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 posterior-most 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 (ujvp_a_1994414_sm6076.zip)). 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 are well-developed (Wilson and Upchurch, 2009; Mannion et al., 2013, 2019a; Moore et al., 2020). However, pre-epipophyses are typically absent in the dorsal vertebrae of sauropods (Wilson and Upchurch, 2009), so the condition in Hudiesaurus might merely reflect a location in the anterior dorsal series.

The transverse processes are short and project laterally and slightly ventrally (Dong, 1997), although it is difficult to ascertain how genuine this morphology is, given the degree of plaster restoration. If the transverse processes are truly pendant, then this is consistent with this specimen being either a cervical or very anterior dorsal vertebra (Upchurch et al., 2004a). For example, the shift from pendant to horizontal transverse processes occurs between Cv18 and Dv2 in Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010), from Cv17 to Dv2 in Euhelopus (Wilson and Upchurch, 2009), and more abruptly between Cv18 and Dv1 in Xinjiangtitan (Wu et al., 2013; Zhang et al., 2020). In Hudiesaurus, the transverse process lies some distance below the level of the zygapophyses (Fig. 2), as is typical for posterior cervical and the most anterior dorsal vertebrae (Moore et al., 2020). Prominent anterior and posterior centrodiapophyseal laminae (ACDLs, PCDLs) extend anteroventrally and posteroventrally, respectively, at approximately 45° to the horizontal (Fig. 2). The presence of an ACDL is consistent with this specimen being either a cervical or anterior dorsal vertebra: for example, in Klamelisaurus, the ACDL is present in Dv1 and 2 as a separate lamina, and in Dv3 and 4 merges into the CPRL (Moore et al., 2020; see also Wilson, 1999). As the ACDL approaches the transverse process in Hudiesaurus, it bifurcates to form two laminae that extend along the ventral and anterior surfaces of the transverse process (potentially as far as the distal articular end) (Fig. 3B). The more posterior of these laminae merges into the posteroventral margin of the transverse process, where it meets the anterodorsal end of the PCDL. This posteriorly bifurcate ACDL appears to be unique to Hudiesaurus. The relatively steeply inclined PCDL is consistent with the identification of the Hudiesaurus vertebra as a posterior-most cervical or an anterior dorsal vertebra: this lamina is typically close to horizontal in cervical vertebrae but tends to become more steeply inclined in the cervicodorsal region (Wilson and Upchurch, 2009). Sauropods display some variation in this regard, although this might also reflect inconsistent identification of the cervical-dorsal junction. For example, PCDLs remain shallowly inclined even in the most posterior cervical vertebrae of Qijianglong (Xing et al., 2015:fig. 12F), but they become increasingly steep from Cv16 to 18 in Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010). In Hudiesaurus, the prezygodiapophyseal lamina (PRDL) extends anterodorsally from the transverse process to the prezygapophysis at a moderate angle (c. 30°) to the horizontal, whereas the postzygodiapophyseal lamina (PODL) is nearly vertical (Fig. 2). The anterior margin of the PRDL forms a convex projection or ‘kink’ (Figs. 2, 3) that is potentially homologous with the apomorphically convex ventral margin seen in the middle and posterior cervical vertebrae of several CMTs (Moore et al., 2020). Unlike the condition in the cervicodorsal vertebrae of Euhelopus, Klamelisaurus, and some additional CMT specimens (Moore et al., 2020), the PODL is not bifid ventrally.

The posterior margins of the postzygapophyses terminate some distance anterior to the posterior margin of the centrum (Fig. 2). This condition is a derived state when it occurs in posterior cervical vertebrae, which is seen in several non-neosauropod eusauropods (e.g., Omeisaurus tianfuensis—He et al., 1988:fig. 23; Mamenchisaurus youngi—Ouyang and Ye, 2002:fig. 18C; Chuanjiesaurus—Sekiya, 2011:fig. 14; Qijianglong—Xing et al., 2015:fig. 12F; Xinjiangtitan—Zhang et al., 2020:figs. 15 and 16; Jobaria—Mannion et al., 2017), and early diverging macronarians (e.g., Camarasaurus; Osborn and Mook, 1921:pl. LXVII), but is typically absent in many diplodocoids, including Apatosaurus ajax (Upchurch et al., 2004b), Dicraeosaurus (Janensch, 1929, 1936:table I, fig. 11a), and Limaysaurus (Calvo and Salgado, 1995:fig. 8B) (see also Tschopp and Mateus, 2013; Tschopp et al., 2015a; Poropat et al., 2016). Epipophyses are greatly reduced or absent in Hudiesaurus, perhaps being represented by small tab-like processes above the postzygapophyses (Fig. 3). Such a condition is typical for the posterior-most cervical vertebrae of sauropods, except Euhelopus (Wilson and Upchurch, 2009), Jobaria (MNN specimens; PDM pers. observ., 2012), Nigersaurus (MNN specimens; PDM pers. observ., 2010), and diplodocines (Tschopp and Mateus, 2013). For example, epipophyses are present in Cv2–16 in Xinjiangtitan, but are absent in Cv17 and 18 (Zhang et al., 2020). Their absence is also consistent with the Hudiesaurus vertebra being an anterior dorsal, since it is even rarer for well-developed epipophyses to be present on such vertebrae (to date they have only been reported in anterior dorsal vertebrae of some turiasaurians (Britt et al., 2017; Mannion, 2019; Mannion et al., 2019a), although they can be traced into the dorsal series as the homologs of the tips of the aliform processes in Euhelopus (Wilson and Upchurch, 2009). Given the uncertainty in the position of the Hudiesaurus vertebra, and the subtlety of its putative epipophyses, we score this character (i.e., presence/absence of epipophyses) as a ‘?’ in our phylogenetic data matrices. The postzygapophyses of Hudiesaurus are relatively large, with concave articular surfaces facing downwards and outwards (Fig. 2D). Their ventral margins merge into the dorsal parts of well-developed centropostzygapophyseal laminae (CPOLs) that descend separately without meeting on the midline; however, the detailed anatomy of this region is obscured by damage and reconstruction. Nevertheless, despite Dong’s (1997) assertion of its presence, there is no hyposphene-hypantrum articulation (see above). On the left side at least, and possibly also the right, the CPOLs bifurcate dorsally, creating a small subtriangular fossa that faces mainly posteriorly (Fig. 2D). A dorsally bifurcated CPOL is sporadically present in the middle and posterior cervical vertebrae of eusauropods (e.g., Cetiosaurus, Patagosaurus, Camarasaurus, Giraffatitan, Rapetosaurus, and some flagellicaudatans), and is generally absent in CMTs apart from the ‘Phu Kradung taxon’ (Tschopp et al., 2015a; Carballido et al., 2017; Moore et al., 2020). However, the medial branch of the bifid CPOL of Hudiesaurus supports the postzygapophysis rather than curving medially to meet its partner on the midline as occurs in other taxa. Similarly, no single vertical midline interpostzygapophyseal lamina (TPOL) can be observed, although it is not clear whether this represents genuine absence or the effects of poor preservation.

The spinoprezygapophyseal laminae (SPRLs) are low ridges that extend medially from the middle of the posterior margins of the prezygapophyses to the anterior bases of the metapophyses (Figs. 2, 4). At this point, each SPRL autapomorphically splits into two branches: one ascends the anterior surface of the metapophysis and fades out at about midheight; the other becomes a thin flange-like ridge that extends along the anterolateral margin of the metapophysis and reaches the summit. These anterolateral flanges are potentially homologous with the ‘scabrous’ projections observed in the middle–posterior cervical vertebrae of Klamelisaurus (which become less ‘ragged’ in the most posterior cervical vertebrae), and the dorsolaterally flattened SPRLs seen in the middle and posterior cervical vertebrae of Bellusaurus (Moore et al., 2020). In Hudiesaurus, there is a large flat space on the anterior surface of the neural spine between the SPRLs and below the bifurcated summit. Near the top of this area, along the midline, is the base of a transversely compressed process (Figs. 2, 4): this is the feature that Dong (1997) described as an 84 mm long, anteriorly directed, ‘sword-like’ process (for which he used the term ‘prepophysis’). We observed this process in our first examination of this specimen in 1995, but by our second examination, in 2007, we found that the process had been broken and lost, so that now only its base is preserved. Dong (1997) suggested that this structure might be for the insertion of muscles, or for articulation with the hyposphene of the preceding vertebra; however, the latter proposal would seem to be impossible because the location of the process on the spine means that it would project into the spinopostzygapophyseal fossa (SPOF: = postspinal fossa) of the preceding vertebra. Moreover, hyposphene-hypantrum articulations have not been observed in the posterior cervical or anterior-most dorsal vertebrae of any sauropod: such structures are restricted to middle and posterior dorsal vertebrae (Upchurch et al., 2004a). We instead interpret this structure to be part of an ossified ligament (see above).

