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ABSTRACT

Brevicaudosaurus jiyangshanensis, gen. et sp. nov., a new nothosauroid, is established on the basis of two nearly complete skeletons from the Middle Triassic (Ladinian) of China. Its skull is diagnostic in having a wide and constricted snout, a large supratemporal fossa slightly larger than the orbit, and a parietal table strongly constricted posteriorly. Postcranial specializations include a short trunk with 14 dorsal vertebrae, a short tail shorter than the skull-neck region in length, a stout anterolateral process on the clavicle, an extremely massive and broadened mid-diaphysis of the humerus, a strongly expanded proximal head of the ulna, seven ossified carpals, and a phalangeal reduction in the pes. In addition, the strongly pachyostotic postcranial skeleton may indicate a slow mode of swimming underwater and a benthic carnivorous feeding habit for B. jiyangshanensis. Phylogenetically, it is the sister taxon of the Nothosaurus-Lariosaurus clade within Nothosauroidea. The discovery of this new nothosauroid not only contributes to local faunal diversity and expands the known range of sauropterygian life styles during the late Middle Triassic, but also provides a chance to test the phylogenetic relationships of the Eosauropterygia hypothesized by previous studies.

INTRODUCTION

Sauropterygia is a diverse group of aquatic diapsid reptiles that flourished in Mesozoic seas and consists of two major groups, Placodontiformes (Neenan et al., 2013) and Eosauropterygia (Rieppel, 2000). The latter includes the European pachypleurosaurs, a number of pachypleurosaur-like forms from China, and the clades Nothosauroidea and Pistosauroidea (the latter incorporating Plesiosauria; Cheng et al., 2012, 2016; Neenan et al., 2013; Ma et al., 2015).

Interrelationships of the Triassic eosauropterygians, especially the European pachypleurosaurs and a number of pachypleurosaur-like Chinese forms, have been a subject to debate for years (Jiang et al., 2008, 2014; Holmes et al., 2008; Liu et al., 2011; Shang et al., 2011, 2015, 2017; Wu et al., 2011; Cheng et al., 2012, 2016; Neenan et al., 2013; Sato et al., 2014a, 2014b; Ma et al., 2015). However, the monophyly of Nothosauroidea (sensu Rieppel, 2000) is broadly accepted by the aforementioned studies. This group includes Simosaurus, Paludidraco, Germanosaurus, Nothosaurus, and Lariosaurus. Among the nothosauroids, only Nothosaurus and Lariosaurus have been reported in China since 2000. They come from the strata of the Anisian (Jiang et al., 2006a, 2006b; Shang, 2006; Liu et al., 2014) and Ladinian deposits (Li et al., 2002; Rieppel et al., 2003; Li and Rieppel, 2004; Ji et al., 2014). The Chinese Ladinian nothosauroids were found in the Zhuganpo Member of the Falang Formation (= the Zhuganpo Formation of Wang et al., 2008). The Zhuganpo Member is broadly exposed in Fuyuan County of Yunnan Province and its adjacent area in the west part of Xingyi City of Guizhou Province in southwestern China. The Zhuganpo Member also yielded a number of taxa of other sauropterygian lineages, including pachypleurosaur-like forms (such as Keichousaurus, Qianxisaurus), placodontiforms (Glyphoderma), and pistosaurids (Yunguisaurus, Wangosaurus) (see Sun et al., 2016).

In this paper, we describe a new nothosauroid, Brevicaudosaurus jiyangshanensis, gen. et sp. nov. based on two nearly complete skeletons collected from the Ladinian Zhuganpo Member of Fuyuan County (Fig. 1). They show a skull with a general nothosauroid pattern characterized by a tiny and splint-like jugal intercalated between the maxilla and the postorbital and excluded from the orbit (Fig. 2). Brevicaudosaurus jiyangshanensis differs from other known nothosauroids mainly in having an unusually short tail, an anterolaterally broadened mid-diaphysis of the humerus, and a process from the anterolateral end of the clavicle. The discovery of B. jiyangshanensis not only indicates increased morphological and ecological diversity among the eosauropterygians during the late Middle Triassic, but also provides further information for testing the phylogenetic relationships of the group obtained by previous studies.

FIGURE 1. A, schematic map of the fossil locality. B, stratigraphic logs showing the geological section measured near Jiyangshan quarry (left), Fuyuan County, Yunnan, and a synthesis of the section (right) of the Zhuganpo Member of the Falang Formation modified from the left log of Sun et al. (2016:fig. 4). Stars indicate the strata from which specimens of Brevicaudosaurus jiyangshanensis, gen. et sp. nov., were collected. Abbreviation: Fm., Formation.

FIGURE 2. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., skeletons in dorsal view. A, IVPP V 18625, holotype; B, IVPP V 26010, referred specimen.

Institutional AbbreviationsCMN, Canadian Museum of Nature, Ottawa, Canada; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; NMNS, National Museum of Natural Science, Taichung, Taiwan, China.

GEOLOGICAL SETTING

The Ladinian Zhuganpo Member is widely exposed on both east and west sides of a local river (Huangnihe), which runs north to south along the boundary between Fuyuan and Luoping counties of Yunnan Province (west) and Xingyi City in Guizhou Province (east). The two specimens were collected separately from two quarries (about 1 km apart) in Shibalianshan Township, Fuyuan County. The type specimen (IVPP V 18625) came from Jiyangshan quarry (latitude and longitude 25.108掳N and 104.684掳E) on the east side of a local syncline. The referred specimen (IVPP V 26010) was collected from Daqingtou quarry (latitude and longitude 25.126掳N and 104.687掳E) on the west side of the syncline (Fig. 1A). We investigated the stratigraphies of the two fossil localities in late 2018 and found that both specimens came from the same fossiliferous unit that is 1.2 and 1.3 m thick in Daqingtou quarry and Jiyangshan quarry, respectively. The unit consists of medium-to-thin layers of medium gray to dark gray argillaceous limestone below the limestone with chert nodules (left log of Fig. 1B). Within the unit, the specimens of B. jiyangshanensis were collected from a laminated stratum of 15 cm thickness, locally identified as the 鈥榝ish-layer鈥 that yields a fauna of other sauropterygian taxa, thalattosaurs, and protorosaurs (see Benton et al., 2013; Lu et al., 2018). The fossiliferous layer of Keichousaurus hui Young, 1958 is positioned about 90 cm below the 鈥榝ish-layer鈥 in the same area. Sun et al. (2016) integrated the biostratigraphy of the Triassic marine vertebrate faunas of Guizhou Province and its adjacent area in Yunnan Province and considered that the Xingyi Fauna came from the lower portion of the middle Zhuganpo Member, that is dominated by bioclastic limestone with chert concretions (see the right log of Fig. 1B). According to the lithological features of the fossil localities, the fossiliferous unit that produced the two specimens of B. jiyangshanensis should belong to the lower portion of the middle Zhuganpo Member. Our further examination suggests that B. jiyangshanensis may represent a new member of the Lower Fossiliferous Assemblage of the Xingyi Fauna of Sun et al. (2016) because it comes from a laminated layer that is well below the medium-sized limestone with chert nodules, but 90 cm above the layer that produced K. hui in the area (Fig. 1B).

