A new species of Xinpusaurus (Reptilia, Thalattosauria) from the Ladinian (Middle Triassic) of Xingyi, Guizhou, southwestern China

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Li, Z.-G., D.-Y. Jiang, O. Rieppel, R. Motani, A. Tintori, Z.-Y. Sun, and C. Ji. 2016. A new species of Xinpusaurus (Reptilia, Thalattosauria) from the Ladinian (Middle Triassic) of Xingyi, Guizhou, southwestern China. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1218340.

Thalattosauria is a clade of marine reptiles exclusively known from the Triassic (Nicholls, 1999;Rieppel et al., 2000). It remained poorly understood until the end of the 20th century, because the only records then known were from North America and Europe. In recent years, abundant, well-preserved thalattosaur material has been excavated in sediments of Middle to Upper Triassic age of southwestern China. Four genera and eight species have been recognized amongst this Chinese material: Anshunsaurus huangguoshuensis Liu, 1999; Xinpusaurus suni Yin in Yin et al., 2000;Xinpusaurus bamaolinensis Cheng, 2003;Xinpusaurus kohi Jiang et al., 2004; Anshunsaurus wushaensis ; Anshunsaurus huangnihensis Cheng et al., 2007a;Miodentosaurus brevis Cheng et al., 2007b; and Concavispina biseridens Zhao et al., 2013. Among the taxa, the type species of the genus Xinpusaurus suni Yin in Yin et al., 2000 was described on the basis of four poorly prepared specimens that were collected from the Carnian (early Late Triassic) Wayao Member of the Falang Formation of Guanling County, Guizhou Province, southwestern China (the lithostratigraphic unit name 'Wayao Formation' used by Yin et al. [2000] was proposed to be invalid by Wang et al. [2002], who alternatively proposed a new lithostratigraphic unit name, 'Xiaowa Formation,' but Jiang et al. [2005] and Li [2006] suggested continuing to use the traditional unit name, as is done in this paper). The original description of the species was brief and short, illustrated with low-quality photographs, and the taxon was incorrectly referred to Ichthyosauria. Liu and Rieppel (2001) identified an isolated skull (IVPP V11860) collected from the same locality and same stratigraphic level as Xinpusaurus cf. X. suni and referred Xinpusaurus to Thalattosauria. Their phylogenetic analysis indicated that Xinpusaurus is the sister taxon of Nectosaurus from North America. Subsequently, this isolated skull and two additional incomplete specimens (IVPP V12673, IVPP V14372) were referred to X. suni by Liu (2001) and , whereas the skull of the holotype was redescribed by Luo and Yu (2002) along with one additional specimen (GGSr001).
The second species of Xinpusaurus, X. bamaolinensis Cheng, 2003, was based on a single specimen (SPCV 30015) that was incompletely prepared and imperfectly described. The third species, X. kohi Jiang et al., 2004, was erected on the basis of a nearly complete skeleton (GMPKU2000/005). Liu and Rieppel (2005) proposed that X. kohi is a junior subjective synonym of X. bamaolinensis, whereas Liu (2013) suggested that both X. bamaolinensis and X. kohi are subjective junior synonyms of X. suni. However, Maisch (2014) countered that X. kohi was valid and could be taxonomically distinguished based on diagnostic characters, whereas the taxonomic status of X. bamaolinensis remained problematic. Our personal observation of GMPKU2000/005 supports Maisch's (2014) interpretation of the taxonomy of Xinpusaurus. Therefore, in this paper, we follow Jiang et al. (2005) and Maisch (2014), in treating X. kohi as a valid species and X. bamaolinensis as a species inquirendae.
All specimens of Xinpusaurus previously known were collected in the Carnian (Upper Triassic) deposits of Guanling County, Guizhou Province. Here, we report a new specimen from the Ladinian (Middle Triassic) of Xingyi City, Guizhou Province, collected during the 2011 excavation season by a collaborative team of the Geological Museum of Peking University, University of California Davis, University of Milan, and the Field Museum of Natural History (Chicago). It can be referred to the genus Xinpusaurus, but it differs in its anatomy from the other species in the genus. This is the first record of the genus Xinpusaurus in the Middle Triassic. In the following, we provide a detailed description of the morphology of this new species and present a phylogenetic analysis of Thalattosauria to test its relationships within this clade.

