LAUSR

Abstract Macrogryphosaurus gondwanicus is the largest Elasmaria (Dinosauria, Ornithischia) known up to the date, from the Upper Cretaceous of Patagonia. The present contribution provides a detailed description of Macrogryphosaurus anatomy based on the holotype and only known specimen. This dinosaur presents several unique features along its vertebral column that distinguishes it from other ornithopods: the neck is notably elongated by the acquisition of an additional cervical vertebra, having 10 cervical plus 14 dorsal vertebrae, being the number of dorsals minor than in other taxa, which usually have 16. This implies that Macrogryphosaurus represents a new ornithopod morphotype, with long neck and short thorax. Moreover, three autapomorphies are here added to the original diagnosis of Macrogryphosaurus: presence of poorly developed diapophyses in the cervical vertebrae, anterior and posterior processes on the boat-shaped haemal arches, and a pubic foot in the postpubic process. The dorsoventral length of haemal arches indicate the ventral limit of the M. caudofemoralis longus and their posterior tapering so as the dorsal limit of the M. ilio-ischiocaudalis and its posterior ascension and dominance in the posterior-most portion of tail, resembling to the tail of some coelurosaur dinosaurs.