A new juvenile sauropod specimen from the Middle Jurassic Dongdaqiao Formation of East Tibet

Geological setting

The specimen described in this article was discovered in Qamdo District, about 10 km from the urban center of Chaya County (Fig. S1). Jurassic strata form an extensively exposed succession in the Qamdo Basin, and mainly comprise lacustrine deposits. The Jurassic strata of the Qamdo Basin include the Lower Jurassic Wangbu Formation, the Middle Jurassic Dongdaqiao Formation, and the Upper Jurassic Xiaosuoka Formation. The new specimen is from the Dongdaqiao Formation, which is about 1.2 km thick and mainly consists of purple red feldspar and quartz-bearing sandstones and siltstones. The dinosaur bones were recovered from red argillaceous siltstone in the middle part of the formation, within a thickness of about 5 m (Fig. 1). The Dongdaqiao Formation has generally been considered to date from the Middle Jurassic, based on its bivalve assemblage (Wang & Chen, 2005).

Systematic paleontology

Description

Four isolated cervical vertebrae, including an axis, have been prepared. The centra are relatively short, with an average elongation index (aEI: ratio of centrum length to the average of posterior centrum height and width) of about 1.5−2.1 (Table 1). This ratio is similar in Shunosaurus (2.0−3.0) from the Middle Jurassic of Sichuan Basin (Zhang, 1988), Tazoudasaurus (1.6) from the Early Jurassic of Morocco, Barapasaurus (about 2) from the Early Jurassic of India (Allain & Aquesbi, 2008; Bandyopadhyay et al., 2010), and Patagosaurus (1−1.7) from the Middle Jurassic of Argentina (Holwerda, Rauhut & Pol, 2021), but larger in Cetiosaurus (2.3−2.7), Bagualia (3.8−5.3) (Pol et al., 2020; Gomez, Jose & Pol, 2021; Holwerda, Rauhut & Pol, 2021), and mamenchisaurids such as Analong (Ren et al., 2021) and Omeisaurus tianfuensis (1.9−6.1) (He, Li & Cai, 1988). The lateral surfaces of the three postaxial cervical centra bear pleurocoels that are partitioned by anterodorsally-trending ridges, as in some cervical vertebrae of Patagosaurus (Holwerda, Rauhut & Pol, 2021), mamenchisaurids and neosauropods (Wilson, 2002). A ventral midline keel is present on the anterior part of each centrum as in the cervical vertebrae of many early-diverging sauropods, such as Shunosaurus (Zhang, 1988), Tazoudasaurus (Allain & Aquesbi, 2008), Omeisaurus (He, Li & Cai, 1988), Patagosaurus (Holwerda, Rauhut & Pol, 2021), Bagualia (Gomez, Jose & Pol, 2021), Spinophorosaurus (Remes et al., 2009), Lapparentosaurus (Upchurch, 1998), Amygdalodon (Rauhut, 2003) and an unnamed sauropod from Morocco (Nicholl, Mannion & Barrett, 2018), as well as in the anteriormost cervical vertebrae of the Rutland Cetiosaurus (Upchurch & Martin, 2002), and also dicraeosaurids such as Lingwulong (Xu et al., 2018). This feature is also present in some non-sauropod sauropodomorphs, including Yizhousaurus (Zhang et al., 2018), Massospondylus (Barrett et al., 2019), Isanosaurus (Buffetaut et al., 2000), and potentially Antetonitrus (McPhee et al., 2014) and Lamplughsaura (Kutty et al., 2007).

Axis. The axis (CGS V001-1) is nearly complete, and is well preserved (Fig. 2), with a length of 12.6 cm. The anterior surface of the axis is rugose, bears a pair of dorsoventral grooves, and is tilted to face somewhat dorsally (Fig. 2C). The odontoid process is not preserved. The ventral part of the anterior surface of the centrum contacts, and is fused with, the axial intercentrum (ic) (Fig. 2A). The latter is a small, irregular bone with a crescentic outline in anterior view (Fig. 2C). The ventral surface is smooth and curved dorsally.

The centrum of the axis is relatively elongate (aEI value of 1.5), and transversely compressed. In anterior view, the centrum is taller than wide (Fig. 2C). The posterior surface of the centrum is strongly concave, accommodating the anterior condyle of the third cervical centrum, and has a subcircular outline with equal width and height. The lateral surface of the centrum bears a shallow, elongate fossa with poorly defined margins and no external pneumatic openings, as in the early-diverging eusauropods Shunosaurus (Zhang, 1988) and Bagualia (Gomez, Jose & Pol, 2021), whereas the corresponding fossa is deeper in more derived sauropods such as Omeisaurus (He, Li & Cai, 1988) and Euhelopus (He, Li & Cai, 1988; Wilson & Upchurch, 2009). The fossa is deepest at the anterior end, and gradually becomes shallower posteriorly. The fossa is undivided, as in Shunosaurus (Zhang, 1988), Cetiosaurus (Upchurch & Martin, 2002), Bagualia (Gomez, Jose & Pol, 2021), Mamenchisaurus hochuanensis (Young & Zhao, 1972) and Xinjiangtitan (Zhang et al., 2020), whereas the lateral fossa on the axis is partitioned by a ridge in Omeisaurus (He, Li & Cai, 1988) and more derived sauropods. The parapophysis, positioned on the anterior margin of the centrum, is a weakly developed structure that takes the form of a convex ridge (Figs. 2A and 2C).

