studies of ceratopsians are under way and we will see how our phylogenetic hypothesis holds up.

Ceratopsian phylogeny. Diabloceratops is listed as the 'Last Chance' specimen. Image courtesy Jim Kirkland.

Given that our hypothesis of the phylogenetic significance of the AAF holds up under further scrutiny, why would something that was surely of adaptive significance in the derived “protoceratopsian grade” neoceratopsians be repeatedly lost in the Ceratatopsidae? Our hypothesis for this is simple given our acceptance of Scott Sampson’s hypothesis that ceratopsid horns were used in intraspecific combat; i.e. wrestling matches. Such combat would put severe strain on the skull of these horn-bearing ceratopsids and lead to selective pressures to strengthen the skull and the subsequent loss of the AAF. Thus, it was good at the time, but on second thoughts…..

Our take home message, to be tested by the paleontological hordes that follow, is that the identification of a significant new character can actually be more important than describing a new dinosaur, no matter how cool it is.

Eaton, J. G., and J. I. Kirkland. 2003. Nonmarine Cretaceous vertebrates of the Western Interior Basin; pp. 263–313 in P. J. Harries (ed.), High-Resolution Approaches in Stratigraphic Paleontology; Topics in Geobiology, Volume 21. Kluwer Academic Publishers, Boston .

KirklanKirkland, J. I., and DeBlieux, D. D. 2010 New basal centrosaurine ceratopsian skulls from the Wahweap Formation (Middle Campanian), Grand Staircase– Escalante National Monument, southern Utah ; in Ryan, M.J., Chinnery-Allgeier, B.J., and Eberth, D.A. (eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Bloomington, Indiana University Press, p. 117 – 140.

Ryan, M. J. 2007a. A new basal centrosaurine ceratopsid from the Oldman Formation, southeastern Alberta . Journal of Paleontology 81:376–396.

Sampson, S. D. 1997. Dinosaur combat and courtship; pp. 383–393 in J. O. Farlow and M. K. Surman (eds.), The Complete Dinosaur. Indiana University Press, Wayne , Indiana .

WolfWolfe, D.G., Kirkland , J.I., Smith, D., Poole , K., Chimmery-Allgeier, B., and McDonald, A. 2010 Zuniceratops christopheri: The North American ceratopsid sister taxon reconstructed on the basis of new material; in Ryan, M.J., Chinnery-Allgeier, B.J., and Eberth, D.A. (eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Bloomington , Indiana University Press, p. 91 – 98.

Witmer, L. M. 1997. The evolution of the antorbital cavity of archosaurs: A study of soft tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Society of Vertebrate Paleontology Memoir 3: 1-73.

You, H.. and P. Dodson. 2004. Basal Ceratopsia; pp. 478–493 in D. B. Weishampel, P. Dodson, and P. Osmólska H. (eds.) The Dinosauria (Second Edition). University of California Press, Berkeley , California .

You, H. and Z. Dong. 2003. A new protoceratopsid (Dinosauria: Neoceratopcia) from the Late Cretaceous of Inner Mongolia, China. Acta Geologica Sinica–English Edition 77:299–303.