Xixiasaurus henanensis, Lü & Xu & Liu & Zhang & Jia & Ji, 2010
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publication ID |
https://doi.org/ 10.4202/app.2009.0047 |
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persistent identifier |
https://treatment.plazi.org/id/03B387FC-B452-FF84-FCAC-F90C607AFAA3 |
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treatment provided by |
Felipe |
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scientific name |
Xixiasaurus henanensis |
| status |
sp. nov. |
Xixiasaurus henanensis sp. nov.
Figs. 2–4 View Fig View Fig View Fig .
Etymology: The specific name refers to Henan Province, in which the holotype site in Xixia County is found.
Holotype: A partial skull, an anterior portion of the lower jaw, and central portions of the ulna and radius, and a partial right manus with a complete first digit.
Type locality: Songgou Village, Zhangying of Wuliqiao Town, northeast region of Xixia County.
Type horizon: Mid−lower part of the Majiacun Formation (Coniacian– Campanian) ( Wang et al. 2006; Chen et al. 2007; Pan et al. 2007).
Diagnosis.—Differs from other troodontids by having fewer maxillary teeth (which total 22) than Byronosaurus (> 30; Makovicky et al. 2003) and Mei (24; Xu and Norell 2004), but more than Sinornithoides youngi (18; Makovicky and Norell 2004), Zanabazar junior (20; Makovicky and Norell 2004; Norell et al. 2009), and S. mongoliensis (19; Makovicky and Norell 2004). It also has a distinct opening on the lateral surface of the base of the nasal process of the premaxilla, the rostral end of the upper jaw shows a more tapered U−shape than in Byronosaurus , and the mandibular symphyseal region is slightly inflected medially. Like Byronosaurus , Mei , Urbacodon , and Anchiornis , this new taxon has unserrated teeth, and like Byronosaurus it has a heterodont dentition and an extensive secondary bony palate. An extensive palate is also scored for Saurornithoides mongoliensis and Troodon formosus (see Varricchio 1997).
Description.—The skull is almost complete except for its posterior portion ( Fig. 2 View Fig ). It is long and generally similar to that of Byronosaurus . The connection between nasals and frontals is displaced by preservation. Part of the braincase is missing; however, most of the rostral portion is preserved. The right side of the upper dentition is well−preserved. The posterior teeth are missing, but the tooth sockets are preserved. Thus, the tooth number can be determined accurately.
Both premaxillae are well−preserved and not fused to each other ( Fig. 2A, B View Fig ). The alveolar margin of the upper jaw forms a tapered U−shape in ventral view ( Fig. 2C View Fig ). The nasal process of the premaxilla extends posteriorly and is wedged between the premaxillary processes of the nasals. The nasal process of the premaxilla ends at the level of the posterior margin of the external narial opening. The process is quadrangular rather than triangular in cross−section, as seen in Byronosaurus jaffei ( Makovicky et al. 2003) . The nasal process of the premaxilla forms the anterodorsal corner of the external narial opening and has a distinct opening on the lateral surface at its base ( Fig. 2B View Fig , premaxillary opening). A few small pits are present on the lateral surface of the premaxilla. The premaxilla/maxilla suture curves gently upward and straightens out below the nares in lateral view. The posteriorly tapering maxillary process of the premaxilla wedges between a small anteriorly extended process of the maxilla and the main body of the same bone. It excludes the maxilla medially from participation in the margin of the narial opening. The maxillary process of the premaxilla extends posteriorly to the same level as the nasal process. As in most theropod dinosaurs, there are four, closely packed premaxillary teeth, of which two crowns are missing on the left side, and three are missing on the right side. The tooth alveoli are sub−oval. The premaxillary teeth are smaller than the posterior maxillary teeth. There is a distinct constriction between the crown and root of the premaxillary teeth and the anterior ten maxillary teeth. The teeth are unserrated, with sharp, smooth carinae ( Fig. 3A View Fig 1, A 2 View Fig ) like those of Byronosaurus jaffei ( Norell et al. 2000) . The buccal surfaces of the tooth crowns are convex and the lingual surfaces are moderately concave. The ventral portions of the premaxillae are not fused, leaving a fissure along the mid−line of the premaxilla. Two small openings are present on the medial surface near the base of the third and fourth premaxillary teeth, but none is present near the first and second premaxillary teeth. These two small openings are connected by a shallow groove.
The maxilla is much longer than high and forms most of the lateral surface of the skull. Anteriorly, it forms nearly half of the floor of the external naris. The naris is suboval, with long axis parallel to the ventral margin of the maxilla ( Fig. 2B View Fig ). The maxilla rises gently posteriorly from the narial floor. The lateral surface below the narial chamber is covered with small pits and foramina as in Byronosaurus jaffei ( Makovicky et al. 2003) . This row of small pits and small openings is just dorsal and parallel to the dentigerous margin ( Fig. 3A View Fig 2 View Fig ), but they do not lie in a groove, differing from the case in Byronosaurus jaffei , where the pits lie in a groove ( Makovicky et al. 2003). The lateral surface of the maxilla is perforated by three large, elongated openings (anterior, maxillary, and antorbital fenestrae). The anterior fenestra is probably completely enclosed by the maxilla as it is in Byronosaurus jaffei ( Makovicky et al. 2003) , but because of preservation damage, this is not definite. Two rows of small pits are present on the lateral surface of the maxilla below the first opening. The maxillary fenestra is backed by a perforated osseous wall, which is called the interfenestral bar ( Fig. 2B View Fig ). The wall separates the maxillary fenestra from the antorbital fenestra as in Byronosaurus jaffei ( Makovicky et al. 2003) . The antorbital fenestra lacks an osseous floor and is large and rectangular in lateral view, although the shape of the posterior margin of the antorbital fenestra is not clear due to poor preservation. Below the antorbital fenestra, the maxilla is low dorsoventrally and is devoid of the small foramina that are so conspicuous anteriorly. The dentition underlies most of the lower margin of the antorbital fenestra.
