Abstract

The osteology of the holotype and referred specimens of the saurolophine hadrosaurid, Gryposaurus latidens, from the lower Two Medicine Formation of Montana (USA), is described in detail. With an estimated early Campanian (Late Cretaceous) age, this dinosaur is one of the oldest known hadrosaurids. Gryposaurus latidens shows several autapomorphies, including dentary tooth crowns, with an apicobasal length/mesiodistal width ratio of 1.7–2.2, a single median carina, and large marginal denticles in sections of the dental battery; deep and oval depression on the posteroventral premaxillary surface; and, in the context of Hadrosauridae, a shallow lacrimal process of the rostral process of the jugal. Furthermore, G. latidens differs from G. notabilis and G. monumentensis in having a broadly arcuate anterolateral oral margin of the premaxilla, a posterior margin of the narial fenestra that is wider than the dorsoventral depth of the proximal region of the narial bar, with a nasal crest that rises above the level of the frontals (in adults) and is located above the posterior margin of the narial fenestra, and wide and shallow posteroventral margin of the rostral process of the jugal, among other characters. This study confirmed a previous hypothesis supporting G. latidens as the sister taxon to a clade composed of the other species of Gryposaurus.

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