Cranial osteology and ontogeny of 'Saurolophus angustirostris' from the Late Cretaceous of Mongolia with comments on 'Saurolophus osborni' from Canada

Reanalysis of the skull of the crested Asian hadrosaurine 'Saurolophus angustirostris' confirms its status as a distinct species from its North American relative, 'Saurolophus osborni'. In addition to its greater absolute size, S. angustirostris is differentiated from 'Saurolophus osborni' by an upturned premaxillary body, a more strongly reflected oral margin of the premaxilla, the absence of an anterior notch in the prenarial fossa, a sigmoidal contour of the ventral half of the anterior process of the jugal, a shallow quadratojugal notch on the quadrate, and by a strongly bowed quadrate in lateral view. Phylogenetic analysis corroborates a sister taxon relationship between 'S. angustirostris' and 'S. osborni'. 'Saurolophus' itself is characterised by a solid, rod-like crest composed of the nasals, frontals, and prefrontals; secondary elongation of the frontal and prefrontal resulting in the backwards extension of the frontal platform; a frontal platform that extends dorsal to the anterior portion of the supratemporal fenestra; a parietal that is excluded by the squamosals from the posterodorsal margin of the occiput; and the presence of two supraorbital elements. Although the palaeobiogeographic history of 'Saurolophus' remains unresolved, at least two possible dispersal events took place across Beringia during the late Campanian leading to the evolution of the clade composed of 'Kerberosaurus', 'Prosaurolophus', and 'Saurolophus'.