MATERIALS AND METHODS
The specimen is an almost complete skeleton that lacks some of the caudal vertebrae. The skull is disarticulated and mostly exposed in ventral view. The postcranial skeleton is mainly preserved in lateral view. The specimen is housed in the Xingyi National Geopark Museum at Wusha.
The phylogenetic analysis was based on the data matrix of Liu et al. (2013), using PAUP* version 4.0b10 (Swofford, 2002). Xinpusaurus bamaolinensis, X. kohi, Anshunsaurus huangnihensis, and the new species were added to the ingroup of Liu et al. (2013), and the coding for X. suni was modified based on our personal observations (Appendix 1). The new data matrix comprised 18 ingroup taxa and three outgroup taxa, and 40 characters that were treated as unordered and unpolarized (Supplemental Data). The branch-and-bound search found two equally parsimonious trees with a tree length of 93 steps, a consistency index of 0.5484, and a retention index of 0.7614.
Institutional Abbreviations-GMPKU,  THALATTOSAURIA Merriam, 1904THALATTOSAUROIDEA Nopcsa, 1928XINPUSAURUS Yin in Yin et al., 2000 Type Species-Xinpusaurus suni Yin in Yin et al., 2000. Emended Diagnosis-Medium to large thalattosaur with moderately long rostrum; rostrum straight or slightly bent dorsally with pointed anterior tip; anterior alveolar margin of maxilla curved dorsally, carrying enlarged procumbent teeth; nasal forming entire dorsal margin of external naris; pineal foramen situated on posterior part of parietals; vomer and pterygoid dentigerous; alveolar margin of dentary straight anteriorly and curved posteriorly; slender mandible with dentary symphyseal region tapering to a narrow tip; strong overbite between upper and lower jaws; anterior teeth (on premaxilla, anterior maxilla and dentary) conical and pointed, more posterior teeth blunt; number of cervical vertebrae no more than seven; neural spines of anterior caudal vertebrae slender and narrow; all caudal neural spines inclined posteriorly; humerus wider proximally than distally; radius strongly broadened.
Horizon and Locality-Middle part of Zhuganpo Member ( D 'Zhuganpo Formation' of Wang et al. [2002]), Falang Formation; about 0.6 m below the base of the ammonite Haoceras xingyiensis Zone, which is in part equivalent to the lower part of the North American Frankites sutherlandi Zone, of the middle Late Ladinian, Middle Triassic (Zou et al., 2015). Nimaigu Village, Wusha Town, Xingyi City, Guizhou Province, People's Republic of China.
Etymology-The specific epithet is derived from the name of the city in the territory from which the specimen was excavated.
Diagnosis-Posterior process of jugal absent; coracoid oval; radius only about half as long as humerus; dorsal and ventral margins of ilium approximately equal in width; femur without mediolateral constriction; fibula without mediolateral expansion.

DESCRIPTION
The specimen is nearly complete except for the tail (Fig. 1), with a preserved length of 2.1 m. Including the estimated length of the missing distal part of the tail, the complete skeleton may have exceeded 3 m in length. The length of the skull may be estimated to exceed 42 cm, as indicated by the length of the mandible. This specimen is larger than any other known specimen of Xinpusaurus, in which the total body length is 2.38 m long, with a skull that is at most 34 cm in length (Yin et al., 2000;Luo and Yu, 2002).