The posterior margin of the neural spine slopes strongly forward in lateral view, and the spine is slightly anterodorsally directed (though not to the same extent as in Dicraeosaurus; Janensch, 1929). The neural spine of Cv16 in Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010) has a nearly vertical anterior margin and gently sloping posterior one, resembling that of Hudiesaurus. This contrasts with the posterior-most cervical vertebrae of some taxa, such as Qijianglong (Xing et al., 2015:fig. 12E, F), in which the neural spine has a fairly symmetrical lateral profile, with posterodorsally sloping anterior and anterodorsally sloping posterior margins. As in the cervicodorsal vertebrae of CMTs, turiasaurians, Camarasaurus, and some titanosaurs, the neural spine is relatively low in Hudiesaurus, projecting only slightly above the level of the postzygapophyses (Mannion et al., 2019a; Moore et al., 2020). The neural spine is bifurcated (Fig. 2C, D), as in the presacral vertebrae of numerous other eusauropods (Klamelisaurus, Mamenchisaurus, Qijianglong, some turiasaurians, flagellicaudatans, Camarasaurus, Euhelopus, and several somphospondylans; Wiman, 1929; Young, 1954; Borsuk-Białynicka, 1977; Zhao, 1993; Wilson, 2002; Harris and Dodson, 2004; Upchurch et al., 2004a; Royo-Torres et al., 2006; Ksepka and Norell, 2006; D'Emic et al., 2013; Mannion et al., 2019a; Moore et al., 2020). In anterior and posterior views (Fig. 2C, D), the metapophyses are divergent, as in diplodocids and most other taxa with bifid neural spines, but unlike the derived condition seen in dicraeosaurids, in which these structures are subparallel or converge towards their summits (Rauhut et al., 2005; Xu et al., 2018). In Hudiesaurus, the notch between the metapophyses is moderately deep and ‘U’shaped, with a median tubercle at its base (Fig. 2C, D). Such a tubercle is variably present in other sauropods with bifid presacral spines: for example, it occurs in the last two cervical vertebrae and Dv1–4 of Euhelopus, where it is drawn out into a large process that is as prominent as the metapophyses (Wilson and Upchurch, 2009); it is present as a low rounded process in the last two cervical vertebrae and Dv1–3 of Barosaurus (Zhang et al., 2020); it is a small bump on the posterodorsal margin of the notch in Klamelisaurus (Moore et al., 2020); it is variably absent/present in specimens of Camarasaurus (Tsuihiji, 2004); and it is absent in Mamenchisaurus, Qijianglong, Suuwassea, and Amargasaurus (Wilson, 2002; Harris and Dodson, 2004; Xing et al., 2015). The metapophyses of Hudiesaurus are knob-like and subtriangular in dorsal view, robust rather than compressed transversely, and relatively short dorsoventrally (not elongated as in derived dicraeosaurids: Janensch, 1929; Xu et al., 2018).

The spinodiapophyseal fossa (SDF), posterior to the SPRL and anterior to the SPOL, is divided into three subtriangular coels by two accessory laminae or ridges (Fig. 4). Dong (1997:103) described these structures as forming “a V-shaped posterolaterally projecting lamina”: in lateral view, the two laminae meet each other at their posterior ends and diverge anteriorly. This ‘V’ is created from a lower horizontal lamina that extends from the PODL to the base of the SPRL, and an upper anterodorsally directed lamina that extends from the posterior end of the horizontal lamina to the posterior margin of the anterolateral branch of the SPRL (see above). Although both of these ridges are found separately on the presacral vertebrae of many sauropods (see below), the presence of both of them in this ‘V’-shaped configuration is only known in Cv18 of Xinjiangtitan (Zhang et al., 2020:figs. 16A, 17B) and Hudiesaurus. The lower, horizontal, lamina is reminiscent of the ‘epipophyseal-prezygapophyseal lamina’ (EPRL) that occurs in the cervical vertebrae of several sauropods, such as Nigersaurus (Sereno et al., 2007) and Euhelopus (Wilson and Upchurch, 2009), as well as some other dinosaurs (Moore et al., 2020). Occasionally, this structure can also occur in the anterior-most dorsal vertebrae, such as Dv1 and 2 in Euhelopus, where it partially divides the SDF into lower and upper portions (Wilson and Upchurch, 2009), and Dv1 of Klamelisaurus (Moore et al., 2020). However, Moore et al. (2020) demonstrated that simply identifying this structure as the EPRL is problematic because it can be formed by either one or both of two separate components. One component is a more anteriorly placed ridge (termed the horizontal accessory lamina) that lies fully within the SDF and was probably formed by pneumatization. The other component is a more posteriorly placed ‘anterior epipophyseal’ epaxial muscle scar that lies on the lateral surface of the postzygapophyseal process and may project anteriorly into the posterior part of the SDF. Here, we identify the lower strut in Hudiesaurus as the horizontal accessory lamina formed by pneumatization. Moore et al.’s (2020) survey of these structures among sauropods suggests that, when considering just posterior cervical vertebrae, the pneumatic strut is currently only known in rebbachisaurids (e.g., Nigersaurus, Limaysaurus), Euhelopus (where it lies below, and separate from, the anterior epipophyseal muscle scar), and some CMTs such as Klamelisaurus and Mamenchisaurus hochuanensis (CCG V 20401; PU and PMB pers. observ., 2010). It can be confirmed as being absent in the posterior cervical vertebrae of some non-neosauropods such as Mamenchisaurus youngi (where it only occurs in middle cervical vertebrae: Zhang et al., 2020), as well as several macronarians in which it has previously been identified, including Camarasaurus lewisi, Europasaurus, Giraffatitan, and Uberabatitan. The anterodorsally directed ridge within the SDFs of Hudiesaurus and Xinjiangtitan is potentially a SPDL, though it contacts the PODL rather than the diapophysis directly. The SPDLs in Dv4 of Klamelisaurus and Dv3 of Euhelopus resemble this anterodorsal lamina, but no such structure occurs in the more anterior dorsal or posterior cervical vertebrae of these taxa (Wilson and Upchurch, 2009; Moore et al., 2020). Despite the presence of two ridges produced by pneumatization within the SDF (i.e., the ?SPDL and horizontal accessory lamina), Hudiesaurus lacks the 3–4 irregular coels in this region seen in several early-branching titanosauriforms and many CMTs (Mannion et al., 2017; Moore et al., 2020). In Hudiesaurus, the SDF is not roofed dorsally by a horizontal rugose line of epaxial muscle scars immediately below the spine summit, unlike the condition in some non-neosauropod sauropods (e.g., Klamelisaurus, Jobaria, Mierasaurus, and Moabosaurus), as well as most diplodocids and many non-titanosaurian macronarians (Tschopp and Mateus, 2013; Mannion et al., 2019a; Moore et al., 2020). The prominent SPOLs of Hudiesaurus extend anteromedially and dorsally to the summit of each metapophysis (Fig. 2). At its posteroventral end (above the postzygapophysis), the SPOL splits into two ridges, with a small subtriangular fossa (SPOL-F) between them (Fig. 4). Such a bifurcated SPOL and cavity is not known in the posterior cervical vertebrae of other sauropods, but SPOL bifurcation in dorsal vertebrae has been listed as a synapomorphy of a clade of eusauropods comprising Barapasaurus, Omeisaurus, Mamenchisaurus, Patagosaurus, Jobaria, and neosauropods (Wilson, 2002). However, the SPOL bifurcation noted by Wilson typically occurs in the middle and posterior dorsal vertebrae and has a very different structure. In the Barapasaurus + Neosauropoda clade, each SPOL is a single structure close to the postzygapophysis and then bifurcates into a lateral SPOL (which usually merges with the SPDL) and a medial SPOL (which usually meets its partner on the midline within the SPOF: Wilson, 1999, 2002). Aside from occurring in a more anteriorly placed presacral vertebra, the condition in Hudiesaurus also differs from other eusauropods in that the SPOL is single over most of the spine length and then bifurcates as it approaches the postzygapophysis. As such, irrespective of whether the Hudiesaurus specimen is a posterior cervical or anterior dorsal vertebra, it appears to possess an autapomorphic condition with regard to its SPOL bifurcation. The SPOF is large, ‘U’-shaped in transverse cross-section, and opens posterodorsally.

