New information on the cranium of Brachylophosaurus canadensis (Dinosauria, Hadrosauridae), with a revision of its phylogenetic position

ABSTRACT

The cranium of the hadrosaurid dinosaur Brachylophosaurus canadensis is redescribed on the basis of abundant and complete material from the Lower Campanian Judith River Formation of Malta, northeastern Montana. The diagnosis of this taxon is emended, and the species Brachylophosaurus goodwini is now considered to be a junior synonym of Brachylophosaurus canadensis. Autapomorphies of Brachylophosaurus are: nasal greatly developed into paddle-like solid crest extending caudodorsally, overhanging dorsal region of skull; nasal possessing anteroposteriorly oriented groove terminating in elongated foramen located medial to prefrontal; prefrontal projecting posteriorly and resting dorsomedially over anterior process of postorbital and, more posteriorly, extending ventromedially under nasal; only anterior sharp tip of lacrimal contacting maxilla; jugal with ventrally projecting semicircular flange; extremely elongated, rod-like anterodorsal process of maxilla projecting medial to narial cavity along most of anteroposterior length of external naris; anteroposteriorly short exoccipital-supraoccipital roof posterior and dorsal to foramen magnum.

Most autapomorphies of the junior synonym species Brachylophosaurus goodwini originated from misplacement of a nasal fragment and individual variation of the jugal. The new osteological information supports Brachylophosaurus canadensis as the sister taxon to Maiasaura peeblesorum, and suggests that the two taxa form a robust and basal clade within the Hadrosaurinae. Two new characters are discussed as phylogenetically informative for hadrosaurids: the mediolateral and anteroposterior predentary proportions and the length of the prequadratic process of the squamosal.