SYSTEMATIC PALEONTOLOGY

SAUROPTERYGIA Owen, 1860 sensu Rieppel, 2000

EOSAUROPTERYGIA Rieppel, 1994b

NOTHOSAUROIDEA Baur, 1889

BREVICAUDOSAURUS, gen. nov.

Type SpeciesBrevicaudosaurus jiyangshanensis, gen. et sp. nov.

Diagnosis鈥擜s for the holotype and only species.

Distribution鈥擫adinian (upper Middle Triassic), Southwest China.

Etymology鈥擳he generic name is derived from Latin 鈥榖revi鈥 for 鈥榮hort,鈥 鈥榗audo鈥 for 鈥榯ail,鈥 and Greek 鈥榮auros鈥 for 鈥榣izard.鈥

BREVICAUDOSAURUS JIYANGSHANENSIS, gen. et sp. nov.

(Figs. 29)

FIGURE 3. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., IVPP V 18625, photographs and outlines of the skull and the mandible in dorsal view. A, B, IVPP V 18625, holotype, in dorsal view; C, D, IVPP V 26010, referred specimen, snout portion of the skull. Zigzag lines indicate broken areas. Abbreviations: art, articular; bo, basioccipital; cqp, cranio-quadrate passage; d, dentary; ec, ectopterygoid; eo, exoccipital; f, frontal; j, jugal; m, maxilla; n, nasal; op, opisthotic; or, orbit; p, parietal; pf, prefrontal; pl, palatine; pm, premaxilla; po, postorbital; pof, postfrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; rap, retroarticular process; sa, surangular; so, supraoccipital; sq, squamosal; st, stapes.

FIGURE 4. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., IVPP V 18625, occipital and anterior cervical regions in dorsal view. Abbreviations: art, articular; at.a, atlantal neural arch; at.c, atlantal centrum; ax.a, axial neural arch; bo, basioccipital; cqp, cranio-quadrate passage; eo, exoccipital; op, opisthotic; p, parietal; p.at, proatlas; prq, pterygoid ramus of quadrate; q, quadrate; qj, quadratojugal; rap, retroarticular process; so, supraoccipital; sq, squamosal; st, stapes.

FIGURE 5. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., IVPP V 18625, cervical region in dorsal view. Zigzag lines indicate broken areas. Abbreviations: cv, cervical vertebra; dv, dorsal vertebra; poz, postzygapophysis; prz, prezygapophysis.

FIGURE 6. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., IVPP V 18625. A, photograph and B, outline of dorsal vertebrae 3 and 4; photographs showing C, the trunk and the sacrum and D, the caudal vertebrae in dorsal view. Abbreviations: car, caudal rib; cr, cervical rib; dr, dorsal rib; dv, dorsal vertebra; dv.c, dorsal vertebral centrum; np, neural spine; poz, postzygapophysis; prz, prezygapophysis; sr, sacral rib; zph, zygosphene; zyg, zygantrum.

FIGURE 7. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., A, B, E, pectoral and B, F, pelvic girdles in dorsal views. AD, IVPP V 18625; E, F, IVPP V 26010. Zigzag lines indicate broken areas. Abbreviations: car, caudal rib; cl, clavicle; clp, clavicle process; co, coracoid; cof, coracoid foramen; cv, cervical vertebra; dr, dorsal rib; dbsc, dorsal blade of scapula; dv, dorsal vertebra; icl, interclavicle; il, ilium; is, ischium; pu, pubis; sc, scapula; sr, sacral rib.

FIGURE 8. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., IVPP V 18625, A, left and B, right forelimbs. Abbreviations: dc1, dc2, dc3, dc4, distal carpal 1, 2, 3, 4; hu, humerus; im, intermedium; ra, radius; se.b, sesamoid bone; ul, ulna; uln, ulnare; I鈥揤, digits I鈥揤.

FIGURE 9. Brevicaudosaurus jiyangshanensis, gen. et sp. nov., left hind limb. A, IVPP V 18625. B, IVPP V 26010. Abbreviations: as, astragalus; ca, calcaneum; dt3鈥+鈥4, distal tarsal 3鈥+鈥4; fe, femur; fi, fibula; mt4, mt5, metatarsal 4, 5; ti, tibia; u.p, ungual phalanx.

Holotype鈥擨VPP V 18625, a complete skeleton exposed in dorsal view except for the posterior portion of the tail, which is slightly twisted.

Referred Specimen鈥擨VPP V 26010, a nearly complete skeleton exposed in dorsal view. The tail has 28 vertebrae preserved and there would be two (the last two) missing, if this specimen also had 30 caudals in life as in the holotype. The last two caudal vertebrae are 3.5 mm long in the holotype and would be proportionally about 3.3 mm long in this specimen (Table 1). The first 14 cervicals and the skull are sharply turned posteriorly and then laterally under the body and the left forelimb, so that the posterior portion of the skull and those cervicals are not exposed. Based on the dimensions of the skull (ca. 43 mm in length compared to 49.3 mm in the holotype) and limb bones, such as the humerus (50 vs. 53 mm) and the femur (48 vs. 52.2 mm), this specimen is slightly smaller than the holotype.

TABLE 1. Measurements (in mm) of selected elements of the holotype, IVPP V 18625, and the referred specimen, IVPP V 26010, of Brevicaudosaurus jiyangshanensis, gen. et sp. nov. Abbreviations: e, estimated; L, left; R, right.

Locality and Horizon鈥擲hibalianshan Township, Fuyuan County, Yunnan Province, China; the holotype from Jiyangshan and the referred specimen from Daqingtou; the lower part of the middle Zhuganpo Member, the Falang Formation, the upper Middle Triassic (Ladinian) (Fig. 1B).

Etymology鈥擳he specific name is derived from the name of the area where the type specimen and many other fossils of the Xingyi Fauna were discovered.

Diagnosis鈥擜 small nothosauroid, distinguishable from others by a set of derived characters: trunk with 14 dorsal vertebrae, tail with 30 vertebrae and shorter than skull-neck length; pre-narial region broad; orbit nearly as big as supratemporal fossa; nasal processes of premaxillae very long, extending posteriorly to meet frontals; postfrontal excluded by postorbital-parietal contact from supratemporal fossa; lateral portion of clavicle hypertrophied and heavily pachyostotic, with a pronounced anterolateral process; humerus with a broadened mid-diaphysis; ulna with a strongly expanded proximal head; seven carpals including a tiny radial sesamoid element; pes with reduced numbers of phalanges (2-3-3-4-2); and the ungual phalanx of each digit dorsoventrally flattened, with a round distal edge.

DESCRIPTION

The holotype of Brevicaudosaurus jiyangshanensis, gen. et sp. nov., is preserved better than the referred specimen, in which sutures between elements are obscured by surface cracks (Fig. 2). The description is based on the holotype unless indicated otherwise.