We could not observe the internal tissue structure of the vertebra. As such, we cannot determine whether the vertebra is camerate, as is the case in most eusauropods (Wedel, 2003), or pneumatized by camellae, which characterizes the presacral vertebrae of titanosauriforms (Wilson, 2002; Wedel, 2003) and many CMTs (Young and Chao, 1972; Moore et al., 2020).

Material—Four teeth, IVPP V11121-2 (Fig. 5; Table 2).

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). Exact locality unknown (see Introduction, above).

Description

The four teeth are not labelled with unique specimen numbers and so are referred to as specimens 1–4 herein. Two of the teeth (identified as premaxillary teeth by Dong [1997]) are embedded in a fragment of very worn, indeterminate bone, and the other two teeth are loose and were interpreted by Dong (1997) as maxillary teeth. It is not possible to determine which elements yielded these teeth, but it seems likely that the three smaller, low-crowned teeth were from the posterior part of the tooth row, whereas the single larger, higher-crowned tooth would have been more anteriorly positioned. No useful morphology can be gleaned from the bone fragment, although it is unlikely to have been the premaxilla on the basis of tooth size. Two of the teeth are quite similar in morphology: these are the larger tooth in the bone fragment (tooth 2) and the smaller of the two loose teeth (tooth 3). These specimens resemble the low broad teeth of Jobaria (Sereno et al., 1999; Chure et al., 2010), Turiasaurus (Royo-Torres and Upchurch, 2012), and Zby (Mateus et al., 2014), whereas the other two teeth (teeth 1 and 4) are more slender (Table 2).

Tooth 1 (smaller tooth in bone fragment: Fig. 5A–D) has been badly damaged and is missing most of the original surface, so its true shape cannot be determined. No informative character states can be observed.

Tooth 2 (larger tooth in bone fragment: Fig. 5A–D) lacks denticles and wear facets. There is no sign of wrinkled enamel texture on either the labial or lingual surface, suggesting some general surficial wear either during life or after the tooth was shed. The apex of the tooth is pointed and is deflected distally: this suggests that it is either an upper right or lower left tooth. The labial surface is gently convex mesiodistally and apicobasally, with the part of the crown mesial to the apex more strongly convex than that section distal to it, creating an asymmetrical ‘D’-shaped cross-section. Mesial and distal grooves appear to be absent on the labial surface. The crown is mesiodistally expanded with respect to the tooth base, but the crown–root junction cannot be precisely determined because most of the tooth below this expansion is obscured by bone. The mesial margin is smoothly convex from apex to base, whereas the distal margin is first concave, then convex, producing a mildly sinuous profile in labial and lingual views (Fig. 5A, B). Most of the lingual surface of the crown is concave mesiodistally and apicobasally: the base of this concavity lies at a point approximately level with the maximum mesiodistal width of the tooth. Basal to this point, the lingual crown surface is swollen and mesiodistally convex. The crown margins are both slightly swollen, with the distal margin possessing a small, low, and elliptical boss that is level with the point of greatest mesiodistal expansion. This boss is in the same position as similar structures in Euhelopus (Wilson and Upchurch, 2009). There is no true lingual ridge, but a slight eminence extends from the tooth apex for a very short distance basally, before merging into the surface of the lingual concavity.

Tooth 3 (the smaller of the isolated teeth: Fig. 5E–H) has the same morphology, in most respects, as tooth 2. The enamel surface is better preserved and has a wrinkled texture. The lingual ‘boss’ is less distinct and is a simple swelling of the distal margin, situated at a point level with the greatest mesiodistal expansion. As in tooth 2, there are no true mesial or distal grooves on the labial surface, but a distinct change in slope distal to the apical swelling does create the impression of a groove in the distal position (the cross-sectional asymmetry mentioned above). The root–crown junction cannot be observed because of breakage. Neither ‘shoulder-like’ nor apical macro-wear are present.

Tooth 4 (largest tooth: Fig. 5I–L) is badly abraded and the enamel surface texture cannot be observed. There is also some damage to the crown margins. No wear facets or serrations can be identified. This tooth is much longer than the others, with a maximum length of 40 mm (Table 2): however, it is not possible to judge the position of the root–crown boundary because of the absence of enamel. It appears to be much slenderer than the other teeth, with a maximum mesiodistal width of 11 mm, and thus a Slenderness Index (SI: sensu Upchurch, 1998; Chure et al., 2010) that is potentially >3, but the true value cannot be determined because of the lack of accurate information on the location of the crown–root junction. The crown has a ‘D’-shaped cross-section but has only a very shallow lingual concavity. There is no sign of a lingual ridge, lingual bosses, or labial grooves, but these absences could be the result of poor preservation.