The posterior half of the ventral surface has a gentle transverse convexity. The anterior half of the centrum narrows ventrally but does not form a sharp midline keel of the kind seen in some non-sauropod sauropodomorphs (e.g., Yizhousaurus (Zhang et al., 2018)) and such early diverging sauropods as Shunosaurus (Zhang, 1988), Tazoudasaurus (Allain & Aquesbi, 2008) and Bagualia (Gomez, Jose & Pol, 2021). The ventral surface of the axis is flat and lacks a midline keel in Barapasaurus (Bandyopadhyay et al., 2010) and Mamenchisaurus hochuanensis (Young & Zhao, 1972). In the mamenchisaurid Xinjiangtitan, the anterior part of the ventral surface of the axial centrum lacks a keel but bears paired fossae whose outer margins are defined by ventrolateral ridges (Zhang et al., 2020).

The neural arch is well developed, but less dorsoventrally tall than the centrum in the mid-region of the vertebra. The part of the neural arch anterior to the apex of the neural spine is taller than the part posterior to the apex (Figs. 2C, 2F). Both diapophyses are largely broken away, but the bases of these structures are anteroposteriorly elongate and located anteroventrally on the neural arch, just above the neurocentral suture (Fig. 2B). No posterior centrodiapophyseal lamina (pcdl) is observable. The anterior opening of the neural canal is large, and taller than wide (Fig. 2C), whereas the posterior opening is relatively small and subcircular, with equal width and height (Fig. 2F; Table 1).

The prezygapophyses are not preserved. The postzygapophyses are large, and extend posterolaterally beyond the posterior part of the centrum. The postzygodiapophyseal lamina (podl) forms a weak ridge extending posterodorsally at an angle of about 30° above the horizontal (Figs. 2A, 2B). A large spinopostzygapophyseal fossa (spof) is present between the postzygapophyses (Fig. 2F), as in Xinjiangtitan (Zhang et al., 2020). The postzygapophyseal articular facets face ventrally, and are elliptical in outline. The long axis of each facet diverges at 45° from that of the centrum. A prominent epipophysis is clearly present on the dorsal surface of the postzygapophysis (Figs. 2A, 2B, and 2F), as in Bagualia (Gomez, Jose & Pol, 2021) and Xinjiangtitan (Zhang et al., 2020). The epipophysis is essentially a dorsal extension of the postzygapophysis, but is separated from the main dorsal surface of the latter by a shallow groove.

The neural spine is weakly developed. The anterior part of the spine is transversely narrow, but a robust laterally projecting ridge extends along the dorsal margin of this portion of the spine and is prominent enough to slightly overhang a deep, distinct fossa (spinodiapophyseal fossa, sdf) situated on the spine’s lateral surface, as in the mamenchisaurids Mamenchisaurus hochuanensis (Young & Zhao, 1972) and Xinjiangtitan shanshanensis (Zhang et al., 2020). The fossa is slightly deeper than that of Bagualia (Gomez, Jose & Pol, 2021), whereas in the Rutland Cetiosaurus and Tazoudasaurus the lateral surface of the neural spine is flattened or convex (Upchurch & Martin, 2002; Allain & Aquesbi, 2008). The height of the neural spine gradually increases posteriorly, reaching a maximum slightly posterior to the midpoint of the centrum as in Shunosaurus (Zhang, 1988) and Mamenchisaurus hochuanensis (Young & Zhao, 1972). In the Rutland Cetiosaurus and Tazoudasaurus, by contrast, the apex of the neural spine is near the posterior margin of the centrum (Upchurch & Martin, 2002; Allain & Aquesbi, 2008). The spinopostzygapophyseal lamina (spol) is straight and robust, and trends ventrolaterally.

Postaxial cervical vertebrae. A nearly complete cervical vertebra (CGS V001-2) is well preserved, except that the posterior portion and the left half of the anterior portion of the neural arch are missing, and the left half of the anterior portion of the centrum has likewise been broken away (Fig. 3). The cervical centrum is strongly opisthocoelous, with a prominent hemispherical anterior condyle. The centrum is about 196 mm long, and the ratio of centrum length to posterior centrum height is about 1.9. The posterior articular surface is tilted to face partly ventrally, rather than being perpendicular to the long axis of the centrum. The parapophyses are missing, owing to damage to the anteroventral part of the centrum. The relatively modest height of the preserved neural arch, and the small size of the centrum as a whole, suggest that this vertebra may be from the anterior part of the cervical series.