Cranial Skeleton

The skull and the mandible of the holotype are closely occluded (Fig. 3). Some palatal elements are visible through the skull openings. The snout (preorbital region) is slightly constricted in the region of the external nares; the pre-narial region is relatively broader than that of other nothosauroids such as Nothosaurus youngi (Li and Rieppel, 2004). The snout is longer than the postorbital region along the dorsal midline, but shorter than the postorbital length to the posterolateral end of the squamosal. The external naris and the orbit are both round or oval in outline. The apparent large size of the external naris is exaggerated by damage in the holotype; in life it was smaller and ovoid in outline and surrounded by a post-narial fossa, as seen in the referred specimen (Fig. 3C, D). The external naris is closer to the orbit than to the anterior tip of the rostrum. The internarial septum formed by the premaxillae is very narrow, about one-fourth of the interorbital width. The weakly rimmed orbit is large, nearly as big as the supratemporal fossa (the opening as well as the surrounding depression). The parietal foramen is round and posteriorly positioned. The supratemporal fossae are well rimmed as well. They would have been triangular in outline in life, with the narrowed end pointing medially as in the referred specimen; their kidney-shaped form would have resulted from an inward push of the supratemporal arch at the postorbital-squamosal contact. The occipital edge of the skull roof is deeply concave. The occiput is typically closed and plate-like as in nothosaurs and pachypleurosaurs or pachypleurosaur-like forms (Rieppel, 2000; Holmes et al., 2008; Cheng et al., 2012).

Skull Roof鈥擳he paired premaxillae are characterized by thin and elongated nasal processes, which separate the external nares and the nasals, and extend posteriorly to contact the frontals at points anteromedial to the orbits. The maxillary process of the premaxilla is short and forms the anterolateral margin of the external naris. The premaxilla-maxillary suture is concave.

The nasal is quadrilateral in outline and longer than wide. Anteromedially, the nasal forms the posterodorsal (posteromedial) border of the external naris. Laterally, it contacts the maxilla anteriorly and prefrontal posteriorly. Posteriorly, it makes an interdigitated suture with the frontal. The nasals do not meet along the midline.

The paired frontals are irregular in outline and form a wide interorbital septum. Their sutures with the premaxilla, the nasal, and the preorbital form an interdigitated pattern. Their posterior portions are forked, with the posteromedial fork longer than the posterolateral fork and approaching the supratemporal fossa as in many other eosauropterygians. Both frontal-postfrontal and frontal-parietal sutures are deeply interdigitated.

The parietals are fused. The upper (roof) part is strongly constricted posteriorly and narrower than the depressed portion that creates the floor of the supratemporal fossae. The broad anterior region receives the frontals medially, and meets the postfrontal and the postorbital anterolaterally, an organization that excludes the postfrontal from the margin of the supratemporal fenestra. An anteriorly facing depression is developed from the margin of the pineal foramen on the parietal roof. The posterior margin of the parietal roof is deeply concave and forms a sharp occipital edge. Posteriorly, the parietal suture with the supraoccipital extends transversely and meets the parietal-squamosal suture laterally on the occipital plane, although the dorsoventral compression of the skull roof slightly obscures the direction of the sutures.

The large maxilla is anteriorly broad and posteriorly narrow. Its broad ascending process meets the nasal and the prefrontal anteromedially, and forms the posteroventral border of the external naris anteriorly and the anteroventral border of the orbit posteriorly. Its long posterior process forms the majority of the ventral border of the orbit and receives the jugal and the postorbital at a level near the posteroventral corner of the orbit.

The prefrontal is a small element and forms the anterodorsal corner of the orbit as in Nothosaurus (Rieppel et al., 1999). It meets the nasal and the frontal dorsally and the maxilla ventrally. No suture indicates the presence of a lacrimal.

The jugal is a narrow splint-like bone intercalated between the maxilla and the postorbital, and is ventral to the postorbital as in Nothosaurus (Rieppel, 2001). Its anterior end reaches a level just posterior to the mid-ventral margin of the orbit, being excluded by the maxilla-postorbital contact from the orbit. Posteriorly, it may terminate at the same level as the maxilla, but this could not be directly observed, and it remains broadly separated from the anterior tip of the squamosal.

The postfrontal is irregular in outline and slightly larger than the prefrontal. It forms the posterodorsal margin of the orbit. Posteriorly, it is excluded by the parietal-postorbital contact from the anteromedial margin of the supratemporal fossa.

The postorbital appears to be a triradiate bone. Its broad anterior process forms the posteroventral margin of the orbit. Its narrow posterior process forms the anteroventral margin of the supratemporal fenestra and meets the squamosal in an oblique suture. Its medial process forms the lateral half of the postorbital arch between the orbit and the supratemporal fenestra.

The squamosal is a large and irregularly shaped element. Anteriorly, it forms the posterior half of the supratemporal arch; medially, the bone extends along the posterior border of the supratemporal fossa to meet the parietal and together with that bone forms the posterior border of the fossa; posterolaterally, it develops a distinct lateral process; posteriorly, it sutures with the quadratojugal and caps the quadrate. Its occipital flange contacts the parietal and the supraoccipital medially, the exoccipital and the opisthotic ventrally, and forms the dorsal margin of the cranio-quadrate passage laterally (Fig. 3A, B).

The quadratojugal is bar-like and tapers dorsally. It abuts against the anterolateral margin of the quadrate down to the mandibular condyle.

Braincase鈥擳he supraoccipital is nearly horizontal in orientation and forms a rectangular plate with a median process entering the foramen magnum posteriorly. No distinct ridge is present along the midline of the dorsal surface of the bone (Figs. 3A, B, 4). The supraoccipital forms the dorsal margin of the foramen magnum and sutures with the squamosal laterally and exoccipital ventrolaterally.

The basioccipital defines the ventral margin of the foramen magnum and contacts the exoccipital dorsolaterally. As in Nothosaurus (Rieppel, 1994a), the occipital condyle is formed only by the basioccipital, with a tubercle located on either side of the bone anterolaterally. As a result of the manner of preservation, the presence of an 鈥榚ustachian foramen鈥 in IVPP V 18625 remains unknown, and if present would lie between the lateral basioccipital tubercle and the pterygoid, as in Nothosaurus (Rieppel, 1994a).

The exoccipital is a relatively large and irregular bone. In contrast to the pillar-like exoccipital in Nothosaurus and Simosaurus (Rieppel, 1994a), only the ventromedial part of the bone sits on the dorsolateral aspect of the occipital condyle in this new species. It has an extensive contact with the squamosal and the supraoccipital dorsally. Medially, the exoccipital defines the dorsolateral margin of the foramen magnum. Laterally, it sutures with the opisthotic and may define the dorsal and medial margins of the jugular foramen.

The opisthotic makes a triangular form exposed ventromedially to the occipital flange of the squamosal. It has a crack along its length on both sides. Its suture with the exoccipital is obscured owing to fusion. A recess sitting between the opisthotic and the exoccipital possibly indicates a muscle attachment. The jugular foramen is obscured due to a dorsoventral compression. The cranio-quadrate passage opening is defined by the opisthotic medially, the mandibular condyle of the quadrate ventrally, the shaft of the quadrate laterally, and the occipital flange of the squamosal dorsally and dorsomedially, as it is in Nothosaurus (Rieppel, 1994a).