Comparisons and Identification

The teeth are too incomplete to be usefully incorporated into a formal phylogenetic analysis. Instead, we assess their affinities by evaluating the potential significance of the putative synapomorphies and symplesiomorphies that they display. Possession of crowns that are basally constricted mesiodistally is a derived state characteristic of Sauropodomorpha (Yates, 2007; McPhee et al., 2014; Peyre de Fabrègues et al., 2015; Apaldetti et al., 2018; Chapelle and Choiniere, 2018), although this is lost in the elongated ‘pencil-like’ teeth of most diplodocoids and derived somphospondylans (Upchurch, 1998; Upchurch et al., 2004a). The labial profile of the IVPP V11121-2 teeth, with convex mesial and sigmoid distal margins, is characteristic of most spatulate sauropod teeth (Carballido and Pol, 2010). Only tooth 3 confirms the presence of wrinkled tooth enamel, but its absence on the other three crowns appears to be the result of poor preservation. Such enamel texturing is absent in the earliest branching sauropodomorphs (e.g., Efraasia), occurs in small patches of fine wrinkles in more derived non-sauropods (such as massospondylids, Melanorosaurus), and occurs over the entire crown as coarse anastamosing ridges and grooves in ‘true’ sauropods (e.g., Pulanesaura, Gongxianosaurus, Tazoudasaurus, and eusauropods) (Yates, 2007; Carballido and Pol, 2010; McPhee et al., 2015; Apaldetti et al., 2018; Chapelle and Choiniere, 2018). The presence of a lingual concavity on tooth crowns is generally regarded as a synapomorphy pertaining to a node between Sauropoda and Eusauropoda (Upchurch, 1995; Yates, 2007; Peyre de Fabrègues et al., 2015; Apaldetti et al., 2018; Chapelle and Choiniere, 2018). For example, this feature occurs in the teeth of all eusauropods (except diplodocoids and those somphospondylans with ‘pencil-like’ teeth), as well as some non-eusauropod sauropods such as Gongxianosaurus and Tazoudasaurus, but is rudimentary in Chinshakiangosaurus and Pulanesaura (Barrett et al., 2002; Upchurch et al., 2007a; Mannion et al., 2013; McPhee et al. 2015). Labial grooves are a synapomorphy of Eusauropoda, being present in Shunosaurus, Barapasaurus, Omeisaurus, Patagosaurus, and many other forms, including most neosauropods (except some diplodocoids and titanosaurs with cylindrical teeth). By contrast, with the exception of Pulanesaura (McPhee et al., 2015), such grooves are absent in non-eusauropod sauropods (e.g., Tazoudasaurus) and non-sauropod sauropodomorphs such as Plateosaurus and Anchisaurus (Upchurch, 1995; Yates, 2007; Peyre de Fabrègues et al., 2015; Apaldetti et al., 2018; Chapelle and Choiniere, 2018). There is some evidence that the distal labial groove evolved before the mesial one, since the teeth of Chinshakiangosaurus and Amygdalodon either possess only the latter, or the distal groove is more marked than the mesial one (Upchurch et al., 2007a; Carballido and Pol, 2010). This character state distribution could be taken as evidence that the IVPP V11121-2 teeth did not belong to a eusauropod: however, Mamenchisaurus sinocanadorum (IVPP V10603) also lacks both mesial and distal grooves (PMB and PU pers. observ., 2010), and this feature might sometimes reflect individual variation and/or position in the jaws (Holwerda et al., 2015). Non-sauropod sauropodomorphs typically have SI values in the range of 1.5–2.0, with some taxa (such as Thecodontosaurus and Anchisaurus) having SIs around 2.2 (Chure et al., 2010). Most sauropods, except diplodocoids and titanosaurs, have SI values between 2.0–2.5, although a few forms (such as Amygdalodon, Patagosaurus, Jobaria, and turiasaurians) have unusually low SIs in the range of 1.3–1.6 (Barrett et al., 2002; Chure et al., 2010). Thus, although caution is warranted given their incomplete preservation, the SI of 1.5 (tooth 2) to ∼3.0 (tooth 4) estimated for the IVPP V11121-2 teeth (Table 2) is consistent with a phylogenetic position anywhere within Sauropodomorpha apart from Diplodocoidea and Somphospondyli. Dong (1997) stated that the teeth of Hudiesaurus are serrated, but we found no such structures on any of the four crowns. Virtually all non-sauropod sauropodomorphs, and many non-eusauropod sauropods, have relatively large serrations on both the mesial and distal margins of their tooth crowns (Upchurch, 1998; Wilson and Sereno, 1998; Upchurch et al., 2004a, 2007a, b; Yates, 2007; Apaldetti et al., 2018; Chapelle and Choiniere, 2018). Well-developed serrations are also present on both mesial and distal crown margins in some non-neosauropod eusauropods, such as the CMT Klamelisaurus (Moore et al., 2020). In a few early-branching eusauropods (e.g., Barapasaurus, Omeisaurus tianfuensis, a referred specimen of Mamenchisaurus hochuanensis), serrations are retained on the mesial margins and lost on the distal margins (Ye et al., 2001; Yates, 2007; Moore et al., 2020). Variation can even occur along the length of the jaw of a single individual: for example, the anterior dentary teeth of Mamenchisaurus sinocanadorum lack serrations, whereas they are present as relatively small projections on just the mesial/apical margins of the posterior teeth (Moore et al., 2020). Thus, the absence of serrations in the IVPP V11121-2 teeth is more typical of a neosauropod (or close relative such as a turiasaurian) (Upchurch et al., 2004a; Royo-Torres and Upchurch, 2012), though this is also seen in Amygdalodon, Shunosaurus, and teeth referred to Kotasaurus (Carballido and Pol, 2010). Given this variation, however, the absence/presence of serrations probably provides only weak evidence of phylogenetic affinities (Upchurch, 1998; Barrett and Upchurch, 2005; Upchurch et al., 2007b; Carballido and Pol, 2010). An apicobasally oriented ridge within the lingual concavity is present in nearly all known spatulate sauropod teeth (Barrett et al., 2002; Mannion et al., 2013), and might be homologous with the mesiodistally convex lingual surface of the crowns of many diplodocoids and somphospondylans (Upchurch et al., 2004a, 2011). The absence of this ridge in the IVPP V11121-2 teeth is shared with just three other taxa with spatulate teeth: Oplosaurus armatus from the Early Cretaceous of England (Upchurch et al., 2004a, 2011), Jobaria from the Middle Jurassic of Niger (Mannion et al., 2017), and Klamelisaurus gobiensis from the Middle Jurassic of China (Zhao, 1993; Moore et al., 2020). However, in most other respects the teeth of the former two taxa are very different from those of IVPP V11121-2 (Upchurch et al., 2011; Mannion et al., 2017). In particular, the lingual surfaces of the IVPP V11121-2 crowns are nearly flat mesiodistally, whereas this surface is concave in Oplosaurus and Jobaria. Perhaps the most informative character state in the IVPP V11121-2 teeth is the presence of a boss on the distal margin of the crown. These resemble those seen in Euhelopus (Wilson and Sereno, 1998; Wilson and Upchurch, 2009). Over the past decade, nearly all studies have recovered Euhelopus within Macronaria, usually as an early-branching somphospondylan (e.g., Wilson and Sereno, 1998; Wilson, 2002; Wilson and Upchurch, 2009; D’Emic, 2012; Mannion et al., 2013; Gorscak and O’Connor, 2019; Carballido et al., 2020). Consequently, the presence of these bosses in IVPP V11121-2 specimens 2 and 3 would previously have been interpreted as indicative of macronarian affinities and potential membership of an Early Cretaceous somphospondylan euhelopodid radiation (sensu D’Emic, 2012; see also Canudo et al. [2002] and Barrett and Wang [2007]). However, Moore et al. (2020) found that most of their phylogenetic analyses placed Euhelopus within CMTs, well outside Neosauropoda. Moreover, the distolingual boss is also present on the dentary teeth of Mamenchisaurus sinocanadorum (Suteethorn et al., 2013; Moore et al., 2020), although it also characterizes the teeth of the Early Cretaceous Chinese taxon Yongjinglong, which has been recovered as a somphospondylan in previous studies (Li et al., 2014; Mannion et al., 2019b).

In summary, the character states present in the teeth of IVPP V11121-2 support their identification as those of a non-neosauropod eusauropod (though somphospondylan affinities cannot be ruled out) and are consistent with Dong's (1997) suggestion that they belonged to a mamenchisaurid. Indeed, apart from the absence of the lingual apicobasal ridge in IVPP V11121-2, these teeth most closely resemble those of Mamenchisaurus sinocanadorum. IVPP V11121-2 lacks any true autapomorphies but does possess a unique combination of features: it is the only taxon currently known that lacks both the apicobasal lingual ridge and clear labial grooves, while also possessing a distolingual boss. Given the inadvisability of naming new taxa on such scant material (e.g., the danger of historical obsolescence described by Wilson and Upchurch [2003]), we refrain from erecting a new genus or species at this time, pending further discoveries.

Diagnosis—As for type species.

Nomenclatural Acts—The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix ‘ http://zoobank.org/.’ The LSID for this publication is: urn:lsid:zoobank.org:pub:A42348FE-ECE6-4524-B536-857AFFD22DB2. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: CLOCKSS.

Species Diagnosis—Rhomaleopakhus turpanensis is diagnosed on the basis of three autapomorphies: (1) humeral deltopectoral crest terminates distally in a transversely narrow ridge that is separated from the main body of the crest by distinct lateral and medial grooves; (2) prominent (100 mm long) ridge, projecting posteromedially, on posterior surface of radial shaft, a short distance below the proximal end; and (3) radial distal articular surface markedly concave in central and medial portions. In addition, Rhomaleopakhus turpanensis possesses one of the most robust ulnae of any known sauropod (maximum proximal end width to proximodistal length ratio is 0.50;  Table S2 in Supplemental Data 1 (ujvp_a_1994414_sm6076.zip)), and is currently the only known non-somphospondylan eusauropod with the long-axes of the proximal and distal surfaces of the radius twisted through ∼90° with respect to each other.

Holotype—A right forelimb, IVPP V11121-1 (Figs. 6–10; Tables 3 and 4), consisting of the humerus, ulna, radius, one carpal, and virtually complete manus of a single individual.

Etymology—Rhomaleos (ancient Greek, masculine) equals ‘robust’ (pertaining to the body), and pakhus (ancient Greek, masculine) equals ‘forearm.’ The species name refers to the Turpan Basin, China, where the holotype was found.

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).

Description and Comparisons

Humerus—The right humerus is nearly complete, apart from a portion of the proximomedial expansion (Dong, 1997) and a small part of the proximolateral corner (Figs. 6, 7A, 8A). The posterior surface of this element could not be examined fully due to its large size and storage within a protective cradle. It is a relatively robust element, with an estimated Humeral Robusticity Index (sensu Wilson and Upchurch, 2003) of 0.35, similar to those of other heavily built taxa such as Mamenchisaurus youngi, Apatosaurus, dicraeosaurids, and Opisthocoelicaudia (Upchurch et al., 2015:table 2). Proximally, the humerus expands laterally relative to the shaft, giving it an hourglass-shaped outline in anterior view; this is the plesiomorphic sauropod condition, contrasting with the more asymmetrical humeri of most titanosauriforms and turiasaurians (Tschopp et al., 2015a; Poropat et al., 2016). The anterior surface of the humerus is too damaged proximally to determine whether a tuberosity for the attachment of the M. coracobrachialis was present.

