A northern occurrence of Hypacrosaurus altispinus (Dinosauria: Hadrosauridae), with discussions on generic monophyly and hypodigm construction

While the Late Cretaceous dinosaur faunas of southern and northern Alberta, Canada, are well studied, comparable faunas of central Alberta are more poorly known. Herein is described a new occurrence of the lambeosaurine hadrosaurid Hypacrosaurus altispinus Brown, 1913 from the Horseshoe Canyon Formation in Leduc, Alberta. The skull of this specimen is well preserved, allowing its identification as the northernmost-known occurrence of this taxon. This specimen preserves an unusual, possibly paedomorphic feature of the premaxilla, as well as a keratinous beak, and two modes of scale preservation. The addition of this specimen to the known sample of H. altispinus reaffirms the derived cranial morphology of this species, calling attention to morphological separation between it and Hypacrosaurus stebingeri. Comparison of the skull and vertebrae of this specimen and other H. altispinus to other Lambeosaurinae reveals problematic past diagnoses for the genus Hypacrosaurus; crest dimensions, fontanelle closure, and neural spine height are non-diagnostic for this genus at present. Hypacrosaurus stebingeri has a problematic history of study and may be a wastebasket taxon; the new combination Corythosaurus stebingeri is proposed.