Xixiasaurus

Xixiasaurus (/ˌʃiːʃiəˈsɔːrəs/) is a genus of troodontid dinosaur that lived during the Late Cretaceous Period in what is now China. The only known specimen was discovered in Xixia County, Henan Province, in central China, and became the holotype of the new genus and species Xixiasaurus henanensis in 2010. The names refer to the areas of discovery, and can be translated as "Henan Xixia lizard". The specimen consists of an almost complete skull (except for the hindmost portion), part of the lower jaw, and teeth, as well as a partial right forelimb.

Xixiasaurus is estimated to have been 1.5 metres (5 ft) long and to have weighed 8 kilograms (18 lb). As a troodontid, it would have been bird-like and lightly built, with grasping hands and an enlarged sickle-shaped claw on the second toe. Its skull was long, with a long, low snout that formed a tapering U-shape when seen from below. The  frontal bone  of the forehead was dome-like in side view, which indicates it had an enlarged  braincase. It differed from other troodontids in that the front of the  dentary bone  of the lower jaw was down-turned. Unlike in most troodontids, the teeth of Xixiasaurus did not have  serrations ; instead, their  carinae  (front and back edges) were smooth and sharp. It was distinct among troodontids in having 22 teeth in each  maxilla  (in other genera the maxillary tooth count was either higher or lower).

The precise relationships of Xixiasaurus with other troodontids are uncertain, but it had some similarities with Byronosaurus. Though troodontids with serrated teeth were once thought to form a clade, the taxonomic significance of this feature has been questioned. Troodontids had large brains, keen senses, and were probably agile. There has been debate about their diet, with some researchers arguing that they were carnivorous, and others that they were omnivorous or herbivorous. The lack of serrated teeth in Xixiasaurus and some other troodontids indicates these were herbivorous, as they had lost the ability to slice meat. Xixiasaurus is known from the Majiacun Formation, the exact age of which is uncertain. These sedimentary rocks were deposited by braided streams and meandering streams, and are noted for containing abundant dinosaur eggs.