A splint-like element lying against the medial surface of the pterygoid ramus of the quadrate on both sides is identified as the stapes (Figs. 3A, B, 4). The right stapes is complete, with an exposed length of 8 mm; its external end is slightly wider than the mid-shaft, but its medial end is not exposed. The stapes is relatively massive compared with that of some aquatic reptiles, such as the thalattosaur Miodentosaurus (Wu et al., 2009:fig. 2) or mesosaurs (Laurin and Pi帽eiro, 2017:fig. 3).

Palate鈥擳he palatine is partly visible in the orbits. It appears as a plate-like bone, but its exact shape and relationships with other palatal elements are unknown.

The pterygoid and the ectopterygoid are also visible in the orbits and the supratemporal fenestra. The pterygoid contacts the ectopterygoid laterally. No further information on these bones is easily ascertained from a dorsal view of the skull.

The ectopterygoid is exposed in the orbits on both sides. It is incomplete and disarticulated with the pterygoid posteriorly. No other features are available because the palatal surface is hidden in the matrix.

The quadrate forms the posterolateral corner of the skull. It contacts the descending process of the squamosal anterodorsally and extends posteroventrally, placing the mandibular joint behind the occipital condyle. The lateral portion of the mandibular condyle of the quadrate is slightly larger than the medial portion. Medially, the pterygoid ramus of the quadrate is broad and forms, together with the quadrate ramus of the pterygoid and the paraoccipital process, the dorsomedial and lateral walls of the middle ear chamber.

Mandible鈥擳he mandible abuts the upper jaw firmly and only the posterior portion is exposed. The surangular forms the dorsal half of the posterior portion of the mandible but anteriorly it is split into two by a crack along its length, which exposes the adductor chamber formed ventrally by the angular. The angular is partly exposed opposite to the supratemporal arch. Relations of the angular with the neighboring bones are hidden under the surangular. The articular is short and broad, and its articular fossa is divided. Its retroarticular process is moderately developed, bar-like in dorsal view, and posteriorly truncated. A longitudinal ridge divides the dorsal surface of the process.

Teeth鈥擳here are five premaxillary teeth. The anterior four teeth are large, fang-like, and strongly procumbent, and the fifth is small. The maxillary teeth are small and sub-conical. There is one maxillary fang exposed. Due to the occlusion with the lower jaw, the exact numbers of maxillary and dentary teeth cannot be determined. The posterior end of the dentary dentition projects to a level opposite to the anterior margin of the supratemporal fenestra.

Postcranial Skeleton

Vertebral Column鈥擳he vertebral column is complete and includes 21 cervicals, 14 dorsals, four sacrals, and 30 caudals (Figs. 2, 5, 6). In the referred specimen, the cervical section is not completely exposed but the last cervical, the 21st, vertebra can be determined based on a set of criteria (see below). Accordingly, the vertebral number of the other sections are the same as in the holotype.

The 21st vertebra is considered as the last cervical for the new form in terms of the following features: (1) the distal half of its rib is medial in position; (2) the shaft of the rib is still thinner in diameter than that of the first dorsal rib; and (3) the distal end of the rib tapers rather than slightly swells for connecting with a cartilaginous section of the first dorsal rib (Figs. 2, 6C, 7A, B, E). These are comparable to conditions seen in other reptiles such as a Triassic turtle (Gaffney, 1990:fig. 108), extant crocodylians (see Alligator sinensis in Cong et al., 1998:fig. 82) and lizards (see CMN 44006, a skeleton of Heleoderma suspectum). Four sacral vertebrae were identified because of their ribs, which are distally convergent towards the ilium and thickened dorsoventrally or anteroposteriorly (Figs. 6C, 7C, D, F).

The neural spines are very low, although the tip of this process is damaged on most of the vertebrae except for some posterior caudals. The broken surfaces of some spines indicate that the neural spines are longitudinally broad and closely approach each other along the midline. All zygapophyseal processes and ribs are pachyostotic.

The proatlas is a small triangular element that is displaced to the right side of the atlantal centrum (Fig. 4). The latter is a short column and slightly dislocated, sitting in front of the axial neural arch. Two atlantal neural arches are present, detached from the atlantal centrum. The rather complete right one shows a pointed anterior process and a pronounced postzygapophysis. Both atlantal neural arches may have met along the dorsal midline in life. The axial neural arch shows no difference from other cervicals in morphology. It is only slightly longer than that of cervical 3.

In cervicals 3 to 21, the neural arch increases in size caudally. It gradually becomes longer and broader. The zygapophyses are all swollen and pachyostotic. The zygosphene-zygantrum articulation is exposed in cervicals 6, 7, and 13鈥18 (Fig. 5). There are small elements visible along the right side of cervicals 3鈥5. All of them are of triangular shape and pointed anteriorly, possibly representing the cervical ribs 3鈥5. No traces of the cervical ribs are preserved from cervicals 6鈥14. The cervical ribs are well preserved from cervical 15 onward. They are swollen and become longer posteriorly. Cervical ribs 16 and 17 have a free anterior process at the proximal end. Cervical ribs 18鈥21 are single headed (Figs. 5, 7A, B, E).

All 14 dorsal vertebrae of the trunk are well articulated in the referred specimen. They increase in size from dorsals 1鈥5 and then decrease from dorsals 10鈥14. The pre- and postzygapophyses are weakly developed, anteroposteriorly very narrow, as shown in dorsal vertebrae 3 and 4 of the holotype (Fig. 6A, B). Curiously, the zygosphene-zygantrum articulation is a dorsoventral articulation and characterized by a unique position. The zygosphene comes from the posterior base of the neural spine as a sharp projection on the same plane of the postzygapophyses, whereas the zygantrum is a triangular depression on the anterior base of the neural spine dorsal to the prezygapophyses, an opposite condition to that present in Simosaurus gaillardoti (see Rieppel, 1994b:fig. 15). The transverse processes are very stout and heavily pachyostotic. There is no evident notch between the transverse process and zygapophyses in the mid-dorsal vertebrae. The neural spines are very low and dorsally taper off, with their anterior margins ending well posterior to the level of the prezygapophyses, whereas their posterior margins extend backwards well beyond the level of the postzygapophyses.

All dorsal ribs are distinctly pachyostotic. They bow posteromedially, with a thickened distal end. The last three dorsal ribs become gradually shorter; the posterior-most two are just slightly longer than the first sacral rib and taper off distally (Figs. 2, 6C). Some gastralia are visible through the rib cage and show no distinctive features.

The four sacral vertebrae become shorter posteriorly following the last dorsal vertebra (Figs. 6C, 7C, D, F). As in the dorsal series, the neural spines are low and longitudinally broadened. The sacral ribs are stout and bar-like; the first is slightly longer than the other three in the holotype but not in the referred specimen. The four sacral ribs converge distally.

Of the 30 caudal vertebrae, the last one tapers posteriorly (Fig. 6D). The 2nd and 3rd caudals are partly damaged, the 4th and 5th have the neural arches partly detached from the centra, but the four are all complete in the referred specimen (Fig. 2B). In size, the caudal vertebrae become wider from the first to the fifth and then become thinner and shorter towards the end. The last caudal vertebra is very thin but it is as long as the penultimate (about 1 mm long). The caudal centra are exposed in the posterior region, with a cylindrical outline and no lateral constriction. The rectangular neural spines, which are well preserved in some of the posterior caudals in the referred specimen, are longitudinally long and oblique in orientation, and meet each other at mid-height.