The deltopectoral crest of Rhomaleopakhus is more prominent than those of most sauropods and is similar to those in Turiasaurus (Royo-Torres et al., 2006) and brachiosaurids (Wilson and Sereno, 1998). The crest lies entirely on the anterolateral margin of the humeral shaft: it does not expand or project medially across the anterior surface (Fig. 7A), unlike those in many titanosauriforms (Wilson, 2002; Mannion et al., 2013). It terminates at ∼44% of humerus length from the proximal end: by comparison, values among other sauropods range between 35–50% (Upchurch et al., 2015:table 2). In this respect, Rhomaleopakhus is almost identical to several other CMTs: for example, these values are 44% in Anhuilong and Omeisaurus tianfuensis, and 43% in Huangshanlong (Ren et al., 2018). In anterior view, the anterolateral margin of the deltopectoral crest has a sigmoid profile and is relatively narrow throughout its length. One unusual feature of the deltopectoral crest is that its distal terminus forms a narrow ridge that is offset medially and laterally from the rest of the crest surface by deep, dorsoventrally oriented grooves or breaks-in-slope: this is provisionally regarded as autapomorphic. Rhomaleopakhus lacks prominent ridges or bulges on the posterolateral surface of the shaft, at the level of the deltopectoral crest. Such projections occur in many titanosaurs, including Alamosaurus, Opisthocoelicaudia, Patagotitan, and Saltasaurus, and have been interpreted as the insertion sites of a number of muscles, including the M. latissimus dorsi, M. scapulohumeralis anterior, and M. deltoideus clavicularis, although these interpretations are debated (e.g., Borsuk-Białynicka, 1977; Otero, 2010, 2018; Upchurch et al., 2015; Moore et al., 2020; Otero et al., 2020; Voegele et al., 2020). In Rhomaleopakhus, as in most sauropods (Wilson, 2002; Mannion et al., 2013; Upchurch et al., 2015), the humeral shaft is wider transversely than anteroposteriorly, producing an elliptical horizontal cross-section at midlength. The transverse width of the shaft at midlength to proximodistal length ratio is estimated at 0.17–0.18. There is a small amount of torsion in the shaft, such that the long-axes of the proximal and distal end surfaces are slightly rotated relative to each other, but Rhomaleopakhus lacks the marked torsion (c. 40°) seen in many diplodocids (Tschopp et al., 2015a) and some CMTs (e.g., at least 30° in Klamelisaurus [Moore et al., 2020] and 25° in Huangshanlong [Huang et al., 2014] and Anhuilong (Ren et al., 2018]). Huang et al. (2014) regarded such humeral torsion as a synapomorphy of Mamenchisauridae, but there is clearly some variation among CMTs and homoplasy within Sauropoda, especially given that a strong degree of torsion of the humeral shaft is the plesiomorphic sauropodomorph condition that is lost in early sauropods (e.g., Yates, 2007; McPhee et al., 2014).

The distal end of the humerus is relatively wide transversely compared with the width of the shaft at midlength, largely because it projects a considerable distance medially (Fig. 7A). The ratio of distal end transverse width to humerus proximodistal length is 0.38, which is equaled or exceeded only by Apatosaurus and a few titanosaurs (Poropat et al., 2016;  Table S2 in Supplemental Data 1 (ujvp_a_1994414_sm6076.zip)). Distally, the anterior surface of the humerus is flat, apart from the relatively large lateral and medial anterodistal processes (sensu Upchurch et al., 2015) (Fig. 8B). Although the relative size of these anterodistal processes is difficult to quantify, they are very reduced or absent in Chubutisaurus and titanosaurs (D'Emic, 2012), and are particularly large in several CMTs (Remes, 2008), such as Chuanjiesaurus (Sekiya, 2011) and Huangshanlong (Huang et al., 2014). Enlarged (Huang et al., 2014) and/or anteriorly directed (Ren et al., 2018) anterodistal processes have been regarded as a synapomorphy of Mamenchisauridae: however, reduction and loss of these processes appears to be the derived state (D'Emic, 2012), and increased process size requires quantification and more comparative work before it can provide support for mamenchisaurid affinities. In Rhomaleopakhus, the distal articular surface is rugose and does not expand up onto the anterior face of the shaft, unlike the humeri of some titanosaurs (Wilson and Carrano, 1999; Wilson, 2002). The ulnar and radial condyles are not strongly divided from each other, and the former is somewhat larger than the latter. Remes (2008) suggested that mamenchisaurids possess a unique distal humeral configuration. In Klamelisaurus, Omeisaurus tianfuensis, and Mamenchisaurus youngi, the lateral condyle (which Remes [2008] termed the ‘radial’ condyle, but which has become the ulnar condyle in sauropods because of the rotation of the antebrachium [Bonnan, 2003]), is larger than the radial one. Moreover, the ulnar and radial condylar surfaces have long axes that are at ∼90° to each other in distal end view, with the former directed anterolaterally. This results in the lateral part of the distal end having a distinct subtriangular profile, formed by fairly straight anterolateral and posterolateral margins that meet each other at an acute angle (e.g., He et al., 1988:fig. 44B; Ouyang and Ye, 2002:fig. 35F; Sekiya, 2011:figs. 38C, 39C). In many other sauropods, this lateral portion is more semicircular or subquadrate in distal view (see Upchurch et al., 2015:fig. 4; N.B., Upchurch et al.’s fig. 4A shows the distal end profile of the right humerus of Mamenchisaurus youngi incorrectly labelled as the left). Rhomaleopakhus possesses the same distal end profile seen in other CMTs (Fig. 8B): however, several non-CMTs also possess this state and, in any case, it is potentially the plesiomorphic eusauropod condition (Mannion et al., 2019a). In Rhomaleopakhus, the lateral third of the flat distal end surface is quite strongly beveled (∼30° relative to the plane lying perpendicular to the proximodistal long-axis of the humerus) (Fig. 7A): as a result, it faces laterodistally. This feature, however, does not seem to have a clear phylogenetic significance; it occurs sporadically in distantly related taxa such as Amargasaurus, Anhuilong, Haestasaurus, Limaysaurus, Mamenchisaurus youngi, and Saltasaurus (Ouyang and Ye, 2002; Upchurch et al., 2015; Ren et al., 2018; Mannion et al., 2019a). The supracondylar (= olecranon or cuboid) fossa, and the medial and lateral ridges that bound it on the distal part of the posterior surface of the shaft, are partially obscured by the packing material upon which the humerus rests (Fig. 8B). However, this fossa is not deep, unlike those of Giraffatitan and several somphospondylans (Upchurch et al., 2004a, 2015; D’Emic, 2012), and the associated ridges are broadly rounded transversely rather than acute.

Ulna—The ulna is complete apart from a small amount of material missing from the proximal end (Figs. 6, 9A–F). It is extremely robust, with one of the highest proximal end maximum width to proximodistal length ratios (0.50) of any sauropod, although Opisthocoelicaudia has a ratio of 0.51 ( Table S2 in Supplemental Data 1 (ujvp_a_1994414_sm6076.zip)). The expanded proximal end is triradiate because of the presence of well-developed anterolateral, anteromedial, and posteromedial processes. As in other sauropods, the anterolateral and anteromedial processes define a deep concavity that receives the proximal end of the radius (Wilson and Sereno, 1998). In proximal view (Fig. 9E), the ulna of Rhomaleopakhus has a ‘V’-shaped profile, rather than the ‘T’-shape seen in several somphospondylans (Upchurch et al., 2015). The angle between the anteromedial and anterolateral processes is ∼70°, which is the derived state (i.e., less than 80°) that occurs in most sauropods (including Chuanjiesaurus, Mamenchisaurus youngi, and Klamelisaurus), except some non-neosauropods, such as Shunosaurus, Omeisaurus tianfuensis, Anhuilong, Huangshanlong, Bellusaurus, and Cetiosaurus, as well as several titanosaurs, in which this angle is greater than 80° and often approaches 90° (Huang et al., 2014; Tschopp et al., 2015a; Poropat et al., 2016; Ren et al., 2018; Moore et al., 2020). In Rhomaleopakhus, the anteromedial to anterolateral process length ratio (sensu Upchurch et al., 2015) is 1.72 (N.B., the measurements in Table 3 give a ratio of 1.25, but these are the maximum lengths of the processes, not their lengths measured to the intersection of process long-axes, as defined by Upchurch et al. [2015:fig. 13A]). This ratio typically ranges between 1.6–1.8 in non-neosauropod eusauropods (e.g., Vulcanodon, Cetiosauriscus, Ferganasaurus), 1.0–1.3 in most diplodocoids and non-titanosauriform macronarians, and >1.5 in titanosauriforms (with values >1.6 in titanosaurs such as Opisthocoelicaudia and ≥2.0 in Epachthosaurus and Cedarosaurus) (Upchurch et al., 2015:table 2). The anteromedial process of the proximal end of the Rhomaleopakhus ulna has a strongly concave articular surface (Fig. 9A–D), as also occurs in many titanosaurs (Upchurch, 1995, 1998), several non-neosauropod eusauropods such as Janenschia and Haestasaurus (Bonaparte et al., 2000; Upchurch et al., 2015; Mannion et al., 2019a), and in a more shallowly concave form in Chuanjiesaurus (Sekiya, 2011). Dong (1997) stated that the olecranon process is relatively low in Rhomaleopakhus, although this region is moderately projected, which is emphasized by the concave proximal surface of the anteromedial process. Similarly developed olecranon processes are seen in Mamenchisaurus youngi (Ouyang and Ye, 2002:fig. 36), Chuanjiesaurus (Sekiya, 2011:fig. 40), Haestasaurus (Upchurch et al., 2015), Janenschia (Bonaparte et al., 2000; Mannion et al., 2019a), and several titanosaurs (Upchurch, 1995; Wilson and Carrano, 1999; Upchurch et al., 2004a). In Rhomaleopakhus, the posteromedially directed process of the proximal end creates a concavity on the posteromedial surface that does not fade out until approximately the midlength of the element, whereas the lateral surface is flat or slightly convex anteroposteriorly. In horizontal cross-section, the proximal portion of the ulna retains the triradiate configuration, but by midlength it is elliptical, with the long-axis of this ellipse oriented anteromedially. There is a prominent ridge for a ligamentous attachment to the radius, located on the anteromedial surface of the shaft at ∼100 mm above the distal end. The distal end of the ulna is expanded both anteroposteriorly and transversely relative to the shaft. In distal view (Fig. 9F), the margins of this surface are strongly convex laterally and posteriorly, but slightly concave anteromedially, resulting in a comma-shaped distal profile, as is typical for most non-titanosaurian sauropods (Upchurch et al., 2015). The distal articular surface is mildly convex anteroposteriorly and transversely.

