The late surviving 'duck-billed' dinosaur 'Augustynolophus' from the Upper Maastrichtian of western North America and crest evolution in Saurolophini

Abstract

We amend the taxonomy and provide new anatomical information on the hadrosaurid dinosaur 'Saurolophus morrisi' (upper Maastrichtian Moreno Formation, central California, USA) derived from full preparation of the referred skull roof. The cranial morphology of this species is distinct enough to justify the new combination 'Augustynolophus morrisi' gen. nov. The morphology of the nasals and surrounding cranial bones indicates that 'A. morrisi' sported a solid nasal crest ending in an elongate triangular plate that extended above the skull roof. Autapomorphies include a crescentic base of the frontal caudodorsal process and extension of the process caudal to the frontal 'dome'; distal end of nasal crest with knob-like process inflected rostrally; circumnarial depression lightly incised and weakly emarginated, adjacent to caudolateral margin of nasal and occupying two-thirds the width of lateral surface of distal region of crest; and caudal surface of distal nasal crest subrectangular. We formally establish the new tribe Saurolophini consisting of 'Prosaurolophus', 'Augustynolophus' and 'Saurolophus'. Saurolophin synapomorphies include a premaxilla with broad arcuate contour of rostrolateral region of thin everted oral margin and flat and steeply inclined occlusal surface of dentary dental battery, among other characters. Saurolophin crests evolved towards increasing caudodorsal length, along with caudal extension of the circumnarial fossa and involvement into the crest of adjacent facial elements. 'Augustynolophus' is the second described genus of North American late Maastrichtian hadrosaurids. Its recognition implies a greater diversity among late Maastrichtian dinosaur faunas than previously recognized and is congruent with hypotheses of endemism and/or provinciality during Late Cretaceous time.