All caudal ribs are extremely pachyostotic and clearly present in the first to 14th caudals. They are straight and broad, wider than the inter-rib spaces in the first 10 caudals. The length of the caudal ribs distinctly changes, showing a lateral wave; i.e., they initially become shorter from the first to fourth and then become longer from the fourth to sixth, and again become shorter up until the 14th. The caudal ribs also vary in shape and orientation. The distal ends of the long caudal ribs (the first and the fifth to tenth) taper off into a sharp point distally, whereas the distal ends of the shorter caudal ribs (second to fourth and tenth to 14th) are truncated. The first three caudal ribs project anterolaterally, the sixth is the longest and slightly curves anterolaterally and the others all project posterolaterally. No chevron is exposed.

Pectoral and Pelvic Girdles鈥擡lements of the pectoral girdle are partly exposed, revealing the clavicle and the scapula to be massive (Fig. 7A, B, E). The interclavicle is broadly covered by the vertebral column. An oblique suture present at its distal portion shows that the interclavicle tightly fits the clavicle dorsolaterally. The elements of the pectoral girdle are dislocated in the referred specimen, which obscured their outlines and relationships.

The clavicle is well developed and very stout. The ventromedial bar of the clavicle is relatively short and narrow. The connection of the two clavicles in the midline is obscured. The dorsolateral portion of the clavicle is massive and heavily pachyostotic; it forms a remarkably stout anterior process along the anterolateral margin of the bone. This process is small and comes from the ventromedial bar in Simosaurus (Rieppel, 1994b), whereas it is sharp and lightly built in the pachypleurosaur-like Diandongosaurus (Shang et al., 2011; Sato et al., 2014a) and Dianmeisaurus (Shang and Li, 2015). Posteriorly, the clavicle broadly and tightly overlaps the dorsomedial surface of the scapula, but its posterior tip is damaged.

The exposed ventral portion of the scapula is broadened. The dorsal blade is damaged in the holotype, but it is preserved in the referred specimen on the right side (Fig. 7E). The dorsal blade is rod-like, with a thin distal end, as in Simosaurus (Rieppel, 1994b:fig. 25a). As preserved, the scapula, the clavicle, and the interclavicle are tightly articulated as a unit.

Both coracoids only show their lateral (glenoid) portions in the holotype and a broad part in the referred specimen (Fig. 7A, B, E). The posterior region of the glenoid portion is thickened. The coracoid foramen is enclosed by the scapula and the coracoid.

The ilium is displaced far from its original region but in life position on the left side of the referred specimen (Fig. 7F). It is small and dorsally constricted as in nothosaurs. The medial surface is slightly concave, but ridged around margins. The ischium is the only element of the pelvic girdle that is nearly completely exposed (Fig. 7C, D, F). As in many other eosauropterygians, it has a thickened glenoid portion and a strongly expanded distal portion. The proximal end of the ischium is stout with an oval iliac facet on its dorsolateral surface. Their posterolateral margin is slightly concave. The pubis is broadly covered; only its thickened glenoid portion is exposed and shows a flat iliac facet.

Forelimbs and Hind Limbs鈥擳he forelimb is stout except for the manus and characterized by the shapes of the humerus and the ulna (Fig. 8). The humerus is pachyostotically enlarged and its shaft is distinctly bowed anterolaterally and much thicker than both the distal and proximal ends. The two ends are similar in width in certain orientations (Fig. 8A), whereas the proximal end is narrower than the distal one in other orientations (Fig. 8B), indicating that the articular surfaces are not circular. The entepicondylar foramen is absent. An ectepicondylar groove is present. On the distal surface, the articular facet for the ulna is slightly concave and much broader than that for the radius.

The radius is medially concave and laterally convex, with both the proximal and distal ends slightly expanded. The distal articular facet is nearly flat, whereas the proximal facet for the humerus is slightly concave. The ulna is approximately as long as the radius but its shaft is over twice as broad as that of the latter. The proximal end is distinctly expanded and much wider than the distal end as in Keichousaurus (Lin and Rieppel, 1998) and some specimens of Lariosaurus (Rieppel, 1998). The proximal facet for the humerus occupies about the medial third of the proximal end. The cross section of the ulna is wedge shaped, with the radial edge being thicker than the lateral margin.

There are six well-ossified carpals in close articulation, i.e., the ulnare and intermedium forming the proximal row and distal carpals 1鈥4 of the distal row (Fig. 8). The kidney-shaped intermedium is the largest. It articulates with all other carpals except for distal carpal 1 and its proximal surface is concave. The ulnare is asymmetrically pentagonal in outline and much smaller than the intermedium. The largest of the distal row is distal carpal 4. A tiny pre-axial accessory element developed anterior to distal carpal 1 is tentatively identified as a radial sesamoid bone.

Metacarpal III is the longest, followed by metacarpals IV and II. Metacarpal I is shortest but has a width similar to that of the others. The phalangeal formula is 2-3-4-3(4?)-2. The ungual is present in digits 1鈥3 and digit 5, but not seen in digit 4 of either forelimb. Comparing with the lower arm, the manus is relatively weakly developed; the longest metacarpal III (9.5 mm) is about 36% of the left radial length.

The hind limb is similar to the forelimb in length. The femur is slender and exhibits a weak sigmoidal curvature; the posteromedial side is slightly concave (Fig. 9). The fibula is slightly longer and thinner than the tibia. Its shaft bows towards the tibia. The modestly expanded distal end has two facets for the calcaneum and the astragalus. The shaft of the tibia bows towards the fibula. Both ends of the tibia are slightly expanded and each has a concave facet for the femoral condyle proximally and for the astragalus distally.

There are three well-ossified tarsals, including the calcaneum, the astragalus, and a distal tarsal (Fig. 9). The astragalus is the largest, with a shoe-shaped outline as in Dawazisaurus (Cheng et al., 2016). The flat dorsolateral aspect articulates with the fibula, the concave dorsomedial aspect receives the tibia, the slightly convex posterolateral aspect contacts metatarsal I, and the distal surface meets metatarsals II, III, and the only distal tarsal. The calcaneum is a rounded polyhedron with its dorsal surface depressed. The distal tarsal is the smallest among the three tarsals. It may represent the fusion of distal tarsals 3 and 4. Distally, the distal tarsal meets metatarsal IV and possibly metatarsal III, too, in life. As in many Chinese pachypleurosaur-like forms (Shang and Li, 2015; Cheng et al., 2016), the shortest and stoutest is metatarsal I, which is less than half of the longest metatarsal in length; metatarsals III and IV are similar in length, and metatarsals II and V are comparable in length. All digits are completely preserved in both feet. It shows a reduced number of phalanges in comparison to that of the fingers. The phalangeal formula of the pes is 2-3-3-4-2. The ungual phalanx is nearly rectangular in outline and much larger than the penultimate phalanx in digits I鈥揑V.

