Discovery and namingEdit
The first fossil remains of Sinornithomimus were uncovered by Dong Zhiming near Ulan Suhai in the context of the Mongol Highland International Dinosaur Project. They contained at least fourteen skeletons found in close association, nine of which are nearly complete and relatively uncrushed. The find consisted of three sub-adult to adult specimens and eleven juveniles. The unweathered state of the bones, preserved in siltstone interspersed with layers of clay and the absence of evidence for post-mortem movement, argue for a catastrophic event that killed all the individuals present in the find simultaneously and instantaneously.
The type species Sinornithomimus dongi was named and described by Yoshitsugu Kobayashi and Lü Junchang in 2003. The generic name means “Chinese bird mimic” while the specific descriptor honours Dong as the discoverer of the fossils. The holotype, IVPP-V11797-10, is one of the subadult skeletons. The other skeletons have been assigned as paratypes.
A second expedition in 2001 at the same site led to the discovery of another fossilized herd of thirteen juveniles and subadults of Sinornithomimus. Their positioning suggest that they died together and over a short interval, likely after having become mired in the mud of a drying waterhole. The second discovery also largely consisted of nearly intact exemplars making Sinornithomimus the most completely known ornithomimid.
Sinornithomimus was a two-meter-long (7 ft) ornithomimid, with a relatively short neck and head for a member of that group. Unique derived traits, autapomorphies, included: the possession of a quadrate with a depression having within a smaller opening which is divided by a vertical sheet of bone; and a depression on the lateral side of the posterior process of the parietal bone. The herbivory of this genus is supported by the rare presence of gastroliths, as such stones were found forming conspicuous masses in the stomach areas of the fossil skeletons. This species was gregarious, which is corroborated by the arrangement of the fossil remains in a small bonebed with the juvenile individuals being approximately of the same age. This is furthermore supported by the increase of running ability as the animal progressed in its ontogeny, thus shown in the more cursorial proportions of the adults which possess relatively longer lower legs. It was assumed by the describers that adults protected themselves and their juveniles from predators by forming familial groups; in 2008 however a study concluded that the all juvenile herds suggest that immature individuals were left to fend for themselves in juvenile groups while adults preoccupied themselves with nesting or brooding.