Key words: Dinosauria, Ornithischia, Marginocephalia, Psittacosaurus, Cretaceous, Yixian Formation, China.

Paul C. Sereno dinosaur@uchicago.edu, Department of Organismal Biology and Anatomy, University of Chicago, 1027 East 57th Street, Chicago, IL 60637, USA; Zhao Xijin zhaolu54@263.net, Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, Beijing 100044, China ; Lorin Brown kidbones@sbcglobal.net, 1650 45th Street, Munster, IN 46321, USA ; Tan Lin firsttan@sina.com, Long Hao Geologic and Paleontological Research Center, Bureau of Land and Resources, Hohhot 010010, Nei Mongol Autonomous Region, China.