Ventrally, the longitudinally extensive shelf of the maxilla contributes to the large secondary palate and it extends posteriorly from the contact with the premaxilla and are joined by a median vomer at the anterior border of the choanae. Only a portion of the vomer is preserved. A portion of the palatine is preserved in both sides. The shape of the palatine is not clear due to the poor preservation.
There are three small openings on the anterior portion of the left palatal shelf of the maxilla. There is a row of nutrient foramina that lie in a groove just dorsal and parallel to the dentigerous margin ( Fig. 3A View Fig 1). The total number of maxillary teeth is 22 (8 are shown by alveoli only). The anterior 7 teeth are much smaller than the posterior ones and are closely packed, with distinct constrictions between the tooth crowns and tooth roots. The anterior and posterior carinae of the crowns are sharp but lack serrations. The lingual surfaces of crowns have distinct grooves adjacent to the carinae, which is similar to the teeth of Urbacodon ( Averianov and Sues 2007) and Mononykus ( Chiappe et al. 1996) . Posterior to the tenth maxillary tooth, the bases of the crowns are less expanded, and the teeth curve posteriorly and are laterally compressed ( Fig. 3A View Fig 2 View Fig ). The maxillary teeth are heterodont, with the fifteenth tooth largest, based on the width of the tooth socket. More posterior teeth are smaller based on the widths of the tooth sockets.
The nasals are very elongated and not fused, indicating that the specimen represents an immature individual. The nasals are 99.9 mm long and 8.8 mm wide. They cover the top of the rostrum for most of its length. Anteriorly, the nasal forms the posterolateral boundary of the large external naris. Anteriorly, the nasal slopes dorsolaterally, while it flattens posteriorly.
The frontals ( Fig. 2A View Fig ) are 65 mm long and are not fused. Each frontal is triangular in dorsal view and reaches its maximum width at the point of contact with the postorbital, as in other troodontids. Rostrally, the lateral border of the frontal is overlapped by the nasal and lacrimal. The anteromedial portion of the frontal contacts the posterior end of the nasals. In lateral view, the posterior portion of the frontal is dome−like, indicating an enlarged braincase. The orbital rim is raised with weak notches along the margin. Medial to the orbital rim, a shallow trough is present on the lateral surface of the frontal.
A small portion of the ventral process of the right lacrimal is missing. The bone is T−shaped in lateral view. The anterior process of the lacrimal is very long, reaching the level of the anterior margin of the antorbital fenestra. It forms most of the posterodorsal border of the antorbital fenestra as in Byronosaurus jaffei ( Makovicky et al. 2003) .
A portion of the left squamosal is preserved. It contacts the parietal and postorbital medially. It is rod−like in dorsal view.
Only the anterior portion of the left dentary is preserved ( Fig. 3B View Fig 1, B 2 View Fig ). Thirteen tooth sockets (including three with broken teeth) are preserved. The preserved teeth and tooth sockets indicate that the teeth are small and closely packed. They are set in an open groove, as in other troodontids, rather than individual sockets ( Currie 1987). The mandibular symphysis is short. The symphyseal region is slightly inflected medially. Two rows of nutrient foramina are present laterally, just below the anterior seven dentary teeth. Posterior to the seventh dentary tooth, only one row remains. The foramina lie in a labial groove, which is a diagnostic character of troodontid dinosaurs ( Currie 1987). The medial surface of the dentary is smooth. A distinct deep, narrow groove (the Meckelian groove) is present just above the ventral margin of the dentary ( Fig. 3B View Fig 2 View Fig ). The Meckelian groove extends toward the mandibular symphysis and continues on the symphyseal surface almost to the anterior end of the dentary. Just posterior to the symphysis and ventral to the Meckelian groove, a distinct foramen is confluent with the inferior alveolar canal ( Fig. 3B View Fig 1, B 2 View Fig ). This is identical to the condition in Urbacodon itemirensis ( Averianov and Sues 2007) . On the lateral side of the bone at the same level as the Meckelian groove, there is a shallow groove with elongate pits.
A partial right forelimb (middle portion of the ulna and radius, distal ends of the second and third metacarpals, complete first digit, and first phalanx of the second digit) is preserved ( Fig. 4 View Fig ). All the preserved parts are naturally articulated. The radius is much thinner than the ulna. The first metacarpal is missing. The third metacarpal is thinner than the second metacarpal and their distal ends are at the same level ( Fig. 4A View Fig 2 View Fig ), indicating that the second and third metacarpals are equal in length. The claw of the first digit is trenchant, with a large flexor tubercle. The first phalanx of the first digit is 3.6 cm long, and the first phalanx of the second digit is 3 cm long.
Stratigraphic and geographic range.—Only known from a single locality in the?Campanian deposits of the Xixia Basin, Henan Province.
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