Radius—The radius is complete and is 63% of the length of the humerus. This is broadly similar to the condition in many other sauropods, which tend to have values ≥65% (Yates and Kitching, 2003; Mannion et al., 2013). For example, this value is ∼66% in Mamenchisaurus youngi (Ouyang and Ye, 2002) and ranges from 65–76% in specimens referred to Omeisaurus (He et al., 1988; Ren et al., 2018). By contrast, this ratio is reduced in titanosauriforms (Mannion et al., 2013) and many CMTs (Ren et al., 2018), with particularly low values of 58% and 50% in Huangshanlong and Anhuilong, respectively (Huang et al., 2014; Ren et al., 2018). The radius of Rhomaleopakhus is a robust element with expanded proximal and distal ends relative to the shaft (Dong, 1997) (Fig. 9G–J). The maximum widths of the proximal and distal ends are subequal, the proximal end transverse width to radius proximodistal length ratio is 0.31, and the distal end is ∼1.3 times as wide as the shaft at its midlength (Table 3). The proximal end surface is flat, with a central shallow concavity and a slightly convex portion around both its anterior and lateral margins. In proximal view (Fig. 9K), the radius has a ‘D’-shaped profile, comprising a straight posterior margin (that becomes mildly concave towards the medial corner), and strongly convex anterior and lateral margins. This proximal profile appears to be plesiomorphic for sauropods, contrasting with the derived subtriangular profile with pointed medial process seen in many titanosauriforms (Upchurch et al., 2015:fig. 9), and the anteroposteriorly narrow morphology that characterizes some turiasaurians (Mateus et al., 2014).

Approximately 100 mm below the mildly concave posteromedial margin of the proximal end, on the posterior surface, there is a prominent 100 mm long ridge that projects posteromedially. Titanosaurs, such as Epachthosaurus, Rapetosaurus, and Saltasaurus, usually have a ridge on the posterior surface of the radius that extends along much of the element's length (Curry Rogers, 2005, 2009; Mannion et al., 2013), and Ren et al. (2018: fig. 4C) described a ‘lateral ridge’ (‘lr’) on the proximal part of the Anhuilong radius. However, the morphology and position of the short, prominent and posteromedially directed ridge seen in Rhomaleopakhus appears to be unique and is provisionally regarded as an autapomorphy. The radius is twisted along its length such that the long-axis of the proximal articular surface is set at about 90° to that of the distal end. As a result, the posterior surface of the shaft turns to face laterally as it approaches the distal end. Such torsion of the radius is rare among sauropods (Mannion et al., 2013), although it has also been observed in the somphospondylan Huabeisaurus (D’Emic et al., 2013) and a few titanosaurs (e.g., Epachthosaurus – Poropat et al., 2016; Malawisaurus – Gomani, 2005; Rapetosaurus – Curry Rogers, 2009). At midlength, the cross-section through the shaft is elliptical in Rhomaleopakhus, with the radius being wider transversely than anteroposteriorly. There is a prominent vertical ridge on the posterolateral surface, located at approximately one-fifth of element length from the distal end. This matches the prominent ridge on the anteromedial surface of the shaft of the ulna, close to the distal end, suggesting that these two ridges marked the location of a strong interosseous ligament (Upchurch et al., 2004a).

In medial view (Fig. 9J), the distal end surface is set at an oblique angle to the long axis of the shaft such that it slopes anteroproximally (N.B., this would be proximolateral beveling of the distal end, in anterior view, if the radius was not twisted through 90° along its length). As a result, the distal end surface is set at ∼15° to the plane perpendicular to the proximodistal long-axis of the radius. Non-neosauropod eusauropods (such as Shunosaurus and Mamenchisaurus), and at least some rebbachisaurids, display no such beveling of the distal radius, whereas turiasaurians and several titanosaurs have angles of ∼25° or higher (Wilson, 2002; Mannion et al., 2019a). The degree of distal radial beveling in Rhomaleopakhus is similar to that seen in several non-neosauropod eusauropods, including Omeisaurus tianfuensis, Chuanjiesaurus, and Jobaria, as well as some neosauropods such as Diplodocus and Giraffatitan (Mannion et al., 2019a). In Rhomaleopakhus, beveling of the distal end extends uniformly across the entire articular surface, as occurs in some titanosaurs such as Opisthocoelicaudia and Saltasaurus (Wilson, 2002; Mannion et al., 2013; Upchurch et al., 2015). This contrasts with the more typical form of distal beveling in other sauropods, in which the medial half of the distal end surface is perpendicular to the long-axis of the shaft, such that the beveled section is limited to the lateral half (Mannion et al., 2013; Upchurch et al., 2015). The distal end has a ‘D’-shaped outline (Fig. 9L), with the derived, nearly straight posterior (= lateral because of shaft torsion) margin observed in other sauropod radii, rather than the plesiomorphic convex margin that occurs in non-sauropod sauropodomorphs (Wilson & Sereno, 1998). In fact, this posterior distal margin is mildly concave between the posterolateral and posteromedial ‘condyles.’ Such distal radial condyles were first discussed by D’Emic (2012, 2013), and their wider distribution among sauropods was further investigated by Upchurch et al. (2015). According to the latter, such condyles tend to occur in neosauropods, but with several reversals in, for example, some titanosaurs. Laterally, the distal surface of the Rhomaleopakhus radius is mildly convex, whereas the central and medial portions are markedly concave: this contrasts with the uniformly convex distal surfaces seen in nearly all other sauropods (Janensch, 1961; Upchurch et al., 2004a). Ren et al. (2018) described the distal end surface of the radius of Anhuilong as also being flat over most of its extent, with a convex area placed posteriorly and medially. Thus, while Rhomaleopakhus and Anhuilong potentially share the unusual flattening of the distal articular surface, the location of the residual convex area differs. Consequently, this concavity is regarded as an autapomorphy of Rhomaleopakhus.

Manus—The right manus is virtually complete, including one carpal element, five metacarpals, and two phalanges per digit except for digit V (see below) (Fig. 10). These elements are preserved in articulation, but many details are obscured by matrix (especially the ‘palmar’ surfaces of the metacarpals – see below for definitions of the orientations of the latter).

