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

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.