Xixiasaurus () 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 unserrated 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.
Discovery
The holotype specimen (catalogued as HGM 41HIII−0201 in Henan Geological Museum, Zhengzhou) was discovered near Songgou Village, which is in the northeast region of Xixia County, in Henan Province of central China. This area of the Xixia Basin exposes the Majiacun Formation. In 2010, the specimen was described as the new genus and species Xixiasaurus henanensis by the palaeontologist Lü Junchang and colleagues. The generic name refers to Xixia County coupled with saurus, meaning "lizard", while the specific name refers to Henan Province. The full name can be translated as "Henan Xixia lizard". Remains of troodontids are very rare compared to those of other small theropod dinosaurs (only thirteen troodontid taxa were known at the time Xixiasaurus was named), and have mainly been found in Asia. The holotype specimen is the only known Xixiasaurus fossil, and consists of an almost complete skull except for the hindmost portion, as well as a partial right forelimb. The connection between the frontal (forehead bone) and nasal (bone running at the upper length of the snout) bones is displaced, and part of the braincase is missing. Most of the snout is preserved, with the dentition of the right side being well-preserved. Only the front part of the left dentary bone (tooth-bearing bone of the mandible) and some of its broken teeth are preserved. Though several teeth are missing from both jaws, their original number can be determined in the upper jaw, since their sockets there are preserved. The forelimb (of which all preserved parts are articulated) consists of the middle part of the radius and ulna (bones of the lower arm), the extremity of the second and third metacarpals (hand bones), the complete first finger, and the first phalanx bone of the second finger. In 2014, the palaeontologist Takanobu Tsuihiji and colleagues stated that a bone Lü and colleagues had originally identified as the vomer (part of the palate) of Xixiasaurus was instead part of the premaxillae or maxillae (the main bones of the upper jaw), based on comparison with the vomer of the more complete troodontid Gobivenator.
Description
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