Skull
The premaxillaries are disarticulated and exposed in medial view. They are elongate elements, with straight dorsal and alveolar margins. Each element is medially thickened, both dorsally and ventrally, resulting in the formation of an elongate groove running along the middle part of the medial surface. Posteriorly, the posterodorsal ramus of the premaxilla is longer than the posteroventral ramus. Three teeth are preserved on the left premaxilla and six on the right one, all of which are conical and pointed. The maxillae are large, elongate triangular elements with long posterior processes. The anterior-most part of the maxillary alveolar margin is curved dorsally, carrying procumbent teeth, similar to those of Xinpusaurus suni (Liu and Rieppel, 2001; pers. observ.), X. kohi pers. observ.), X. bamaolinensis (Cheng, 2003), and Concavispina biseridens Zhao et al., 2013;pers. observ.). There are four teeth exposed on each maxilla. All teeth are subthecodont, relatively blunt, and located on the anterior part of the maxillae. Both jugals possess long anterior and dorsal processes, but unlike in other thalattosaurs, a posterior process is uniquely absent. The smooth posteroventral margin on both jugals indicates that no process that FIGURE 1. Skeleton of the holotype of Xinpusaurus xingyiensis, sp. nov. (XNGM WS-53-R3). Scale bar equals 10 cm. might originally have been present was broken off during fossilization or preparation. All previously known specimens of Xinpusaurus are smaller in size, but a posterior process of the jugal is well developed in all of them. The absence of a posterior process of the jugal in the new specimen thus cannot be explained by ontogenetic variation because it is unlikely that the process was resorbed with growth. Both quadrates are dislocated. One is mostly obscured by other skull elements, whereas the other one is well exposed (Fig. 3). The quadrate is a large and robust element, with a prominent anterior flange.
The vomers appear fused along the midline at least posteriorly and form the anterior margins of the internal nares. The posterior end of the vomer is embraced by the anterior ramus of the pterygoids. Each side of the fused vomers bears three blunt teeth. The palatine is poorly preserved. The pterygoids are triangular, each with a slender anterior ramus, a broad transverse process, and a long posterior ramus. Many teeth concentrate on the transverse process of the pterygoids, as is also the case in X. suni (Liu and Rieppel, 2001;Luo and Yu, 2002;pers. observ.), X. bamaolinensis (Cheng, 2003), and X. kohi (Maisch, 2014;pers. observ.). The interpterygoid vacuity is narrow. The parabasisphenoid is partly preserved. Anteriorly, it appears to extend into a long and narrow cultriform process located within the interpterygoid vacuity. Posterolaterally, it possesses prominent paired basipterygoid processes like those of Anshunsaurus huangguoshuensis (Liu and Rieppel, 2005), but only the left one is complete.
The lower jaw is exposed in ventral view. The right mandibular ramus is well preserved, although partially concealed by overlapping bones. The anterior part of the left ramus is missing. The length of the mandibular symphysis is unknown because of the incomplete preservation. The dentary bears a long posterodorsal process that contacts the underlying surangular, as is also the case in X. suni (Liu and Rieppel, 2001;pers. observ.), whereas a posteroventral process as observed in X. kohi pers. observ.) and Concavispina biseridens  pers. observ.) is absent. No dentary tooth can be observed. The splenial is an elongate element located ventral to the angular in the medial view of the mandible. The angular is a relatively elongate element as well. It contacts the dentary anterodorsally, the surangular dorsally, and the articular posterodorsally.

Postcranial Skeleton
The vertebral column is partially disarticulated. There are 54 vertebrae preserved, including three cervical, 18 dorsal, and 33 caudal elements. We cannot identify any sacral vertebrae. The number of cervical vertebrae is unknown in most specimens of Xinpusaurus. In X. suni, four cervical vertebrae are preserved in Gmr010 (Yin et al., 2000), and no more than seven in IVPP V12673 (Liu, 2001). In the specimen described here, three cervical vertebrae can be identified by three isolated neural arches exposed in dorsal view (Fig. 2). The most robust one among them probably is the axial neural arch. The pre-and postzygapophyses of the cervical vertebrae are prominent, and the neural spines are extremely low. Some neural spines of the dorsal vertebrae possess a concave dorsal margin, differing from those of X. suni (Liu, 2001; pers. observ.) and X. kohi pers. observ.), but the concavity is not as strongly expressed as it is in Concavispina biseridens Zhao et al., 2013;pers. observ.). The neural spines and hemal arches of the caudal vertebrae are extremely slender, as in other thalattosaurs. All caudal neural spines are posterodorsally inclined, a feature that Jiang et al. (2004) considered a diagnostic feature of the genus Xinpusaurus. The fused neurocentral suture throughout the vertebral column indicates that the specimen here described is an adult.
