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HUDIESAURUS SINOJAPANORUM Dong, 1997 (Figs. 2–4) Original Diagnosis — Re-written from Dong (1997:102): (1) top of neural spine of anterior dorsal vertebra forms a ‘U’-shaped shallow cleft; (2) wing-like process between bases of postzygapophyses and lateral margin of neural spine; (3) anteriorly directed laterally compressed ‘sword-like’ process on anterior face of neural spine; (4) deep pleurocoels on lateral faces of the centrum; (5) midline keel on the ventral surface of the centrum. Comments on Original Diagnosis — The original diagnosis provided by Dong (1997) can now be shown to be inadequate. Putative autapomorphies 1, 4, and 5 are present in several other sauropod genera. For example, shallow ‘U’-shaped bifurcation of the posterior cervical and anterior dorsal neural spines also occurs in Mamenchisaurus (Young and Chao, 1972), Klamelisaurus (Zhao, 1993; Moore et al., 2020), Euhelopus (Wiman, 1929; Wilson and Upchurch, 2009), several turiasaurians (Royo-Torres et al., 2006, 2017; Britt et al., 2017), Camarasaurus (Osborn and Mook, 1921; Gilmore, 1925), and Opisthocoelicaudia (Borsuk-Białynicka, 1977), among others. Deep lateral pneumatic openings (= ‘pleurocoels’) are widespread in the presacral centra of many eusauropods (Upchurch et al., 2004a), and a ventral keel is also present in the cervicodorsal region of several other taxa, including Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ. 2010), Klamelisaurus (Moore et al., 2020), and Euhelopus (Wilson and Upchurch, 2009). It is not entirely clear what Dong (1997) meant by the ‘wing-like’ processes (putative autapomorphy ‘2’), as their location was neither fully described nor annotated in his figures. However, it seems likely that these are merely the typical posterolateral projection of the postzygapophyses, rather than unusual processes. Finally, the ‘sword-like’ anterior process is not part of a novel articulation with the hyposphene of a preceding vertebra (contra Dong, 1997: see Description, below); rather, it appears to be a transversely compressed sheet of ossified intervertebral ligament. Ossification of such ligaments and tendons is rare, but not unheard of, among sauropods (e.g., Camarasaurus [= ‘ Cathetosaurus ’] lewisi [Jensen, 1988]; Diplodocus [USNM 10865; Gilmore, 1932; PU pers. observ., 1991]; see also Cerda, 2009; Klein et al., 2012; Cerda et al., 2015). Thus, the presence of such a feature is more likely to represent individual variation, pathology, and/or unusual preservation, rather than an autapomorphy. If this feature is to be accepted as having some diagnostic value, this must wait until it is found repeatedly in other individuals of Hudiesaurus. Revised Diagnosis —Hudiesaurus can be diagnosed on the basis of the following autapomorphies: (1) small projection on neurocentral junction above lateral pneumatic opening; (2) ACDL splits into upper and lower branches (the former extends to anterodorsal margin of the diapophysis, and the latter to posteroventral margin of the diapophysis, where it meets the anterior end of the PCDL); (3) approximately transverse row of 5–6 small coels on dorsal surface of prezygapophyseal process, immediately posterior to articular facet; (4) SPRLs bifurcate close to the base of the metapophysis, with one branch extending up anterior surface and fading out before reaching the summit, and the other branch forming a thin sheet that extends along the anterolateral margin of the metapophysis to the summit; and (5) SPOL bifurcates into two distinct ridges immediately above postzygapophysis (or this could be described as a short lamina extending dorsomedially from the PODL to the SPOL). N.B., portions of the PRDLs and diapophyses have been heavily restored with plaster, so autapomorphy 2 should be treated with caution. Holotype — A nearly complete vertebra from the cervicodorsal region (estimated to be the last cervical vertebra; IVPP V11120) (Figs. 2–4; Table 