The earliest-known duck-billed dinosaur from deposits of late Early Cretaceous age in northwest China and hadrosaur evolution

A new dinosaur of Early Cretaceous age was recently discovered in the Gobi Desert of northwest China. It is more closely related to Late Cretaceous hadrosaurids than to Early Cretaceous iguanodontids. It occupies the most basal position in the phylogeny of all duck-billed dinosaurs, or the Hadrosauroidea. This early hadrosauroid sheds new light on the origin of the herbivorous feeding specializations of the Late Cretaceous duck-billed dinosaurs, and corroborates the view that the Iguanodontidae and the Hadrosauroidea are monophyletic clades, with the former characterized by an enlarged maxilla as the main mechanism for mastication, and the latter diagnosed by a smaller yet more mobile maxilla with an elaborate dental battery, separated by a diastema from the enlarged premaxilla. Our study also suggests that the Hadrosauroidea had most likely originated in Asia in the Early Cretaceous before this clade diversified and spread to other Laurasian continents during the Late Cretaceous. (C) 2003 Elsevier Ltd. All rights reserved.