MAMENCHISAURIDAE Young and Chao, 1972

Upchurch, Paul, Mannion, Philip D., Xu, Xing & Barrett, Paul M., 2021, Re-assessment of the Late Jurassic eusauropod dinosaur Hudiesaurus sinojapanorum Dong, 1997, from the Turpan Basin, China, and the evolution of hyper-robust antebrachia in sauropods, Journal of Vertebrate Paleontology (e 1994414) 41 (4), pp. 1-31 : 9-12

(?) MAMENCHISAURIDAE Young and Chao, 1972

GEN. ET SP. INDET.

( Fig. 5 )

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 macrowear 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., 2004 a, 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., 2004 a, 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.

Figures

FIGURE 1. Map showing Xinjiang Autonomous Region in China, with a magnified inset showing the approximate location of the Hudiesaurus specimens within Shanshan County.

FIGURE 5. Teeth previously referred to Hudiesaurus sinojapanorum (IVPP 11121-2) but regarded as?Mamenchisauridae indet. herein. A–D, Two tooth crowns within a broken jaw element in lingual (A), labial (B), distal (C), and mesial (D) views. E–H, Isolated tooth crown in lingual (E), labial (F), distal (G), and mesial (H) views. I–L, isolated tooth crown in lingual (I), labial (J), distal (K), and mesial (L) views. Abbreviation: lb, lingual boss. Scale bars equal 10 mm.