1). N.B., Dong (1997) identified this specimen as an anterior dorsal vertebra, but we regard it as being more probably a posterior cervical vertebra (see below). Locality and Horizon — Lower part of the Kalazha Formation (Upper Jurassic: upper Kimmeridgian–Tithonian) of Qiketai, Shanshan County, Turpan Basin, Xinjiang Uyghur Autonomous Region, China (Dong, 1997; Deng et al., 2015; Fang et al., 2016; Fig. 1). Description and Comparisons Dong (1997) identified the holotype of Hudiesaurus as an anterior dorsal vertebra; however, it also resembles a posteriormost cervical vertebra in several features. Even with well-preserved presacral series, it is often difficult to define the point where the neck meets the trunk in sauropods: this is because the morphology of the posterior cervical vertebrae gradually transforms into that of the most anterior dorsal vertebrae (Wilson and Upchurch, 2009; Moore et al., 2020). Despite some occasional doubts and apparent inconsistencies, we have generally accepted the identifications of the cervical-dorsal junction proposed by previous workers for other taxa. However, in the case of Mamenchisaurus hochuanensis (CCG V 20401), we note that the suggested 19 cervical and 12 dorsal vertebrae (Young and Chao, 1972) is likely to be incorrect. This is because ‘Dv2’ possesses a hyposphene (PU and PMB pers. observ., 2010), which would be atypical for such an anterior dorsal vertebra: a hyposphene does not usually appear until Dv3 or Dv4 in sauropods (Upchurch et al., 2004a). We therefore propose provisionally that Mamenchisaurus hochuanensis had 18 cervical and 13 dorsal vertebrae. Given the difficulties of pinpointing the cervical-dorsal junction in even well preserved and complete presacral series, identifying the precise position of an isolated vertebra (such as Hudiesaurus) is even more problematic. Below, we compare the Hudiesaurus vertebra with both the posterior cervical and anterior dorsal vertebrae of other sauropods. The majority of features support a position as either the last cervical or the first dorsal vertebra, with the former being more probable based on some features that are uniquely shared by Hudiesaurus and the last cervical vertebra (Cv18) of Xinjiangtitan. This identification, of course, depends on the assumption that Zhang et al. (2020) were correct when they placed the cervical-dorsal junction of Xinjiangtitan between the 18th and 19th presacral vertebrae (counting from the head). The Hudiesaurus vertebra is relatively complete, although the PRDLs and transverse processes have been partly reconstructed (see also Dong, 1997). As in the cervical and anterior dorsal vertebrae of most eusauropods, it has a strongly opisthocoelous centrum (Dong, 1997) (Fig. 2), differing from the amphiplatyan/amphicoelous presacral vertebrae of most non-gravisaurian sauropodomorphs (Upchurch, 1995; Wilson, 2002; Upchurch et al., 2007a; Yates, 2007; Allain and Aquesbi, 2008; McPhee et al., 2014). In anterior or posterior view, the centrum is subcircular in outline, being slightly wider transversely than dorsoventrally (Table 1), as is typical for the cervicodorsal vertebrae of neosauropods (Mannion et al., 2019a) and some earlier-branching forms such as Qijianglong, Mamenchisaurus youngi, and Bellusaurus (Moore et al., 2020 and references therein). This contrasts with the transversely compressed middle–posterior cervical centra of many other East Asian eusauropods, including Shunosaurus, Erketu, Euhelopus, Mamenchisaurus hochuanensis (CCG V 20401), and Xinjiangtitan (Upchurch, 1998; Mannion et al., 2013; Moore et al., 2020; Zhang et al., 2020; PU and PMB pers. observ., 2010), as well as most rebbachisaurids (Mannion et al., 2019a). The Functional (i.e., excluding the anterior convexity) Average Elongation Index (FAEI) is 1.0 in the Hudiesaurus vertebra. FAEIs tend to decrease towards the cervical-dorsal junction compared with those for middle cervical vertebrae, and a value close to 1.0 is compatible