COMPARISONS

As suggested by the phylogenetic analysis of this study (see below), B. jiyangshanensis is a nothosauroid. Therefore, Brevicaudosaurus will be compared here mainly with the members of Nothosauroidea. Nothosauroidea at present includes five genera as defined by this study; they are Nothosaurus, Lariosaurus, Simosaurus, Wangosaurus, and the newly described Paludidraco (de Miguel Chaves et al., 2018), and the new genus Brevicaudosaurus. In addition to the aforementioned genera, the following comparison also includes Germanosaurus because the latter has often been considered as a nothosauroid (Rieppel, 2000; Jiang et al., 2014). Brevicaudosaurus has a relatively small supratemporal fossa about the same size as the orbit, differing from the huge and elongate supratemporal fossa of all other nothosauroids listed above (Rieppel and Wild, 1996; Rieppel, 1998, 2000; Li and Rieppel, 2004; Shang, 2006; Jiang et al., 2006b). With this feature, Brevicaudosaurus cannot be confused with any of the aforementioned nothosauroids.

Rieppel (1998, 2000) considered Silvestrosaurus buzzii and Ceresiosaurus calcagnii as two species (L. buzzii, L. calcagnii) of the genus Lariosaurus. Lariosaurus buzzii appears similar to Brevicaudosaurus in exhibiting a relatively small supratemporal fossa (equal in length to the orbit), a broad parietal skull table that shows only a weak constriction in its posterior part, and the pineal foramen located slightly behind the midpoint of the parietal (Tschanz, 1989). However, the oval supratemporal fossa, the weakly expanded anterolateral corner of the clavicle, and the absence of pachyostosis are very different from those of the new species. As for L. calcagnii, it clearly differs from Brevicaudosaurus in the supratemporal fossa that is much larger than the orbit, the relatively large forelimbs, and the presence of hyperphalangy in the pes (Rieppel, 1998; H盲nni, 2004:fig. 9).

The nothosaurid genera Simosaurus and Germanosaurus have not yet been found in China. Although Simosaurus has a brevirostrine skull, it is similar to Brevicaudosaurus in that the mandibular articulation is at a level well behind the occipital condyle and the squamosal possesses a distinct lateral process at the posterolateral corner. However, in Simosaurus (Rieppel, 1994b:figs. 8, 9), the supratemporal fossa is not as large as in Nothosaurus or Germanosaurus or Lariosaurus, but it is still about twice as large as the orbit, which is very different from the condition in Brevicaudosaurus. In addition to the relatively small supratemporal fossa, Brevicaudosaurus further differs from Simosaurus in the constricted rostrum, the relatively anterior position of the maxillary tooth row, and the closed and plate-like occiput (Rieppel, 2000). Compared with Germanosaurus (see Rieppel, 2000), Brevicaudosaurus differs in the fused parietals and posteriorly constricted parietal septum between the supratemporal fossae.

In the postcranial skeleton, Brevicaudosaurus differs from known nothosauroids mainly in the trunk that has only 14 dorsal vertebrae, the tail that is shorter than the skull-neck region, the vertebral column and the ribs that are all pachyostostic, the clavicle has a stout and massive anterolateral process, and the pes exhibits fewer phalangeal elements. Brevicaudosaurus and Lariosaurus curionii (Rieppel, 1998:fig.14) appear similar in the pachyostostic zygapophyses and the morphology of the humerus and the ulna, but the former genus differs from the latter in other postcranial characters mentioned above, such as the short tail and the stout and massive anterolateral process of the clavicle. The clavicle has an anterior process in both Brevicaudosaurus and Simosaurus, but in the latter this process is small rather than massive and extends from the anterior margin rather than the anterolateral end of the anteroventral portion (Rieppel, 2000).

Brevicaudosaurus appeared to coexist with Keichousaurus, and its ulna is comparable in the morphology of that of the latter genus, but the proximolateral edge of the ulna is round rather than the square shape in Keichousaurus. It is obvious that Brevicaudosaurus is also different from Keichousaurus in that the supratemporal fossa has a triangular outline and is slightly larger than the orbit, the clavicle possesses a massive anterolateral process, and the dorsal (trunk) and caudal sections of the vertebral column are reduced in number.

PHYLOGENETIC RELATIONSHIPS

In order to test the relationships of the new nothosauroid, we conducted a phylogenetic analysis based on the data matrix of Shang et al. (2017), which is the most complete in the inclusion of the Chinese eosauropterygians after modification from those of Neenan et al. (2013) and Cheng et al. (2016). The study of a new eosauropterygian by Jiang et al. (2019) is the most recent work on the Chinese eosauropterygians, but this study did not include a newly described eosauropterygian, Dawazisaurus brevis Cheng et al., 2016, and other Chinese taxa closely related to the Sauropterygia in their phylogenetic analysis. With the addition of five extra taxa, including Bobosaurus (Dalla Vecchia, 2006) as in de Miguel Chaves et al. (2018), Wangosaurus (Ma et al., 2015), Paludidraco (de Miguel Chaves et al., 2018), Panzhousaurus (Jiang et al., 2019), and the new species described here, our data matrix thus includes 54 taxa and 145 characters. Of the 145 characters, four are new and one (character 97) was modified following Sato et al. (2014a, character 80). Coding of each character for the previous ingroups, except for Simosaurus, Eusaurosphargis, Dianmeisaurus, Diandongosaurus, and Wangosaurus, changes little (see Appendix S1 in Supplemental Data), but the Chinese genera Chinchenia, Kwangsisaurus, and Sanchiaosaurus were excluded owing to their fragmentary nature, as in Shang et al. (2017), de Miguel Chaves et al. (2018), and Jiang et al. (2019). Coding of the four new characters for each taxon was based on personal examination of specimens (mostly Chinese taxa) or the photographs and figures in publications, or following de Miguel Chaves et al. (2018). This study is not intended to test a new diapsid phylogeny or relationships between Sauropterygia and other terrestrial groups, which is beyond the scope of the study. We limit our analysis to the establishment of the phylogenetic relationships of Brevicaudosaurus within the Eosauropterygia.

