Sinornithomimus

Discovery and naming

The first fossils of Sinornithomimus were found in 1997 by Dong Zhiming, working in the Ulansuhai Formation as part of the Mongol Highland International Dinosaur Project. The site yielded at least fourteen skeletons lying close together, with nine nearly complete and not badly crushed. Among them were three sub-adult to adult individuals and eleven juveniles. The bones show little weathering, were preserved in siltstone mixed with clay layers, and lack any signs of having been moved after death.

This points to a sudden, catastrophic event that killed the entire group at once. The type species, Sinornithomimus dongi, was named in 2003 by Yoshitsugu Kobayashi and Lü Junchang. The genus name means “Chinese bird mimic,” and the species name honors Dong as the discoverer. The holotype, IVPP-V11797-10, is one of the sub-adult skeletons; the others became paratypes.

A later expedition in 2001 at the same spot uncovered another fossilized herd of thirteen juveniles and sub-adults. Their arrangement suggests they perished together over a short period, probably after getting stuck in mud at a drying waterhole. This second find also consisted mostly of intact specimens, making Sinornithomimus the best-known ornithomimid.

Classification

When Sinornithomimus was first described, it was placed within Ornithomimidae. It was seen as a more advanced ornithomimid than Archaeornithomimus, though later studies have flipped that relationship. Its hand structure matches Archaeornithomimus, placing it between the primitive ornithomimosaur Harpymimus and later ornithomimids. This dinosaur shows that some ornithomimid traits are actually primitive, and it also reveals differences between Asian and North American ornithomimid beaks, based on the presence of maxillary vascular openings in the latter.

Paleobiology

We know Sinornithomimus was an herbivore because of the gastroliths—stomach stones—found in clumps where its digestive tract would have been. This dinosaur was also social. Its fossils were discovered in a small bonebed where all the juveniles were roughly the same age. Running ability improved as they grew, with adults having longer lower legs for faster movement.

The scientists who first described the species thought adults formed family groups to protect the young from predators. However, a 2008 study suggested otherwise. Since the bonebed contained only juveniles, it appears the young were left to fend for themselves in these groups while the adults were busy nesting or brooding.

Did You Know?

• Sinornithomimus had a relatively short neck and head for an ornithomimid.
• Autapomorphies include a quadrate with a depression containing a smaller opening divided by a vertical sheet of bone.
• A depression on the lateral side of the posterior process of the parietal bone is another unique trait.