with a position either as the last cervical or one of the first two dorsal vertebrae of a non-diplodocine sauropod (Table S1 in Supplemental Data 1). As in Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010), Klamelisaurus (Moore et al., 2020; contra Zhao, 1993), Euhelopus (Wilson and Upchurch, 2009), and many flagellicaudatans (Upchurch et al., 2004a), the ventral surface of the Hudiesaurus centrum is strongly concave transversely as well as anteroposteriorly over its whole length, and is bounded by ventrolaterally directed ridges (Dong, 1997). A prominent midline ridge is present within the ventral concavity, as also found in dicraeosaurids (Upchurch, 1998; Wilson, 2002), Cv17–Dv1 of Euhelopus (Wilson and Upchurch, 2009), posterior cervicals to Dv2 in Klamelisaurus (Moore et al., 2020), Cv13–18 in Xinjiangtitan (Zhang et al., 2020), and Dv1 (= ‘Cv19’) in Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010). The parapophysis is located at the anteroventral corner of the lateral surface of the centrum (Fig. 2). This position is typical for sauropod cervical vertebrae, although it also occurs in Dv1 in most taxa (Upchurch et al., 2004a), including Klamelisaurus (Moore et al., 2020), Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010), and Xinjiangtitan (Zhang et al., 2020), and in Dv1 and 2 in Euhelopus (Wilson and Upchurch, 2009) and Apatosaurus ajax (Upchurch et al., 2004b). In Hudiesaurus, there is no indication that the shallowly concave articular surface of the parapophysis was fused to a rib: this is more consistent with this specimen being a dorsal, rather than cervical, vertebra (Hatcher, 1901; Gilmore, 1936; McIntosh, 1990; Upchurch, 1998; Upchurch et al., 2004a; Zhang et al., 2020). However, rib–vertebra fusion is not an infallible indicator that a vertebra is a cervical (Moore et al., 2020): for example, the ribs of Cv17 and 18 of Mamenchisaurus hochuanensis (CCG V 20401) are not fused to the parapophyses (PU and PMB pers. observ., 2010). The dorsal surface of the parapophysis is excavated in Hudiesaurus, and this depression is continuous with the lateral pneumatic opening, as seen in the cervical vertebrae of many non-neosauropod eusauropods, such as Cetiosaurus and Chebsaurus (Upchurch and Martin, 2002, 2003; Upchurch et al., 2004a; Mahammed et al., 2005). Many neosauropods also have dorsally excavated cervical parapophyses, but such taxa typically possess a ridge that divides this depression from the lateral pneumatic opening (Upchurch, 1998; Upchurch and Martin, 2002, 2003). The lateral pneumatic opening of Hudiesaurus is small and deep, with a rounded, wide anterior margin that is positioned dorsal to the parapophysis (Fig. 2). Posteriorly, this opening is bounded dorsally by a sharp ridge that runs posteroventrally, giving the posterior margin an acute profile. Such a ridge is unusual in sauropods, only being reported previously in Cv17 and 18 of Xinjiangtitan (Zhang et al., 2020:figs. 15, 16, and 18), and confirmed as absent in Mamenchisaurus youngi by the latter study. Dorsal vertebrae 1 and 2 of Apatosaurus ajax have a ridge bounding the lateral pneumatic opening dorsally (Upchurch et al., 2004b), but this differs from the condition in Hudiesaurus and Xinjiangtitan by extending further anteriorly (i.e., to the anterior end of the opening) and being horizontal rather than posteroventrally inclined. In Hudiesaurus, this ridge merges into the centrum-arch junction, where there is a small, laterally extending projection on each side (Fig. 2): the latter is unique and is regarded as an autapomorphy. The presence of lateral pneumatic openings with oval outlines (i.e., strongly rounded and dorsoventrally wide anterior margins and acute posterior ends) in anterior dorsal vertebrae has frequently been regarded as a derived character state uniting Macronaria or a slightly less inclusive clade (e.g., Upchurch, 1998; Mannion et al., 2013). However, they are also seen in Dv1 and 2 of Klamelisaurus (Moore et al., 2020), the anterior dorsal vertebrae of Bellusaurus and Haplocanthosaurus priscus (Mannion et al., 2019a), and indeterminate cervicodorsal vertebrae from the Late Jurassic Shishugou Formation of China (Moore et al., 2020). In Hudiesaurus, the lateral pneumatic opening is not as elongate as those found in either the cervical centra of Cetiosaurus (Upchurch and Martin, 2002) or several Jurassic Chinese taxa (such as Dashanpusaurus and Daanosaurus; Peng et al., 2005; Ye et al., 2005). Indeed, Hudiesaurus possesses a lateral pneumatic opening that is largely restricted to the anterior two-thirds of the centrum (excluding the anterior articular convexity), a derived condition seen in the cervical vertebrae of many CMTs (e.g., Klamelisaurus, Mamenchisaurus youngi, Qijianglong, Xinjiangtitan), Euhelopus, and several titanosauriforms (Whitlock, 2011; Moore et al., 2020). However, the relatively small size and anterior location of the lateral pneumatic opening is also consistent with the Hudiesaurus vertebra being from the anterior dorsal region. The oblique accessory lamina that divides the lateral pneumatic opening into anterior and posterior sections in the cervical vertebrae of several non-neosauropod eusauropods (e.g., Mamenchisaurus, Klamelisaurus, Xinjiangtitan) and many neosauropods (Wilson, 2002; Upchurch et al., 2004a; Moore et al., 2020) is not present in Hudiesaurus (Fig. 2). While its absence is more compatible with an identification of the Hudiesaurus specimen as being an anterior dorsal vertebra, the oblique lamina is also sometimes absent in posterior-most cervical vertebrae, such as Cv18 of Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010), Cv17 and 18 of Xinjiangtitan (Zhang et al., 2020), and Cv17 of Euhelopus (Wilson and Upchurch, 2009). The lateral pneumatic opening becomes shallower posteriorly in Hudiesaurus, as is typical for most sauropod cervical vertebrae (e.g., Cetiosaurus, Patagosaurus, and the CCG V 20401 specimen of Mamenchisaurus hochuanensis: Bonaparte, 1986; Upchurch and Martin, 2002, 2003; PU and PMB pers. observ., 2010). Measured on the anterior surface, the ratio of the dorsoventral height of the neural arch (from the dorsal surface of the centrum to the ventromedial tips of the prezygapophyses) to centrum height is low (∼0.35) in Hudiesaurus. With the exception of comparably low neural arches in some somphospondylans and Omeisaurus tianfuensis, this ratio is ≥0.5 in the posterior cervical vertebrae of other eusauropods (Bonaparte et al., 2006; Mannion et al., 2013). In Hudiesaurus, the prezygapophyses project forward to a point beyond the anterior end of the condyle (Fig. 2). Such projection is typical for the posterior cervical and anterior dorsal vertebrae of many sauropods: for example, in Klamelisaurus it is only posterior to Dv5 that the prezygapophyses no longer project beyond the anterior articulation of the centrum (Moore et al., 2020). However, this contrasts with the condition in taxa like Apatosaurus ajax, where the prezygapophyses no longer project beyond the anterior end of the centrum from Cv12 rearwards (Upchurch et al., 2004b). In Hudiesaurus, the prezygapophyses are large and broad, with transversely convex articular surfaces (Fig. 3A). Sauropods typically have flat prezygapophyseal articular surfaces plesiomorphically, but the derived, strongly convex condition is also present in the cervical vertebrae of diplodocines (Upchurch, 1995; Tschopp et al., 2015a) and the CMTs Klamelisaurus (Moore et al., 2020) and Xinjiangtitan (Zhang et al., 2020), as well as the anterior dorsal vertebrae of Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010). The zygapophyses have several small, irregularly shaped coels on their dorsal surfaces (Dong, 1997). In the case of the prezygapophyses, these coels form a line of 5–6 adjacent pits, separated from each other by small anteroposteriorly directed ridges, located immediately posterior to the articular facet (Fig. 