We analyzed the data matrix using TNT version 1.5 (Goloboff et al., 2008). Multistate characters were left unordered. All characters were equally weighted. A traditional search was performed, with 1,000 replications of Wagner trees, followed by tree bisection recognition as a swapping algorithm, saving 100 trees per replication as in de Miguel Chaves et al. (2018). Phylogenetic analysis yielded 14 most parsimonious trees (MPTs), each with a tree length of 864 steps, a consistency index (CI) of 0.291, and a retention index (RI) of 0.657. As indicated in the strict consensus of the 14 MPTs (Fig. 10), the monophyly of the Eosauropterygia was again recognized, as in recent studies (such as Neenan et al., 2013; Cheng et al., 2016; de Miguel Chaves et al., 2018; Jiang et al., 2019). Brevicaudosaurus was grouped in a monophyletic Nothosauroidea. Within that group, Brevicaudosaurus was positioned as the sister taxon of Nothosauridae (Nothosaurus and Lariosaurus). Wangosaurus, which was considered to be closely related to pistosauroids (Ma et al., 2015; Jiang et al., 2019), is the sister group of the Nothosauridae-Brevicaudosaurus clade within the Nothosauroidea. As in de Miguel Chaves et al. (2018), Simosaurus and Paludidraco formed a monophyletic clade, which is parallel to the clade ((Nothosauridae鈥+鈥Brevicaudosaurus) Wangosaurus) within Nothosauroidea. Germanosaurus, which has been often grouped in the Nothosauroidea before, was excluded from the group and is positioned here as the sister taxon of Cymatosaurus within the Pistosauroidea (sensu Rieppel, 2000). Pistosauroidea and Nothosauroidea are sister group according to our results, which differ from the results of Shang et al. (2017) in which the Nothosauroidea, the European pachypleurosaurs, and Chinese pachypleurosaur-like forms formed a monophyletic group. There are 10 Chinese pachypleurosaur-like forms included in our analysis, of which Diandongosaurus and Dianmeisaurus form successive outgroups to the Pistosauroidea-Nothosauroidea clade, and then these clades were followed by five Chinese forms and two European pachypleurosaurs with unresolved relationships. The Chinese genera Qianxisaurus, Wumengosaurus, and Hanosaurus were found by this analysis to be progressively basal within the clade of the Eosauropterygia. Phylogenetic relationships for Eusaurosphargis, the placodontiforms (including Helveticosaurus), and Saurosphargidae were also resolved. Compared with the study of Cheng et al. (2016), phylogenetic relationships within Eosauropterygia are better resolved but not as well as in Shang et al. (2017). In the study of de Miguel Chaves et al. (2018), Wumengosaurus and Diandongosaurus of the Chinese pachypleurosaur-like forms were more closely related to the Pistosauroidea than to Nothosauroidea, Germanosaurus was closely related to Nothosauroidea, and Helveticosaurus was very basal in phylogenetic position, although the phylogenetic relationships of other non-sauropterygian groups are comparable to those of this study. Compared with the study of Jiang et al. (2019), the most striking differences include the uncertain relationships of the aforementioned five Chinese forms and two European pachypleurosaurs, as well as the nothosauroid status of Wangosaurus and the pistosauroid status of Germanosaurus. Furthermore, compared with those studies including the Ichthyopterygia and Thalattosauria in their phylogenetic analyses (Neenan et al., 2013; Cheng et al., 2016; Shang et al., 2017; de Miguel Chaves et al., 2018), the interrelationships of turtles (represented by Testudines and Odontochelys), Lepidosauromorpha, and Archosauromorpha obtained in this study are comparable to those of all studies listed above except Shang et al. (2017), in which the three groups were hypothesized as successive outgroups of the clade formed by the included aquatic groups. The aforementioned differences among studies may have resulted from the different sizes of the data matrices used. Synapomorphies of major groups are provided in Appendix S1 in Supplemental Data.

FIGURE 10. Strict consensus of 14 MPTs depicting the phylogenetic relationships of Brevicaudosaurus. Archosauriformes, Choristodera, Prolacertiformes, Rhynchosauria, and Trilophosaurus are summarized as Archosauromorpha. Kuehneosauridae, Rhynchocephalia, and Squamata are consolidated here as Lepidosauromorpha. Numbers by each clade represent Bremer support values (left) and bootstrap support values (right), respectively. Abbreviations: Anaro , Anarosaurus; Dactylo , Dactylosaurus; Largo-polycarpon , Largocephalosaurus polycarpon; Largo-qianensis , Largocephalosaurus qianensis; Neustico , Neusticosaurus; Serpiano , Serpianosaurus.

DISCUSSION

Ontogenetic Age

The unfused condition of the neurocentral sutures suggests that IVPP V 18625 may represent a young or even juvenile individual. However, the fused parietals and the well-ossified carpals and tarsals may indicate that IVPP V 18625 is, at least, a young adult. There are many examples in reptiles where the parietals are paired in juvenile or even young adults and become fused in adults. In general, carpals or tarsals are the last to ossify in eosauropterygians, as in extant reptiles. For example, a specimen of Neusticosaurus peyeri that is considered as a hatchling shows no carpals but an almost complete phalangeal count (Sander, 1989:fig. 33). Although a tarsal started to ossify in an embryo and three are present in a young juvenile of the taxon (Sander, 1988:fig. 2), all of these tarsals show a roundish outline with no finished edges. In addition, another line of evidence may also suggest that IVPP V 18625 is an adult individual: in the pectoral girdle, the interclavicle, the clavicle, and the scapula are tightly sutured to form a rather rigid, articulated unit. Considering the disarticulation in the series of vertebrae 22鈥25 of the same region, the elements of the pectoral girdle should have also been displaced from each other by the same distortion if IVPP V 18625 would have been a juvenile individual, in which a loose connection was expected between the elements. The referred specimen (IVPP V 26010) is slightly smaller than the holotype, but its carpals and tarsals are also well ossified and its parietals are fused. These suggest that the sizes of the holotype and the referred specimen may represent the actual size of adults in this species.

Stapes

Carroll and Gaskill (1985:fig. 14g) postulated the presence of a cylindrical stapes in a specimen of Neusticosaurus sp., but the interpretation of that fossil remains equivocal (Rieppel, 1989). Although the stapes is known in neither Simosaurus nor Nothosaurus, Rieppel (1994a) predicted that the stapes, if present, would be thin and slender in view of the facts that sauropterygians are neodiapsid reptiles and the pachypleurosauroids are the sister group of all other sauropterygians.

The stapes is clearly identifiable in B. jiyangshanensis. It is in the form of a relatively thickened, elongate bar; such a massive stapes may have attached laterally to a 鈥榯ympanum鈥 of skin or muscle rather than a membrane as a sound transmitting system in the water.

Short Tail

Dawazisaurus brevis is most comparable in the count of the total vertebrae among the known non-pistosauroid eosauropterygians that have the vertebral column completely preserved. Dawazisaurus brevis and B. jiyangshanensis both have 36 presacral vertebrae, but the former has 37 caudal vertebrae, which is seven more than the 30 of B. jiyangshanensis. In general, the tail is always much longer than the combination of the neck and the skull in non-pistosauroid eosauropterygians, including D. brevis, but B. jiyangshanensis displays the opposite condition, i.e., the tail is uniquely shorter than the neck-skull region, and much shorter than the precaudal region. With such a short tail, B. jiyangshanensis may not have been able to swim fast. On the other hand, the transversely elongated caudal ribs (transverse processes) indicate that the proximal half of the tail was certainly not laterally compressed but strongly flattened dorsoventrally, with its width comparable to that of the trunk based on the length of the sixth caudal rib (Fig. 2). Such a short and flat tail may have functioned as a balancer during activities of the animal (see more below).

Reduction of Phalangeal Count

Though the fore- and hind limbs of Brevicaudosaurus jiyangshanensis are comparable in many aspects to those of some species of Lariosaurus in which the limbs are complete (Rieppel, 1998), the phalangeal morphology of both fore- and hind limbs is very different. The hyperphalangy of the limbs of Lariosaurus is well established (Rieppel, 1998), displaying a phalangeal formula of 4-5-5-4(5?)-3 and a complete pedal phalangeal formula of 2-3-4-5-4, which is little different from the primitive pattern of terrestrial reptiles. It is true that some species of Lariosaurus show a mild hyperphalangy, such as Lariosaurus hongguoensis (Jiang et al., 2006b). The phalangeal count remains unknown in the manus and incompletely known for the pes in L. hongguoensis, but most likely corresponds to the plesiomorphic count 2-3-4-5-4 as seen in Nothosaurus (Rieppel, 2000). The limbs retain the primitive phalangeal formula in most Chinese pachypleurosaur-like forms although some show a trend toward hyperphalangy (such as Diandongosaurus: Shang et al., 2011).