The left scapula is partly preserved; it appears elongated judging by its outline (Fig. 3). Its posterior margin is concave, whereas most of the dorsal and anterior margins are convex. In X. suni, both the anterior and posterior margins of the scapula are almost straight (Liu, 2001: fig. 1; pers. observ.). Both coracoids are slightly deformed, but they appear to be oval in shape. In contrast, X. suni possesses a more elongate, trapeziform coracoid (Liu, 2001: fig. 1; pers. observ.). The interclavicle is cruciform, with a short anterior process and an elongate posterior shaft. The shaft is much narrower posteriorly than anteriorly, resembling that of Miodentosaurus (Wu et al., 2009). The clavicle is robust, with the shaft curved at an angle of about 120 (Fig. 2). The humeri are relatively short, with expanded proximal and distal ends. Similar to Xinpusaurus suni (Yin et al., 2000;Liu, 2001;pers. observ.), X. bamaolinensis (Liu, 2013), X. kohi pers. observ.), and Concavispina biseridens pers. observ.), the proximal end of the humerus is wider than the distal end. The deltopectoral crest is  well developed and exposed in ventral view on the right humerus. The radius is short (32 mm) and broad, with a convex lateral margin and a concave medial margin, showing a roughly reniform shape. The length of the radius is 0.54 that of the humerus, whereas in the holotype of X. suni (Gmr010), this ratio is 0.63 (Liu, 2001:table 1). Specimen Gmr010 is in some respects a smaller skeleton (34 cm in skull length, 2.38 m in total length), but its radius is longer (39 mm). In other known specimens of X. suni, this ratio ranges from 0.63 to 0.69 (Liu, 2001:table 1) and is 0.63 in X. kohi (pers. observ.). The difference in radius/humerus length ratio between these taxa and among these specimens does not reflect ontogenetic variation, but is probably taxonomically significant. The anteroproximal notch of the radius is absent, a trait that is different from X. suni (specimen GGSr001; pers. observ.) and X. kohi pers. observ.), but similar to X. suni specimen IVPP V12673 (Liu, 2001: fig. 1; pers. observ.).
The ilium (Fig. 3) is curved, bearing a rectangular dorsal blade. The ventral (acetabular) margin is approximately as wide as the dorsal margin, a morphology similar to that observed in Concavispina biseridens pers. observ.), but dissimilar from X. suni (Gmr010), in which the ventral and dorsal margins of the ilium measure 20 and 12 mm, respectively (Yin et al., 2000). The pubis is incomplete, but a small obturator foramen can be discerned. The cylindrical femur is much longer than the humerus. The shaft of the femur shows no mediolateral constriction, which is different from X. suni (Liu, 2001: fig. 1; pers. observ.) and X. kohi  fig. 3C; pers. observ.). Although the surface of the femur is damaged, a well-developed internal trochanter can be recognized. Both tibiae are well preserved; the left one is preserved in articulation with more distal limb elements. The tibia has a strongly expanded proximal end, and slightly expanded distal end, as is also the case in other thalattosaurs. The fibula is of nearly the same length as the tibia. It . A, C, pectoral girdle and forelimbs; B, D, pelvic girdle and hind limbs. Abbreviations: as, astragalus; cal, calcaneum; co, coracoid; dt, distal tarsal; fe, femur; fi, fibula; hu, humerus; il, ilium; icl, interclavicle; mt, metatarsal; obt. fo, obturator foramen; pu, pubis; q, quadrate; ra, radius; sc, scapula; ti, tibia. All scale bars equal 2 cm. is expanded distally but not laterally, which is different from the condition of X. suni and X. kohi (Liu, 2001: fig. 1; Jiang et al., 2004: fig. 3C; pers. observ.). Three tarsals are present: the astragalus and one distal tarsal of the right side, and the calcaneum of the left side. Both the astragalus and calcaneum are reniform, the former larger than the latter. The distal tarsal is oval and smaller than the calcaneum. Five metatarsals of the left hind limb are preserved, of which the first is the shortest, showing a distinct proximal expansion, and the fourth is the longest. Several phalanges are preserved, including two ungual phalanges, but the phalangeal formula remains unknown. The ungual phalanges are triangular in shape, with a sharp distal tip.