A large, flat, block-like carpal is situated above metacarpals I and II (Fig. 10A, D) (N.B., Dong [1997] stated that this element also articulated with metacarpal III, but this is not supported by our observations of the specimen). Possession of block-like carpals is a synapomorphy of Eusauropoda according to Wilson and Sereno (1998), contrasting with the carpals of non-sauropod sauropodomorphs, which tend to have proximodistally more rounded margins, and proximal and distal surfaces that are less parallel (Yates, 2007). Sauropods have often been interpreted as possessing ossified distal carpals only (e.g., Gauthier, 1986; Wilson and Sereno, 1998; Upchurch et al., 2004a), although an ossified proximal carpal is probably present in at least ‘Bothriospondylus madagascariensis’ and Apatosaurus (Läng and Goussard, 2007; Tschopp et al., 2015b). The Rhomaleopakhus carpal resembles the ‘medial distal carpal’ in Camarasaurus (Tschopp et al., 2015b). With the exception of Apatosaurus (Hatcher, 1902; Gilmore, 1936), the largest carpal in the sauropod wrist is generally placed over metacarpals I and II and articulates closely with them. This element could represent: a single enlarged distal carpal I; a fusion of distal carpals I and II; or the fusion of the intermedium, one or two centrales, and distal carpal I (as proposed for ‘Bothriospondylus madagascariensis’ by Läng and Goussard, 2007). If the latter interpretation is correct, then we cannot regard the carpal of Rhomaleopakhus as being either a proximal or distal carpal since it would be a composite with contributions from each of the three rows of carpals found in the plesiomorphic archosaurian wrist.

The margins of the Rhomaleopakhus carpal are damaged, such that its outline can only be estimated as subcircular to elliptical, with the long axis running transversely. The approximate transverse:anteroposterior width ratio is 1.23, similar to the values seen in several non-neosauropod eusauropods such as Shunosaurus and turiasaurians, but differing from the higher values (>1.4) observed in many neosauropods (Royo-Torres et al., 2014; Mannion et al., 2017). The proximal surface of the carpal is irregularly flat, with a slight convexity near the posterior and lateral margins. The posterolateral edge has a small vertical groove, suggesting that this portion is possibly a small medial part of a more lateral carpal, perhaps supporting the view that this large medial element is a composite structure (Läng and Goussard, 2007). The distal surface of the carpal cannot be examined because of the presence of matrix and the proximal ends of the metacarpals.

The true number of ossified carpals in Rhomaleopakhus cannot be determined. Sauropods appear to show a trend towards loss and/or fusion of carpals through their evolutionary history, with five and three-to-four elements in the early-diverging taxa ‘Bothriospondylus madagascariensis’ and Shunosaurus, respectively, two in non-neosauropod eusauropods and non-titanosauriform macronarians, one in diplodocids (such as Apatosaurus and Diplodocus) and Giraffatitan, and complete loss in some titanosaurs such as Alamosaurus and Opisthocoelicaudia (Janensch, 1961; Upchurch, 1998; Upchurch et al., 2004a; Apesteguía, 2005; Remes, 2008; Tschopp et al., 2015b). The single carpal in Apatosaurus (Gilmore, 1936; Bonnan, 2003) is placed centrally over metacarpals II–IV and has a proximal surface that conforms closely to the distal ends of the ulna and radius (Tschopp et al., 2015b). Although it is possible that Rhomaleopakhus only possessed one carpal and that this taxon differed from Apatosaurus in having this placed medially over metacarpals I and II, we consider it more likely that there was at least one additional (lateral) carpal placed over metacarpal III (as in Mamenchisaurus youngi: Ouyang and Ye, 2002) or metacarpal V (as in Camarasaurus, Atlasaurus, and possibly Argyrosaurus: Apesteguía, 2005; Tschopp et al., 2015b). This view is supported by the possible presence of a small portion of a more lateral carpal (as described above) which, if correctly identified, would suggest that the wrist of Rhomaleopakhus most closely resembled that of Mamenchisaurus youngi (Ouyang and Ye, 2002).

The stout metacarpals have a semicircular or horseshoe-shaped arrangement with their long axes oriented vertically (Fig. 10); this is a eusauropod synapomorphy (Upchurch, 1995, 1998; Yates, 2007; McPhee et al., 2014; Apaldetti et al., 2018). The arc of a circle covered by this metacarpal arcade is ∼270°, as occurs in neosauropods (Upchurch, 1998; Wilson and Sereno, 1998; Bonnan, 2003; Apesteguía, 2005; Remes, 2008) and several taxa close to the neosauropod radiation, such as Mamenchisaurus youngi (Ouyang and Ye, 2002) and ‘Bothriospondylus madagascariensis’ (Läng and Goussard, 2007). This contrasts with the apparently less strongly curved arcades (∼90–180°) seen in other non-neosauropod eusauropods, such as Omeisaurus tianfuensis (Bonnan, 2003), Shunosaurus (ZDM T5402; PU pers. observ., 1995), and possibly Ferganasaurus (Alifanov and Averianov, 2003) (N.B., we are skeptical about the accuracy of the reconstruction of the manus of the latter based on, for example, an anomalous arrangement of the metacarpals as reconstructed in distal end view: see Alifanov and Averianov, 2003:fig. 9C). The vertically oriented metacarpals, in a ‘tubular colonnade,’ make conventional directional anatomical terms ambiguous unless care is taken to define them (e.g., see Upchurch, 1994). Here, we treat the metacarpals as if they were laid on a flat surface side-by-side. As such, ‘lateral,’ ‘medial,’ ‘dorsal,’ and ‘ventral’ refer to surfaces on the shafts of the metacarpals, rather than how these surfaces would face in the articulated manus. As a result, the dorsal surfaces face outwards, ventral surfaces face towards the center of the tubular colonnade, and metacarpals typically contact each other via portions of their lateral and medial surfaces. In correct articulation, the phalanges are placed in a more conventional orientation, with their ventral surfaces facing approximately downwards. Therefore, no additional definitions are required for phalanges, although it should be borne in mind that, for example, the medial surface of the pollex claw would have faced posteriorly or posteromedially in life with respect to the sagittal plane of the animal (Fig. 10).

The proximal ends of metacarpals I and II in Rhomaleopakhus are obscured by the overlying carpal. In anterior view (Fig. 10A), the proximal ends of metacarpals I–III are level with each other, whereas that of metacarpal IV is displaced distally. The proximal end of metacarpal V is, in turn, displaced distally with respect to metacarpal IV. These displacements of metacarpals IV and V are presumably the result of post-mortem distortion rather than an unusual morphology possessed by the living animal. In metacarpals III–V, the exposed proximal end surfaces are generally flat and mildly rugose.

Metacarpal I is short compared with the other metacarpals (e.g., it is only 0.67 of the averaged length of metacarpals II and III: Table 4) and shorter than the ungual on digit I. Such a relatively short metacarpal I is the plesiomorphic state that occurs in non-sauropod sauropodomorphs, non-neosauropod eusauropods (such as Shunosaurus, Omeisaurus tianfuensis, and Mamenchisaurus youngi), and, to a lesser extent, in diplodocines ( Table S2 in Supplemental Data 1 (ujvp_a_1994414_sm6076.zip)). In Rhomaleopakhus, metacarpal I is substantially longer along its medial margin than on its lateral one (Table 4): this reflects the beveling of the distal end relative to the long-axis of the shaft. This condition is a derived state that occurs in most eusauropods except Shunosaurus, with a reversal to the plesiomorphic state in most titanosauriforms (Wilson, 2002; Mannion et al., 2013). As in Chuanjiesaurus (Sekiya, 2011), Turiasaurus (CPT-1195-1210; PU and PDM pers. observ., 2009), and many neosauropods (Wilson, 2002), the distal end of metacarpal I is not divided into two distinct condyles.

In dorsal view, the proximal end of metacarpal II is strongly expanded to overhang the medial surface of its shaft (Fig. 10A, C). This feature is absent in taxa such as Mamenchisaurus youngi (Ouyang and Ye, 2002:fig. 38B), Apatosaurus ajax (Upchurch et al., 2004b:pl. 8, fig. D), and Camarasaurus (Tschopp et al., 2015b: fig. 11), but a medial process appears to be developed to some extent in Ferganasaurus (Alifanov and Averianov, 2003:figs. 9, 10), Giraffatitan (Janensch, 1961:194, fig. 1a), and Alamosaurus (Gilmore, 1946:fig. 10). A minimum shaft width to proximodistal length ratio of <0.2 in metacarpal II was proposed as a diagnostic character of Chuanjiesaurus by Sekiya (2011); however, this ratio is 0.19 in Rhomaleopakhus, similar to those of several other non-neosauropod eusauropods, such as Omeisaurus tianfuensis, Mamenchisaurus youngi, and Turiasaurus (Poropat et al., 2016).

The proximal articular surface of metacarpal III is subtriangular in outline (Fig. 10D). This element is the longest of the five metacarpals, as is the case in most eusauropods (Poropat et al., 2015a), although it only slightly exceeds the length of metacarpal II (Table 4). The length of metacarpal III is 0.42 of radius length, similar to the condition in taxa such as Mamenchisaurus youngi and Apatosaurus, but lower than the derived 0.45 ratio employed as a synapomorphy of Macronaria by Wilson and Sereno (1998;  Table S2 in Supplemental Data 1 (ujvp_a_1994414_sm6076.zip)). Its proximal end lacks the mediolaterally expanded morphology that characterizes brachiosaurids, as well as Atlasaurus and Jobaria (Mannion et al., 2017).

