A New Basal Hadrosaurid (Dinosauria: Ornithischia) From the Latest Cretaceous Kita-ama Formation in Japan: the Rise of Hadrosaurs

<jats:title>Abstract</jats:title> <jats:p>Here we describe a partial hadrosaurid skeleton from the marine Maastrichtian Kita-ama Formation of the Izumi Group in Japan as a new taxon, <jats:italic>Yamatosaurus izanagii</jats:italic> gen. et sp. nov. Our phylogenetic analysis demonstrates it belongs to the Hadrosauridae, composed of <jats:italic>Hadrosaurus foulkii</jats:italic> + (<jats:italic>Yamatosaurus izanagii</jats:italic> + (Saurolophinae + Lambeosaurinae). <jats:italic>Yamatosaurus izanagii</jats:italic> is unique in having only one functional tooth/row in the middle of the dentary dental battery and complete absence of the branched ridges on the occlusal surfaces. The coracoid lacks a biceps tubercle as in non-hadrosaurid hadrosauroids, suggesting its presence is a key feature for the clade of Saurolophinae and Lambeosaurinae. The evolutionary rate analysis further supports that shoulder and forelimb features, which are likely to be involved in locomotion, are important for the early evolution of Hadrosauridae. Our biogeographic analyses show that the basal Hadrosauridae was widely distributed in Asia and Appalachia, and the clade of Saurolophinae and Lambeosaurinae originated in Asia. The contemporaneous occurrence of basal (<jats:italic>Yamatosaurus izanagii</jats:italic>) and derived (<jats:italic>Kamuysaurus japonicus</jats:italic>) hadrosaurids during the Maastrichtian in Japan is the first record in Asia. Because of a long geographical distance between these localities, they may not co-exist but show some level of provinciality.</jats:p>