Ruixinia and convergences in vertebral morphology among East Asian mamenchisaurids and titanosauriforms

The past decade or so has seen a considerable increase in the number of titanosauriform taxa known from the Early Cretaceous (Berriasian-early Aptian) of East Asia (e.g. Wang et al. 2007; You et al. 2008; Azuma and Shibata 2010; Zhou et al. 2018; Wang et al. 2021), shedding light on the evolution of both titanosaurs and non-titanosaurian titanosauriforms in this region during the Berriasian-early Aptian. However, most of those taxa are based on fragmentary and/or incomplete holotypes, in some cases preserving minimal vertebral material, and although D'Emic (2012) and Mannion et al. (2019) recovered Euhelopus and some non-titanosaur somphospondyls from the Berriasian to Aptian of East Asia in a monophyletic Euhelopodidae, the recovery of Daxiatitan, Dongbetitan, Euhelopus, and Xianshanosaurus outside Titanosauriformes in some cladistic analyses of Klamelisaurus by Moore et al. (2020) combined with the fact that no caudals are preserved for Euhelopus raise questions about the degree of the convergence of cervical and caudal vertebral characters among different lineages of eusauropods which have been found in East Asia. Mo et al. (2023) have recently described a new titanosaur from the Yixian Formation of Liaoning in northeastern China, Ruixinia zhangi, on the basis of a partial articulated postcranial skeleton, constituting the first East Asian titanosauriform of the Early Cretaceous to preserve a complete caudal series. Thus, a discussion of the implications of Ruixinia for the degree of convergent evolution in the morphology of the cervical and caudal vertebrae among East Asian eusauropods found in strata from the Aalenian-Albian interval is warranted.

Elements of the holotype of Ruixinia zhangi (ELDM EL-J009) (from Mo et al. 2023)

As I have noted elsewhere, D'Emic (2012) considered bifurcated cervical neural spines and the presence of thick, subhorizontal epipophyseal–prezygapophyseal laminae on the cervical vertebrae as diagnostic for Euhelopodidae, but those characters are either homoplastic among derived eusauropods or are not exclusively shared by the taxa included by D'Emic (2012) in Euhelopodidae. When describing the cervical vertebrae of Ruixinia, Mo et al. emphasize that the preservation of the cervicals in ELDM EL-J009 make it uncertain whether the neural spines of the middle and posterior cervicals for this taxon are bifurcated, and it should be noted that very few cervical vertebrate are known for Phuwiangosaurus and Tangvayosaurus, making it uncertain if the Southeast Asian taxa assigned to Euhelopodidae by D'Emic (2012) had bifurcated neural spines or not. Also, unpublished cladistic results reported in an abstract by D'Angelo (2022) recover Erketu as a basal titanosaur closely related to Huabeisaurus, indicating that whether or not Ruixinia had bifurcated cervical neural spines, the presence or absence of bifurcation of the neural spines of the cervical vertebrae varies among basal somphospondyls and some members of  Titanosauria. The occurrence of strongly procoelous anterior caudal vertebrae in Ruixinia that was once used to diagnose derived titanosaurs but is now recognized as being present in mamenchisaurids (e.g. the holotype of Wamweracaudia keranjei was once used to assign Janenschia to Titanosauridae before being recognized as a mamenchisaurid), turiasaurians (Royo-Torres et al. 2017; Mannion et al. 2019), the basal titanosaur Hamititan (Wang et al. 2021) and the basal macronarian Bellusaurus (Jacobs et al. 1993) may suggest that strongly procoelous anterior and middle caudal vertebrae evolved convergently among different clades of derived eusauropods by the Tithonian-Berriasian. Considering that Wang et al. (2021) recover Hamititan as more derived than the basal titanosaur Andesaurus despite the younger age of the latter, it is possible that Hamititan might be closely related to Ruixinia because both taxa possess marked ventrolateral ridges on the second caudal vertebrae (compare Mo et al. 2023, fig. 6 with Wang et al. 2021, fig. 4), and Wang et al. note that the well-marked ventrolateral ridges of the anterior caudals of Hamititan are comparable to those of Xianshanosaurus. The fusion of the last six caudals of Ruixinia also deserves attention because Mo et al. point out that this morphological feature has been previously described from mamenchisaurids and the basal eusauropod Shunosaurus, but since the distal end of the tail of Ruixinia is distinct from Shunosaurus and mamenchisaurids in being rod-shaped, the fusion of the distalmost caudal vertebrae constitutes yet another caudal vertebral feature that evolved convergently in Titanosauria and non-neosauropod eusauropods.