The lateral surface of the centrum is strongly excavated by a long depression. A thin, sharp, anterodorsally-trending septum (plr) divides the depressed area into two deep pleurocoels (Figs. 3A and 3B). The anterior pleurocoel extends into the centrum in all directions, except posteriorly. The external opening of the anterior pleurocoel is elliptical and anteroposteriorly elongate, whereas the posterior pleurocoel is relatively shallow and subcircular. The ventral surface of the centrum is strongly concave anteroposteriorly, and slightly concave transversely. The ventral surface bears two shallow fossae anteriorly, and a shallow midline keel along its full length.

On the right side of the neural arch, the distal end of the diapophysis is missing, but the basal part of the diapophysis is flattened dorsoventrally, with a thick mid-region and thin anterior and posterior margins. The diapophysis projects laterally and slightly ventrally, and is supported by a well-developed anterior centrodiapophyseal lamina (acdl), which is stout and oriented posterodorsally (Fig. 3A). The partially preserved prezygodiapophyseal lamina (prdl) extends anterodorsally from the diapophysis to the ventrolateral surface of the prezygapophysis (Fig. 3D). The dorsoventrally compressed basal part of the postzygodiapophyseal lamina (podl) is preserved, and extends posterodorsally (Fig. 3D).

The prezygapophyses are broken away, but a stout, vertically aligned centroprezygapophyseal lamina (cprl) is preserved on the right side of the neural arch (Fig. 3E). An intraprezygapophyseal lamina (tprl) extends ventromedially from the prezygapophysis towards the middle of the dorsal edge of the neural canal. The centroprezygapophyseal lamina (cprl) and intraprezygapophyseal lamina (tprl) come together dorsally to define a large, deep fossa. The prezygapophyseal centrodiapophyseal fossa (prcdf) is elliptical, and is situated between the prezygodiapophyseal lamina (prdl) and the anterior centrodiapophyseal lamina (acdl) on the lateral side of the neural arch (Fig. 3A). The postzygapophyses are not preserved, and most of the neural spine is likewise missing. The anterior side of the base of the neural spine is incised by a deep, wide vertical groove (Fig. 3E). A robust spinoprezygapophyseal lamina (sprl) is preserved on the right side, and extends posteriorly, medially and slightly dorsally from the prezygapophyseal area to the neural spine (Fig. 3D).

A second postaxial cervical vertebra is well preserved, but the anterior part, and much of the right side, of the centrum are missing (CGS V001-3, Fig. 4). This vertebra is relatively large and has a well-developed neural spine, suggesting that it may be from the mid-cervical region. The left lateral surface is excavated by a shallow, elliptical, anteroposteriorly elongate pleurocoel (Fig. 4B). A stout, anterodorsally oriented ridge (plr) forms the pleurocoel’s anterior margin. A second pleurocoel was probably originally present anterior to this ridge, as in mamenchisaurid and neosauropod cervical vertebrae. Based on the position of the ridge in typical cervical vertebrae, in fact, it is likely that the anterior half of the centrum is missing. The ventral part of the right half of the centrum is similarly broken away to expose the centrum’s internal structure. The large, anteroposteriorly elongate pleurocoel (Fig. 4A) resembles that of the early-diverging macronarian Camarasaurus (Wedel, 2003), but lacks the camellate internal cavities that occur in derived titanosaurs. The preserved part of the ventral surface is strongly concave both transversely and anteroposteriorly. The preserved part of the centrum lacks a prominent ventral keel, but a ventral keel may have been present more anteriorly, as in the cervical vertebrae of other early-diverging sauropods and some massopodans.

On the left side of the neural arch, the diapophysis is well preserved, has a subtriangular outline in dorsal view, and tapers ventrolaterally (Fig. 4B). The dorsal surface of the diapophysis is flattened. The prezygodiapophyseal lamina (prdl) is partially preserved as a sheet of bone arising from the base of the anterior edge of the diapophysis, with a thin edge that extends anterodorsally (Fig. 4B). The postzygodiapophyseal lamina (podl) extends from the base of the diapophysis to the lateral margin of the postzygapophysis, forming an angle of about 40° with the long axis of the centrum (Fig. 4B). A prominent, tapering process protrudes posteroventrally from the base of the diapophysis (Fig. 4B: ppr), resembling the costal spurs present in the neosauropod Euhelopus zdanskyi. However, the costal spurs of Euhelopus are less prominent and more distally located (Wilson & Upchurch, 2009).

