Cranial anatomy and systematics of Hypacrosaurus altispinus, and a comparative analysis of skull growth in lambeosaurine hadrosaurids (Dinosauria: Ornithischia)

Abstract

The cranial anatomy of the helmet-crested lambeosaurine Hypacrosaurus altispinus (Ornithischia: Hadrosauridae) is described, with a focus on ontogenetic and individual variation in phylogenetically significant characters of the cranial crest, braincase, and facial skeleton. Cranial material of H. altispinus represents a relatively complete growth series that includes crestless juveniles of less than half the size of large individuals with fully developed crests. Cranial ontogeny is compared with other lambeosaurines using bivariate morphometrics and through qualitative comparison of a size-standardized cranial growth series. Bivariate analyses reveal that the relative growth of the skull and cranial crest of H. altispinus and H. stebingeri are similar, and that Hypacrosaurus more closely resembles Corythosaurus than Lambeosaurus. Hypacrosaurus altispinus is systematically revised. The taxon is characterized by five autapomorphies, most of which are concentrated in the skull, as well as an enlarged terminal ischial foot. Maximum parsimony and Bayesian likelihood (Mk+gamma) phylogenetic analyses were conducted to test the monophyly of the genus. Hypacrosaurus monophyly is corroborated in light of new anatomical data. Although H. stebingeri and H. altispinus share few derived characters of the skull, the hypothesis that H. stebingeri is a metaspecies that represents the ancestor of H. altispinus cannot be rejected. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159, 398–434.