In Brevicaudosaurus, the left manus shows a preserved phalangeal formula of 2-3-4-3-2 and the right manus shows a preserved formula of 1-3-4-3-1. The ungual is not preserved in digit 4 on both sides in the holotype. With the ungual element, digit 4 and digit 5 should have had at least four and two phalanges, respectively, although one more cannot be ruled out in the two digits, i.e., Brevicaudosaurus may have had a manual phalangeal formula of 2-3-4-4/5-2/3. As shown in the holotype and the referred specimen, the feet show a phalangeal formula of 2-3-3-4-2 in Brevicaudosaurus. Therefore, hind limbs had no hyperphalangy but rather a reduction of phalangeal count in pedal digits III to V. This is the only example so far known in eosauropterygians. On the other hand, each phalangeal element of the digits of the manus and the pes also shortens; especially those of the foot, many of which have a length nearly as broad as or even narrower than the width (see Fig. 9). None of these digital morphological changes supports the view that the hind limb played an important role in propulsion in B. jiyangshanensis.

Pachyostosis and Functional Significance

Pachyostosis is common in Triassic eosauropterygians (such as pachypleurosaurs, Lariosaurus, Keichousaurus, Wumengosaurus, Dianopachysaurus, and Dawazisaurus), but with different degrees of intensity and often restricted to the ribs and pre- and postzygapophyses (Houssaye, 2009, 2013). Brevicaudosaurus shows most of the body affected by pachyostosis, in not only the vertebrae and the ribs, but also the girdles and the humerus, as in the filter-feeding Paludidraco multidentatus (de Miguel Chaves et al., 2018) or extant herbivorous dugongs (Houssaye et al., 2016). The heavy skeleton of P. multidentatus and extant sirenians was argued to act as ballast for benthic feeding and as a part of a hydrostatic control mechanism (Domning and de Buffr茅nil, 1991). Taylor (2000) considered that pachyostosis increased stability but lost buoyancy quickly with depth, which was often seen with a corresponding increase in lung volume for positive buoyancy and, therefore, to reach neutral buoyancy at shallower depths. On the other hand, animals exhibiting pachyostosis are usually interpreted as being restricted by their low swimming speeds and lack of agility to living in shallow water environments, in which their increased stability is advantageous (de Ricql猫s and de Buffr茅nil, 2001).

Among the non-pistosauroid eosauropterygians, most elements of the presacral axial skeleton display pachyostosis in the European pachypleurosaurs and some Chinese pachypleurosaur-like forms (Houssaye, 2009; Liu et al., 2011; Cheng et al., 2016). They are small (0.2鈥1.2 m), lizard-like sauropterygians that have a small head, a relatively long neck, flipper-like limbs, loose attachments of girdles to the vertebral column, and a long tail probably used for propulsion (Mazin, 2001).

Nothosaurs display reduced limbs, an elongated neck and body, and a laterally compressed tail (Houssaye, 2009). The forelimbs were probably used for swimming, whereas the more reduced hind limbs are poorly specialized for aquatic locomotion and were probably not involved in propulsion (Carroll, 1988). Pachyostosis has been identified in Lariosaurus in most of the body and in the zygapophyses or the ribs of a few species of Nothosaurus (Rieppel, 2000; Renesto, 2010).

Brevicaudosaurus jiyangshanensis has a broad, flat body, and a very short and flattened tail that is also pachyostotic. Such a tail cannot play a main role in propulsion as in long-tailed species of nothosauroids. As in some species of Lariosaurus, the forelimb is more strongly developed than the hind limb, with a swollen humerus and a widened ulna, and therefore it possibly played a more active role in swimming.

The reduction of phalangeal length, extreme shortening of the tail, and extensive pachyostosis of body elements indicate that B. jiyangshanensis could not swim quickly. It most probably adapted to a way of life as a benthic feeding carnivore, using its short and flattened tail as a balancer in shallow water, without expending energy to maintain a position while searching for food.

However, external pachyostosis itself might not indicate bone mass (ballast) increase in marine animals (Houssaye, 2013; Klein et al., 2016). To verify the correlation of the pachyostosis to internal osteosclerosis in B. jiyangshanensis, it will be necessary to conduct a study on the histology of the species as on others, such as Klein et al. (2016) and Houssaye et al. (2016) did for certain marine reptiles.

Interrelationships

Brevicaudosaurus is a nothosauroid, which is supported by four unequivocal character states: 22(2), parietals fully fused; 40(0), dorsal wing of epipterygoid approximately as broad as its base (unknown in the new species); 121(1), pubis with concave ventral (medial) margin; and 132(1), proximal concavity of astragalus present. Within Nothosauroidea, Brevicaudosaurus is further grouped with Nothosaurus-Lariosaurus clade. This is based on four unequivocal synapomorphies including characters 4(1), distinct snout constriction in adult; 19(2), jugal restricted to a position behind orbit without reaching the posterior margin of the orbit; 89(1), distal head of sacral ribs not expanded; and 142(1), supraoccipital sutured with the parietal horizontally. However, like many other eosauropterygians, the nothosauroid assignment of Brevicaudosaurus is not well established in light of low Bremer and bootstrap values (Fig. 10).

Germanosaurus is here hypothesized as a pistosauroid based on four unequivocal synapomorphies including characters 35(1), 43(1), 82(1), and 103(1). Such a relationship of Germanosaurus cannot be stable because all of the aforementioned synapomorphies are unknown in the taxon. In other words, it is too early to obtain convincing phylogenetic relationships of Germanosaurus and other poorly represented taxa before new materials are found for them.

The pistosauroid Wangosaurus (sensu Ma et al., 2015) is considered as a nothosauroid in this study, an identification that is supported by four unequivocal character states (see those for Brevicaudosaurus above). Furthermore, it is more closely related to the clade (Brevicaudosaurus (Nothosaurus鈥+鈥塋ariosaurus)) than to the clade Simosaurus鈥+鈥Paludidraco within Nothosauroidea based on two unequivocal synapomorphies: 17(0), frontal with no distinct posterolateral process, and 24(2), pineal foramen strongly posteriorly positioned.

Supplemental material

Supplemental Material

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ACKNOWLEDGMENTS

We thank Y.-F. Chen for his skillful preparation of the specimens used in this paper, W. Gao for photographic work, L.-T. Wang for field assistance, D.-Y. Jiang for providing references related to his work, and S. J. Rufolo for editorial and linguistic assistance. We also want to thank two anonymous referees who reviewed the manuscript, offering critical comments and suggestions that led to its great improvement. The research was supported by grants from the Strategic Priority Research Program of Chinese Academy of Sciences (XDB26000000), the National Natural Science Foundation of China (41372028), the Chinese Academy of Sciences (cash in kind), and Canadian Museum of Nature (RS09).

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