3A). These might represent a pneumatized internal tissue structure that has been revealed by erosion of the surface bone: however, their presence in the same position on both prezygapophyses suggests that they are not taphonomic artifacts. We therefore regard these coels as external pneumatic features and as autapomorphic for Hudiesaurus. The thin, medial edges of the prezygapophyses descend steeply to meet each other on the midline and form a single lamina extending down to the top of the small, subcircular neural canal (Fig. 2C); this is probably the “well developed medial lamina” of Dong (1997:103), here termed the interprezygapophyseal lamina (TPRL) according to a revised version of Wilson’ s (1999) system (see Tschopp and Mateus, 2013). This TPRL partially subdivides the centroprezygapophyseal fossa (CPRF) into left and right subfossae. A TPRL is absent from the posterior cervical vertebrae of Euhelopus (Wilson and Upchurch, 2009) and Xinjiangtitan (Zhang et al., 2020), and the anterior dorsal vertebrae of Klamelisaurus and Mamenchisaurus youngi (Moore et al., 2020), although it is present in several other sauropods (e.g., there is a short, stout version on the posterior cervical vertebrae of Apatosaurus ajax; Upchurch et al., 2004b). The centroprezygapophyseal laminae (CPRLs) of Hudiesaurus are large and stout (as in Cetiosaurus; Upchurch and Martin, 2003) and do not bifurcate at their dorsal ends, unlike those of the cervical vertebrae of several diplodocids (Upchurch, 1998) and many non-neosauropod eusauropods (Moore et al., 2020), such as those on Cv18 in Xinjiangtitan (Zhang et al., 2020). The stout, single CPRLs of Hudiesaurus more closely resemble those of anterior dorsal vertebrae in taxa such as Klamelisaurus, although the former lacks the accessory laminae seen in the PRCDF of the latter taxon (Moore et al., 2020). In lateral view, the CPRLs slope anterodorsally and are subparallel with the PCDLs (Fig. 2A, B), a configuration also seen in the cervical and anterior-most dorsal vertebrae (i.e., Dv1 and 2) of many sauropods. By contrast, in Dv3 and 4 of most taxa, these laminae become more vertical, and are fully vertical from around Dv5 onwards, as seen in Klamelisaurus (Moore et al., 2020). Thus, the orientation of the CPRLs further supports the view that the Hudiesaurus vertebra is either a cervical or one of the most anterior dorsal vertebrae. As in the cervical vertebrae of some non-neosauropod eusauropods (including Shunosaurus, Omeisaurus tianfuensis, Chuanjiesaurus, and Cetiosaurus) and many diplodocoids, pre-epipophyses are absent in Hudiesaurus. This contrasts with most CMTs, such as Klamelisaurus and Mamenchisaurus youngi, as well as Bellusaurus, Euhelopus, and many other neosauropods, in which these projections ar, Published as part of Upchurch, Paul, Mannion, Philip D., Xu, Xing & Barrett, Paul M., 2021, Re-assessment of the Late Jurassic eusauropod dinosaur Hudiesaurus sinojapanorum Dong, 1997, from the Turpan Basin, China, and the evolution of hyper-robust antebrachia in sauropods, pp. 1-31 in Journal of Vertebrate Paleontology (e 1994414) (e 1994414) 41 (4) on pages 3-9, DOI: 10.1080/02724634.2021.1994414, http://zenodo.org/record/5839134, {"references":["Dong, Z. 1997. A gigantic sauropod (Hudiesaurus sinojapanorum, gen. et sp. nov.) from the Turpan Basin, China; pp. 102 - 110 in Z. Dong (ed.), Sino-Japanese Silk Road Dinosaur Expedition. China Ocean Press, Beijing.","Young, C. C., and X. - J. Chao. 1972. Mamenchisaurus hochuanensis sp. nov. 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Upchurch, Paul, Mannion, Philip D., Xu, Xing, Barrett, Paul M. (2021): Re-assessment of the Late Jurassic eusauropod dinosaur Hudiesaurus sinojapanorum Dong, 1997, from the Turpan Basin, China, and the evolution of hyper-robust antebrachia in sauropods. Journal of Vertebrate Paleontology (e1994414) 41 (4): 1-31, DOI: 10.1080/02724634.2021.1994414, URL: http://dx.doi.org/10.1080/02724634.2021.1994414