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 long and to have weighed . 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 of the forehead was dome-like in side view, which indicates it had an enlarged . It differed from other troodontids in that the front of the of the lower jaw was down-turned. Unlike in most troodontids, the teeth of Xixiasaurus did not have ; instead, their (front and back edges) were smooth and sharp. It was distinct among troodontids in having 22 teeth in each (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 (forehead bone) and (bone running at the upper length of the snout) bones is displaced, and part of the is missing. Most of the snout is preserved, with the of the right side being well-preserved. Only the front part of the left (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 ) consists of the middle part of the and (bones of the lower arm), the extremity of the second and third (hand bones), the complete first finger, and the first of the second finger. In 2014, the palaeontologist Takanobu Tsuihiji and colleagues stated that a bone Lü and colleagues had originally identified as the (part of the ) of Xixiasaurus was instead part of the or (the main bones of the upper jaw), based on comparison with the vomer of the more complete troodontid Gobivenator.
Description
Xixiasaurus is estimated to have been around long, and to have weighed about . Since the nasal bones of the holotype specimen were not fused, it may not have been a mature individual. As a troodontid maniraptoran, it would have been bird-like, lightly built, with raptorial (grasping) hands and an enlarged sickle-shaped claw on the hyper-extendible second toe. Fossils of other troodontids, such as Jianianhualong, show that members of the group were covered in pennaceous feathers, with long feathers on the arms and legs, and frond-like feathering on the tail (similar to the avialan Archaeopteryx).
The skull of Xixiasaurus was long, similar to that of Byronosaurus overall, and the snout was distinctly long and low. The premaxillae were not fused to each other, and bore a few small pits on their side surfaces. When viewed from below, the front margin of the upper jaw formed a tapering U-shape, distinct from the shape in other troodontids; the U-shape tapered less in Byronosaurus and was very elongated in Sinornithoides, for example. Viewed from above, the nasal processes that extended hindwards from the premaxillae were wedged between the premaxillary processes of the nasal bones, which extended forwards. The nasal process of the premaxilla ended at the level of the hind margin of the external (bony nostril), and was quadrangular in cross-section rather than triangular, as in Byronosaurus. The nasal process of the premaxilla formed the upper hind corner of the narial opening, and Xixiasaurus was distinct in having an opening on the side surface at the base of the nasal process. The suture between the premaxilla and maxilla curved upwards from the front of the snout, and straightened out under the narial opening when seen from the side. The maxillary process of the premaxilla tapered hindwards and wedged between a small forwards-extended process of the maxilla and the main part of that bone. The maxilla was kept from being part of the margin of the narial opening by the maxillary process. The maxillary process of the premaxilla extended hindwards to the same level as the nasal process. Due to not being fused together, the premaxillae had a fissure along their lower midline. There were two small openings (connected to each other by a shallow groove) near the base of the third and fourth premaxillary teeth, but none near the first and second.
The maxilla was much longer than high, forming most of the skull's side surface, and to the front, most of the lower margin of the narial opening. The narial opening was roughly oval, with its long axis (the longest stretch from its front to the back) being parallel with the lower margin of the maxilla. The side surface of the maxilla below the narial opening was covered with small pits and foramina (openings), as in Byronosaurus, and the row of these was just above and parallel to the margin of the tooth-row. The side surface of the maxilla had three large, elongated openings, called the anterior, maxillary, and . The anterior fenestra appears to have been entirely enclosed by the maxilla, and there were two rows of small pits below it. The back of the maxillary fenestra had a bony wall called the interfenestral bar, which separated it from the antorbital fenestra, as in Byronosaurus. The antorbital fenestra (the largest of the three openings, located in front of the orbit) was rectangular in side view, and the part of the maxilla below it was low and did not have small foramina, unlike the front part. The maxillary teeth were placed along most of the lower margin of the antorbital fenestra. On the lower side, the maxilla formed an extensive internal shelf which contributed to a large (which separated the nasal airways from the mouth), extending backwards from the contact with the premaxilla. The front part of the palatal shelf had three small openings, and there was a row of (which allowed blood to supply the bone with nutrients) in a groove just above and parallel to the tooth row margin.
The nasal bones were elongated, long and wide. They were not fused together, and covered the top of the (snout) for most of their length. At the front, the nasal formed the boundary at the lower back of the narial opening. The nasal sloped downwards at the front, whereas it flattened hindwards. The frontal bones were long, and were not fused. Each frontal was triangular when viewed from above, and as in other troodontids, the frontals were widest at the point where they contacted the (bone behind the eye socket). The frontal was overlapped by the nasal and (bone in front of the eye socket) at the middle front, and its frontmost part contacted the hind end of the nasal. Seen from the side, the hind part of the frontal was dome-like, which indicates that Xixiasaurus had an enlarged braincase. The rim of the (eye socket) was raised, with weak notches along the margins. The lacrimal bone was T-shaped in side view; its front process was very long and reached the hind margin of the antorbital fenestra, forming most of the latter's upper hind border, as in Byronosaurus.
It differed from other troodontids in that the front end of the dentary of the lower jaw was downturned (also called "a chin-like eminence"), similar to what is seen in therizinosaurian theropods. The (the area where the two halves of the mandible connected at the front) was short, and this region was slightly curved towards the middle. Two rows of foramina ran along the outer side of the dentary, just below the first seven dentary teeth (only one of the rows continued hindwards past the seventh of these teeth). The foramina lay in a groove, which is a distinct feature of troodontids, while the inner surface of the dentary was smooth. A narrow, deep ran at the inner side of the dentary, just above its lower margin, towards the front of the mandibular symphysis. Just behind the symphysis and below the Meckelian groove, a distinct foramen was present, similar to the condition in Urbacodon. On the outer side of the dentary, at the level of the Meckelian groove, there was a shallow groove with elongated pits.
Unlike most troodontids, Xixiasaurus did not have on its teeth, and their (front and back edges) were instead smooth and sharp, as in Byronosaurus. Xixiasaurus had four closely packed teeth in each premaxilla, as in most other theropods, with roughly oval (tooth-sockets). The premaxillary teeth were smaller than the hindmost teeth of the maxilla. There was a distinct constriction between the crown and root of the premaxillary and frontmost ten maxillary teeth. The inner surfaces of the premaxillary tooth crowns were convex and the outer surfaces were somewhat concave, which created a D-shape when viewed in cross-section, a feature shared with a few other troodontids. It was distinct among troodontids in having 22 teeth in each maxilla (though not all teeth were preserved in the holotype, their number can be determined from the alveoli); other genera had either a higher or lower maxillary tooth count than Xixiasaurus. The first seven maxillary teeth were tightly packed and much smaller than those further back, and had distinct constrictions between their crowns and roots. The outer surfaces of the crowns had distinct grooves close to the carinae, similar to Urbacodon. The teeth curved backwards and were compressed from side to side, and the bases of the crowns were less expanded behind the tenth maxillary tooth. As in Byronosaurus, the maxillary teeth were (differentiated), with the fifteenth tooth being largest.