(Harpymimus) and the derived (Anserimimus, Gallimimus, Struthiomimus, Dromiceiomimus, and Ornithomimus) condi− tions. The monophyly of Ornithomimidae is supported by a single synapomorphy (arctometatarsalian condition) in this analysis, indicating that the family is not as strongly supported as previously suggested. The analysis also implies that the shape of the rhamphotheca in North American taxa may have been different from that in Asian taxa. Previous study suggests herbivorous habits of this dinosaur based on characteristics of the gastroliths. The skeletons of Sinornithomimus were col− lected from a single monospecific bonebed with a high ratio of juvenile individuals (11 of the 14), suggesting gregarious be− havior for protection from predators. The abundance of juveniles indicates high mortality of juveniles or a catastrophic mass mortality of a population with a high proportion of juveniles. An increase in the relative ratio of the tibia to femur through the ontogeny of Sinornithomimus suggests higher cursoriality in adult individuals than in juveniles.