Metacarpal IV also has a subtriangular proximal end but differs from metacarpal III by possessing a ventromedially directed palmar process (Fig. 10D). Unlike the metacarpal IVs of several brachiosaurids and a few titanosaurs, that of Rhomaleopakhus lacks the chevron-shaped proximal end profile that wraps around the proximal end of metacarpal V (D'Emic, 2012; Mannion et al., 2013).

The proximal end of metacarpal V is semicircular to slightly subrectangular in outline, with a flattened medial surface that articulates with metacarpal IV (Fig. 10D). Metacarpal V is twisted along its length such that the long-axes of its proximal and distal ends lie at ∼90° to each other, and this degree of twisting has also been reported in Ferganasaurus (Alifanov and Averianov, 2003). Some torsion of metacarpal V also occurs in neosauropods but is less extreme than in Rhomaleopakhus and Ferganasaurus (Apesteguía, 2005; Bedwell and Trexler, 2005; Tschopp et al., 2015b). For example, in Camarasaurus and Diplodocus the amount of torsion is ∼25–30° (Bedwell and Trexler, 2005; Tschopp et al., 2015b), and in the titanosaur Epachthosaurus it is ∼45° (UNPSJB-PV 920; PU and PDM pers. observ., 2013).

The phalanges are hyper-extended such that they lie on the dorsodistal parts of each metacarpal, except in metacarpal I where the phalanx obscures the distal end (resulting in the distal end surfaces being visible in metacarpals II–V) (Fig. 10E). In general, the distal articular surfaces of the metacarpals are expanded dorsoventrally, and especially transversely, and have a rounded subrectangular outline. These surfaces are gently saddle-shaped, with mild midline grooves between slightly expanded lateral and medial condyles. The ventral portions of the distal ends are flattened and have a rugose texture. Generally, the distal articular surfaces do not extend onto the dorsal surfaces of the shafts: this is a derived state seen in titanosauriforms (Gimenez, 1992; Salgado et al., 1997; Apesteguía, 2005; D'Emic, 2012; Mannion et al., 2013) that also occurs convergently in rebbachisaurids (Mannion et al., 2019a). Rhomaleopakhus lacks the additional flanges, close to the distal ends of the metacarpals, that helped bind them together in some titanosaurs (Apesteguía, 2005).

Dong (1997) stated that IVPP V11121-1 has a phalangeal formula of 2-2-2-1-1; however, it is actually 2-2-2-2-1 (Fig. 10E). Retention of two phalanges on manual digit IV occurs in early-branching sauropods such as Shunosaurus, but in most neosauropods the phalangeal formula has been reduced to 2-2-2-1-1, 2-2-1-1-1, or 2-1-1-1-1 (in diplodocoids and early-diverging macronarians), or the phalanges are completely lost (apart from a rudimentary phalanx IV-1) in titanosaurs such as Epachthosaurus, Alamosaurus, and Opisthocoelicaudia (Gilmore, 1946; Borsuk-Białynicka, 1977; Salgado et al., 1997; Bonnan, 2003; Martínez et al., 2004; Upchurch et al., 2004a, b; Mannion et al., 2013; Poropat et al., 2015b; Tschopp et al., 2015b). The phalanges (except for the ungual of digit I) of Rhomaleopakhus are wider transversely than they are proximodistally, which is a eusauropod synapomorphy (Wilson, 2002; Upchurch et al., 2004a, 2007b; Yates, 2007). The phalanges in the proximal row have flattened or mildly concave ventral surfaces. These phalanges are also expanded transversely at their distal ends, so that they are wider at this point than they are at midlength.

Phalanx I-1 is subrectangular in dorsal view, decreasing only slightly in proximodistal length towards its medial margin. Similar subrectangular manual phalanx I-1s are seen in several other non-neosauropod eusauropods, such as Ferganasaurus (Alifanov and Averianov, 2003:fig. 11) and Omeisaurus tianfuensis (He et al., 1988:pl. XIV, fig. 6), as well as the titanosauriform Giraffatitan (Janensch, 1961). Thus, Rhomaleopakhus retains the plesiomorphic manual phalanx I-1 dorsal profile, rather than the derived trapezoidal outline seen in Turiasaurus (Mannion et al., 2019a) and Jobaria (Läng and Goussard, 2007), or the even more strongly wedge-shaped outline seen in several diplodocids and the non-titanosauriform eusauropod specimen MfN MB.R. 2093 (previously referred to Janenschia but removed from that genus by Mannion et al. [2019a]) (Upchurch et al., 2004a; Tschopp et al., 2015b). The proximal and distal ends of phalanx I-1 are obscured by the metacarpal and ungual respectively, but the general outline of the transverse cross-section is an irregular ‘D’-shape, with rounded medial, dorsal, and lateral surfaces, and a flattened ventral surface. There is no lappet-like projection from the proximodorsal margin. Such a lappet occurs as the plesiomorphic condition in early-branching eusauropods such as Shunosaurus, Omeisaurus tianfuensis, Turiasaurus, and Zby, but is absent in most neosauropods (Mannion et al., 2019a). Distally, the phalanx terminates in well-developed, rounded lateral and medial condyles.

Phalanx I-2 is a large, robust ungual that is transversely compressed. As in most other sauropods, this ungual is much longer than phalanx I-1 (Fig. 10E), whereas in Giraffatitan the two elements are subequal in length (Janensch, 1922). In dorsal view, the proximal articular surface of the Rhomaleopakhus ungual is approximately perpendicular to the long axis of the claw: this is the plesiomorphic state, whereas in neosauropods (e.g., Apatosaurus—Upchurch et al., 2004b; Camarasaurus—Tschopp et al., 2015b; Giraffatitan—Janensch, 1961) this surface is set at an oblique angle to the long-axis such that it faces proximolaterally. The Rhomaleopakhus ungual bears a groove on each of the lateral and medial surfaces, with the former being positioned lower than the latter. The ventral side merges smoothly into the medial surface but meets the lateral surface at a sharper edge.

Phalanx II-1 is subrectangular in dorsal view. The medial, lateral, and dorsal surfaces round smoothly into each other, although the medial edge meets the ventral surface at a slightly more acute angle than the lateral edge. The ventral surface is nearly flat. Phalanx II-2 is larger than phalanx II-1 (Table 4) (contra Dong, 1997) but seems to have a pathological distal termination. It appears damaged and ends irregularly, with a cavity running down the central part of its ventral surface (Fig. 10E).

Phalanx III-1 is large and dorsoventrally compressed, with two distinct distal condyles. Whereas the dorsal surface meets the proximal and distal end surfaces at an obtuse angle in lateral or medial views, the articular surfaces expand ventrally to make the ventral surface concave proximodistally. In dorsal view, this element narrows slightly in transverse width towards its distal end. Phalanx III-2 is similar to phalanx III-1, but is slightly smaller, with its distal end rounding transversely in dorsal view so that it curves into the corners of the proximal end. It is therefore more semicircular, rather than rectangular, in dorsal profile. This element is also bowed upwards in distal end view.

Phalanx IV-1 is large, dorsoventrally compressed, and subrectangular in dorsal outline. The medial condyle is large and rounded, and projects more distally than the lateral one. In dorsal view, the medial margin is mildly concave, whereas the lateral one is straighter. This element tapers slightly transversely towards the distal end in dorsal view. Phalanx IV-2 is a very small, flattened hemisphere of bone that sits in the intercondylar groove on the distal end of phalanx IV-1. The dorsal and ventral surfaces are slightly concave longitudinally because of the expansion of both ends. The lateral condyle of the distal end is enlarged dorsoventrally, but the medial condyle is indistinct.

Phalanx V-1 is large, subrectangular, and dorsoventrally compressed. The dorsal and ventral surfaces are slightly concave longitudinally because of the expansion of the proximal end. The element tapers in dorsoventral thickness towards its distal end. The distal surface is generally convex both dorsoventrally and transversely, with little division into two separate condyles. Thus, this phalanx in Rhomaleopakhus still resembles the other proximal phalanges, as it does in several other sauropods such as Apatosaurus (Upchurch et al., 2004b): this contrasts with phalanx V-1 of Camarasaurus, which is very irregular and rather different from the other proximal phalanges (Tschopp et al., 2015b).