A sub-adult skull ofHypacrosaurus 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.

Ontogeny in the tube-crested dinosaurParasaurolophus(Hadrosauridae) and heterochrony in hadrosaurids

“…7A). This contrasts with the more sinuous suture seen in juvenile and adult Hypacrosaurus , Corythosaurus , and Lambeosaurus (Evans, 2010; Brink et al, 2011), but more closely matches the fairly straight suture (where it can be discerned) in specimens of Parasaurolophus (Sullivan & Williamson, 1999). Similarly, the sutures with the maxilla, lacrimal, and prefrontal, where they can be discerned, are straight, much closer to the condition in Parasaurolophus than in lambeosaurins.…”

“…7A). The presence of a premaxilla-nasal fontanelle contrasts with its absence in adult Parasaurolophus and Hypacrosaurus altispinus of all ontogenetic stages, but is similar to juvenile Corythosaurus , Lambeosaurus , and Hypacrosaurus stebingeri , and probably also Kazaklambia convincens (Bell & Brink, in press; Horner & Currie, 1994; Evans, Forster & Reisz, 2005; Brink et al, 2011). Unlike juvenile lambeosaurins or Kazaklambia convincens , the fontanelle is exceptionally dorsally placed relative to the rest of the crest in RAM 14000.…”

“…The caudomedial corner of the squamosal hooks upward in lateral view, and the dorsal surface of the squamosal is entirely convex. The medial extents of the squamosals are not preserved, so we cannot determine if they contacted each other as in most lambeosaurins, Kazaklambia convincens and adult Parasaurolophus , or were separated by the parietals as in Velafrons (Bell & Brink, in press; Gates et al, 2007; Brink et al, 2011). …”

“…8D). This morphology is unique relative to the rounded or squared contact in lambeosaurins and adult Parasaurolophus , where the sutures can be determined (Evans, Reisz & Dupuis, 2007; Brink et al, 2011). It also differs from Kazaklambia convincens , where a prong of the paired frontals inserts between the nasals on the midline (Bell & Brink, in press).…”

“…12C–12E). Although the ceratobranchials of hadrosaurids (including Hypacrosaurus sternbergii , adults of Saurolophus osborni , Lambeosaurus lambei , and Corythosaurus casuarius , as well as juveniles of Hypacrosaurus altispinus and H. stebingeri ) previously have been described as generally uniform (Ostrom, 1961; Gates et al, 2007; Brink et al, 2011), the morphology of these elements in RAM 14000 has some unique aspects. These differences may be taxonomic or perhaps ontogenetic.…”