    

Reduced consensus cladistic analysis of Ruixinia zhangi (after Mo et al. 2023).

The cladistic analysis of Ruixinia by Mo et al. provides some very important insights into the degree of convergence in the vertebral morphologies of titanosauriforms and non-neosauropod eusauropods found in East Asia. In this phylogeny, Ruixinia is recovered as a derived titanosaur closely related to the taxa Daxiatitan and Xianshanosaurus, while Dongbeititan is placed as a non-titanosaur somphospondylan, whereas Liaoningotitan is clustered with Baotianmansaurus and Diamantinasaurus in the majority of he most parsimonious trees created by the Ruixinia phylogenetic analysis. It is interesting to note that Daxiatitan, Dongbeititan, and Xianshanosaurus are recovered along with Euhelopus within the "Core Mamenchisaurus-like Taxa" clade of Mamenchisauridae in some phylogenetic analyses by Moore et al. (2020) because Dongbeititan and Euhelopus are both similar to mamenchisaurids in having more than 15 cervical vertebrae, and the anterior and middle caudal vertebrae of Daxiatitan, Dongbeititan, and Ruixinia are strongly procoelous like those of mamenchisaurids and the basal titanosaur Hamititan. Since no cervical vertebrae are preserved for Xianshanosaurus, and the Daxiatitan holotype preserves 10 cervicals, the placement of Dongbeititan outside Titanosauria in contrast to Daxititan, Ruixinia, and Xianshanosaurus being recovered as derived titanosaurs suggests that vertebral characters shared with mamenchisaurids by Daxiatitan, Dongbeititan, Ruixinia, and Xianshanosaurus are best regarded as being convergent because Mo et al. (2023) note morphological differences between Ruixinia and other titanosauriforms found in Liaoning and a few titanosaur taxa have been described as having more than 15 cervical vertebrae. For instance, although Ruixinia and Dongbeititan have a cervical vertebral count exceeding 15 and strongly procoelous anterior caudals, the presence of bifid anterior caudal neural spines in both Ruixinia and mamenchisaurids in contrast to the undivided neural spines of the anterior caudals of Dongbeititan indicates that some early-branching somphospondylan titanosauriform taxa from the Early Cretaceous of East Asia evolved strong procoelous anterior caudals convergent with those of derived titanosaurs. Additionally, the anterior caudals of the basal macronarian Bellusaurus are also strongly procoelous and were used to suggest titanosaurian affinities for this taxon by Jacobs et al. (1993), and the lithostrotian titanosaur Rapetosaurus has a total of 17 cervicals (Rogers 2009), in which case the presence of more than 15 cervical vertebrae occurs not just in mamenchisaurids, Euhelopus,  Dongbeititan, and Ruixinia but also in lithostrotian titanosaurs. Considering that Euhelopus is recovered as a basal macronarian by Dai et al. (2022), and Moore et al. (2020) note that the anterior margin of the neural spine in posterior dorsal vertebrae being level with or posterior to the rear margin of the neural arch is shared by Dongbeititan with some mamenchisaurids, it is highly probable that the anterior margin of the neural spine in posterior dorsal vertebrae being level with or posterior to the rear margin of the neural arch evolved convergently in Ruixinia and some mamenchisaurids because Daxiatitan and Euhelopus lack this character as pointed out by Moore et al. (2020).

In summary, Ruixinia is not only the third titanosauriform to be described from the Early Cretaceous of Liaoning, China, but is also the first titanosaur from the Early Cretaceous of East Asia to preserve a complete caudal vertebral series. Although several morphological features of this taxon are also found in non-titanosauriform taxa, including a high cervical vertebral count and strongly procoelous anterior and middle caudal vertebrae, the titanosaurian placement of Ruixinia indicates that the most primitive derived titanosaurs and non-titanosaurian somphospondylans from East Asia convergently evolved those characters with mamenchisaurids, turiasaurians, basal macronarians, and even some titanosaurs. By displaying an unusual combination of autapomorphic characters of the caudal region, Ruixiana itself promises to shed additional light on the presence or absence of strong procoely of the anterior and middle caudals among derived eusauropods, including somphospondylans and mamenchisaurids.

References:

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