Rhomaleopakhus turpanensis Upchurch & Mannion & Xu & Barrett 2021, sp. nov

RHOMALEOPAKHUS TURPANENSIS, sp. nov. (Figs. 6–10; Tables 3 and 4) 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 The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: CLOCKSS.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), 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). 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., 2004 a, 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). 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 nonneosauropods, 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 onefifth 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 longaxis 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 nonneosauropod eusauropods, including Omeisaurus tianfuensis, Chuanjiesaurus, and Jobaria, as well as some neosauropods such as Diplodocus and Giraff