A sub-adult skull of Hypacrosaurus stebingeri (Ornithischia: Lambeosaurinae): anatomy and comparison

Abstract

A well-preserved skull from a previously unknown growth stage for the lambeosaurine hadrosaurid Hypacrosaurus stebingeri from the Campanian of western North America is described. This skull is equivalent in size to the smallest known growth stages for Corythosaurus, Hypacrosaurus altispinus and Lambeosaurus, and allows for a direct comparison of the juvenile growth stage of all four taxa for the first time. H. stebingeri is not diagnosed by any recognised autapomorphies, and the morphology of juvenile skulls is similar for all four taxa. Juveniles and sub-adults of H. stebingeri are characterised by an incipient cranial crest formed predominantly by the nasal, elongate narial openings with a reduced premaxilla-nasal fontanelle and a conspicuous bifurcation at the rostral end of the nasal. The bifurcation of the nasal is virtually identical to juveniles and sub-adults of Corythosaurus. These cranial morphologic similarities support the phylogenetic hypothesis that Hypacrosaurus is more closely related to Corythosaurus than to Lambeosaurus.

Keywords:

• Hadrosauridae
• ontogeny
• juvenile
• Campanian
• Two Medicine Formation

Acknowledgements

For helpful discussions we thank J. Anderson, C. Brown, P. Currie, R. Cuthbertson, E. Snively and J. Theodor. We are grateful to T. Gates and D. Weishampel for manuscript reviews and valuable discussion. For access to collections and specimen information, the authors would like to thank the following people: S. Conaghan, A. Dzindic and J. Issa of Canada Fossils; M. Feuerstack, A. McDonald and K. Shepherd of the CMN; C. Ancell and B. Baziak of the MOR; M. Manabe of NSM; B. Iwama and K. Seymour of the ROM; T. Courtenay, K. Leiper, R. Russell, B. Strilisky and J. Wilkie of RTMP; and D. Evans and W. Ripley of TCMI. This research was funded by grants to KSB from the Dinosaur Research Institute, the Jurassic Foundation and the University of Calgary.

Notes

^† Current address: Department of Ecology and Evolutionary Biology, University of Toronto at Mississauga, 3359 Mississauga Road N., Mississauga, ON, Canada L5L 1C6.