Xixiasaurus

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.^[1] 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.^[3]

Description

Xixiasaurus is estimated to have been around 1.5 metres (5 ft) long, and to have weighed about 8 kilograms (18 lb).

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 narial opening (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.^[1]

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

The nasal bones were elongated, 100 millimetres (4 in) long and 8.8 millimetres (0.35 in) wide. They were not fused together, and covered the top of the rostrum (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 65 millimetres (2.6 in) 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 postorbital bones (bone behind the eye socket). The frontal was overlapped by the nasal and lacrimal bones (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 orbit (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.^[1]

It differed from other troodontids in that the front end of the dentary of the lower jaw was downturned, similar to what is seen in

Unlike most troodontids, Xixiasaurus did not have serrations on its teeth, and their carinae (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 alveoli (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 heterodont (differentiated), with the fifteenth tooth being largest.^[1][7]

The radius bone of the lower arm was much thinner than the ulna. The third metacarpal was thinner than the second, and their outermost edges were at the same level, which indicates these two metacarpals were equal in length. The claw of the first finger was sharp, and had a large flexor tubercle (where a tendon was inserted). The first phalanx of the first finger was 3.6 centimetres (1.4 in) long, and the first phalanx of the second was 3 centimetres (1.2 in).^[1]

Classification

Lü and colleagues assigned Xixiasaurus to the

The following cladogram shows the position of Xixiasaurus within Troodontidae according to a 2017 analysis by the palaeontologist Caizhi Shen and colleagues:^[9]

In 2019, the palaeontologist Scott Hartman and colleagues recovered Xixiasaurus as the sister taxon of Sinusonasus, in a clade with Daliansaurus and Hesperornithoides (sharing features such as a straight ulna and having an upwards projected curve on the claw of the first finger).^[10]

Troodontids have mainly been discovered in the

Troodontids had some of the highest

In 2001, the palaeontologists Philip J. Currie and Dong Zhiming rejected the idea that troodontids could have been herbivorous. They stated that troodontid anatomy was consistent with a carnivorous lifestyle, and pointed out that the structure of their serrations was not much different from those of other theropods. They noted that troodontid features such as sharply pointed serrations that curved up towards the tip of the teeth, razor sharp enamel between the serrations, and blood grooves at the bases, were not seen in herbivorous dinosaurs, which had simpler, cone shaped serrations.^[13] Lü and colleagues discussed the previous studies of troodontid diet, and suggested that the loss of serrations in the teeth of Xixiasaurus and some other troodontids was related to a change in their diet. Since the teeth would appear to have lost their typical ability to slice meat, at least these troodontids may therefore have been either herbivorous or omnivorous.^[1] In 2015, the palaeontologist Christophe Hendrickx and colleagues suggested that basal (or "primitive") troodontids with unserrated teeth were herbivorous, whereas more derived troodontids with serrated teeth were carnivorous or omnivorous.^[5]

Palaeoenvironment

Xixiasaurus is known from the mid-lower part of the Majiacun Formation of China, which dates to the Late Cretaceous

• Timeline of troodontid research

References