DISCUSSION
The new specimen, XNGM WS-53-R3, described above can be easily identified as a thalattosaur based on its strongly elongated caudal neural spines and hemal arches, and the stout and flat limbs, which are important diagnostic features of Thalattosauria (M€ uller, 2007). Furthermore, XNGM WS-53-R3 clearly belongs to the genus Xinpusaurus, because it shares the following synapomorphies of the genus: the anterior alveolar margin of the maxilla curves dorsally and carries procumbent teeth; the proximal end of the humerus is wider than the distal end; the lateral margin of the radius is convex; and the medial margin is concave. Specimen XNGM WS-53-R3 also shares some morphological similarities with Concavispina, such as the neural spines of dorsal vertebrae that show a concave dorsal margin. However, Concavispina possesses two rows of large and blunt maxillary teeth, with a height/basal diameter ratio of the largest tooth crown of only about 0.6 (pers. observ.), corresponding to crushing teeth as defined by Massare (1987) on the basis of a height/basal diameter ratio of 1.0 or smaller. In contrast, the maxillary teeth of XNGM WS-53-R3 are relatively small, and the height/basal diameter ratio of the largest tooth crown is larger than 1.5, thus corresponding to crunching teeth as defined by Massare (1987). Furthermore, Concavispina differs from specimen XNGM WS-53-R3 here described by its dentary, which carries a long posterodorsal process, and by its fibula, which shows a strongly constricted shaft. In conclusion, the new specimen XNGM WS-53-R3 here described must be referred to the genus Xinpusaurus, rather than to Concavispina.
Within the genus Xinpusaurus, X. xingyiensis, sp. nov., can be distinguished from X. suni by the following features: the posterior process of the jugal is absent; the coracoid is oval rather than trapeziform; the radius is relatively short; the ventral and dorsal margins of the ilium are nearly equal in width; the femur shows no mediolateral constriction; and the fibula shows no mediolateral expansion. Most of these features also distinguish X. xingyiensis, sp. nov., from X. kohi. The original description of X. bamaolinensis is brief, rendering a detailed comparison difficult, but at least it can be distinguished from X. xingyiensis, sp. nov., by the unique morphology of the jugal that characterizes the latter species. We therefore conclude that X. xingyiensis, sp. nov., represents a separate species of Xinpusaurus.
To test the phylogenetic position of X. xingyiensis, sp. nov., within its genus, a phylogenetic analysis was carried out. The strict consensus tree shows the genus Xinpusaurus to be a FIGURE 4. Strict consensus tree indicating relationships between Xinpusaurus xingyiensis, sp. nov. (XNGM WS-53-R3) and other thalattosaurs (see text for more details). Values above branches leading to the nodes represent Bremer support. monophyletic grouping, in which X. xingyiensis, sp. nov., appears in a basal position (Fig. 4). This clade is supported by the following characters: 22(1), posteroventral process of dentary indistinct or absent (reversed in X. kohi); 32(1), neural spine height of proximal caudal vertebrae at least triple neural spine anteroposterior length (unknown in X. bamaolinensis); and 35(0), anterior margin of scapula approximately straight or slightly convex (unknown in X. bamaolinensis and X. kohi). Concavispina biseridens is the sister taxon to this clade, confirming the close affinity of Concavispina with Xinpusaurus that was proposed by Zhao et al. (2013), but which was not well supported in the analysis conducted by Liu et al. (2013). In previous studies, Xinpusaurus was found to be phylogenetically close to Nectosaurus from North America (Liu and Rieppel, 2001;Jiang et al., 2004;M€ uller, 2005Wu et al., 2009), but this trans-Pacific relationship is not supported in the present analysis, nor by the results presented by Liu et al. (2013).
The Ladinian Xingyi Fauna and the Carnian Guanling Fauna were thought to be quite different, but recent investigations revealed that the taxonomic composition of marine reptiles from the upper assemblage of the Xingyi Fauna closely resembles that of the Guanling Fauna (Ma et al., 2013). The askeptosauroid Anshunsaurus had previously been found both in Xingyi and Guanling (Liu, 1999;Rieppel et al., 2000Liu and Rieppel, 2005;Cheng et al., 2007Cheng et al., , 2011Liu, 2007), but the thalattosauroid Xinpusaurus had been reported only from Guanling. The present report of X. xingyiensis, sp. nov., from the Ladinian Xingyi Fauna expands the distribution range of the genus both temporally and geographically, reconfirming the similarity between the Upper Assemblage of the Xingyi Fauna and the Guanling Fauna.