Both the pre- and postzygapophyses are missing. The spinoprezygapophyseal lamina (sprl) is sharp, its margin curving posterodorsally from the prezygapophyseal area to merge with the anterior edge of the neural spine (Fig. 4B). The neural spine is well preserved, subrectangular in outline in lateral view, and transversely compressed. The anterior neural spine groove is present and transversely narrow. In posterior view, the spinopostzygapophyseal fossa (spof) is deep and tall (Fig. 4C). The neural canal is subcircular and much smaller than the posterior surface of the centrum.

A third postaxial cervical vertebra can be recognized as a posterior member of the cervical series, based on the relative shortness of the centrum and pleurocoel (ratio of centrum length to posterior centrum height of only about 1.68) (CGS V001-4, Fig. 5). The centrum is strongly opisthocoelous, with a prominent hemispherical anterior condyle. The lateral surface of the centrum is strongly excavated by a deep, elliptical pleurocoel (Fig. 5A). The right parapophysis is broken away, but the left parapophysis is located on the anteroventral region of the centrum and tapers laterally, having a triangular outline in lateral and anterior views (Fig. 5B).

The ventral surface is strongly concave anteroposteriorly, the apex of the concavity being located in the anterior half of the centrum. A strong ventral midline keel is present, and extends along the entire length of the centrum. The midline keel is sharp and deep anteriorly, and progressively becomes wider and less prominent towards the centrum’s posterior end (Fig. 5D). The centroprezygapophyseal lamina (cprl) is a simple stout ridge, extending anterodorsally from the diapophysis to support the prezygapophysis (Fig. 5C).

The left prezygapophysis is well preserved, with a facet that faces dorsomedially and is teardrop-shaped, tapering posteromedially to a point. The articular surface is flattened. A large, shallow fossa is present on the underside of the prezygapophysis (Fig. 5C). The spinoprezygapophyseal lamina (sprl) forms a prominent ridge extending posterodorsally from the prezygapophysis (Fig. 5A). The diapophysis and the posterior part of the prezygodiapophyseal lamina (prdl) are broken away. The anterior part of the prezygodiapophyseal lamina (prdl) is preserved as a stout ridge, whereas the postzygodiapophyseal lamina (podl) appears thinner and more sheet-like, based on the preserved basal part of the latter (Figs. 5B, 5E). The anterior centrodiapophyseal lamina (acdl) is thick and extends posterodorsally, and the posterior centrodiapophyseal lamina (pcdl) extends anterodorsally. Together with the dorsal lamina on the centrum (dlc), they define a deep fossa situated ventral to the diapophysis and visible in lateral view (Fig. 5B) as in the posterior cervical vertebrae of Europasaurus (Carballido & Sander, 2014). The prezygapophyseal centrodiapophyseal fossa (prcdf) is deep and narrow anteroposteriorly (Fig. 5B). Both postzygapophyses are missing, as is the neural spine. The lateral centropostzygapophyseal lamina (lcpol) is robust and vertically directed.

Scapula. The left scapula is nearly complete, lacking only small portions of the proximal plate and distal expansion (CGS V001-5, Fig. 6), and is flat and elongate. The lateral and medial surfaces of the proximal plate are both shallowly excavated, but the acromial ridge that is present in most neosauropods (Upchurch, Barrett & Dodson, 2004) is lacking. The dorsoventral height of the strongly expanded proximal plate is estimated to be more than 50% of the total length of the scapula (about 0.6), as in mamenchisaurids and more advanced sauropods (Upchurch, Barrett & Dodson, 2004). The acromial process is moderately developed and its posterior margin is slightly convex, which is similar to the condition in Lingwulong (Xu et al., 2018), Lapparentosaurus, the Rutland Cetiosaurus and Patagosaurus (Upchurch & Martin, 2002; Holwerda, 2019). Comparatively, the acromial process is better developed in mamenchisaurids (Fig. 7), but poorly developed in Shunosaurus (Zhang, 1988). The long, anteroventrally protruding glenoid region is transversely thick. The glenoid region is rectangular in lateral view, and bears a slightly rugose articular surface. The lateral and medial surfaces of the scapular blade are both convex, creating a lenticular cross section, although the convexity of the lateral surface is more pronounced. The blade is slightly deflected medially, relative to the proximal plate. The distal end of the blade is strongly expanded dorsoventrally, though the dorsal part of the expanded area is slightly damaged. The distal end of the scapular blade is also strongly expanded in Omeisaurus, Mamenchisaurus, Yuanmousaurus (Lu et al., 2006), the Rutland Cetiosaurus (Upchurch & Martin, 2002), and Patagosaurus (Holwerda, 2019), but only slightly expanded in the early-diverging sauropods Shunosaurus (Zhang, 1988) and Barapasaurus (Bandyopadhyay et al., 2010), and in the diplodocoid Lingwulong (Fig. 7).