A new species of basal hadrosaurid (Dinosauria, Ornithischia) from the Cenomanian of Texas

Abstract

ABSTRACT Protohadros byrdi, gen. et sp. nov., from the Cenomanian Woodbine Formation, Texas, is described on the basis of a disarticulated skull and isolated postcrania. The possession of the fewest synapomorphies among hadrosaurids places Protohadros at the basal position within Hadrosauridae. The combination of a derived maxillopalatal unit and a primitive quadrate indicate a level of cranial kinesis intermediate between primitive iguanodontians and other hadrosaurids. Both the phylogenetic and chronological distributions of these characteristics describe an evolutionary sequence of adaptations in which modifications for increased masticatory ability of the maxillary region precede the evolution of both streptostylic cranial kinesis and a more derived mandible in Hadrosauridae. The taxon is unique among hadrosaurids in possessing a ventrally deflected muzzle, including a deep, rostrally massive mandible, that is interpreted to represent an exclusively low-browsing pattern of food acquisition in a deltaic environment. At 95 Ma, Protohadros is the oldest definitive hadrosaurid. This plus the phylogenetic and biogeographic position of the specimen indicate that a North American origin of Hadrosauridae is equally as likely as the prevalent Asian origin hypothesis.