Over the past two decades, numerous somphospondyl sauropod taxa have been described from the Early-Middle Cretaceous of East Asia (Dong et al. 2001; Mannion et al. 2019), augmenting the previously described taxa Euhelopus zdanskyi (Wiman, 1929), Asiatosaurus mongoliensis Osborn, 1924, Mongolosaurus haplodon Gilmore, 1933, and Asiatosaurus kwangshiensis Hou, Yeh, and Zhao, 1975, in terms of illuminating somphospondyl evolution in the Far East during the first 45 million years of the Cretaceous. However, the fossil record of East Asian titanosaurian taxa from the Berriasian-Albian has for the most part remained patchy and sparse compared to the multitude of euhelopodids from the region, and this is one special reason why titanosaur interrelationships are still a work in progress despite advances towards understanding the cladistic diversity of lithostrotian titanosaurs. Wang et al. (2021) have recently described two new somphospondylian taxa from the Early Cretaceous (early Barremian-early Aptian) Shengkinkou Formation of Xinjiang, the euhelopodid Silutitan sinensis and the titanosaur Hamititan tianshanensis, constituting one of the first instances of euhelopodids and titanosaurs being coeval in the Berriasian-Barremian of East Asia. Hence, it should be worth discussing the implications of Hamititan for the macroevolution of titanosaurs in East Asia during the early to middle Cretaceous.
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| Cladogram (using Mannion et al. 2019 dataset) showing the positions of Hamititan and Silutitan (after Wang et al. 2021) |
To begin with, the cladistic analyses conducted by Wang et al. for Hamititan and Silutitan, especially with respect to the datasets from Mannion et al. (2019), offer some new insights into somphospondyl evolution in East Asia during the Early Cretaceous. The most noteworthy aspect of the strict consensus cladogram in figure 8 where Hamititan and Silutitan are treated as distinct operational taxonomic units (OTUs) is the recovery of Hamititan as a derived lithostrotian closely related to Aeolosaurus and Rapetosaurus, more derived than Nemegtosaurus, Tapuiasaurus, and Xianshanosaurus. Regardless of a spree of cladistic studies by Poropat et al. (2015), Wilson et al. (2016), and Carballido et al. (2017) whereby Nemegtosauridae sensu Zaher et al. (2011) is recovered as paraphyletic/polyphyletic in part due to the widespread distribution of putative cranial synapomophies of Nemegtosauridae among different lithostrotian lineages, the cladistic composition of derived Lithostrotia as per Wang et al. with a Nemegtosaurus+Tapuiasaurus+Xianshanosaurus sister to a larger clade formed by titanosaur taxa currently assigned to Saltasauridae and Aeolosaurini would indicate that derived titanosaurs were widespread in Eurasia by the Hauterivan-early Aptian, as indicated by the colossosaurian affinities of Tengrisaurus and Volgatitan from the Hauterivian and Barremian-early Aptian of Russia respectively (Averianov and Efimov 2018; Averianov et al. 2021) and the lithostrotian nature of NHMUK R5333, a specimen comprising a few caudal vertebrae from the Barremian-age Wessex Formation of the Isle of Wight in southern England (Upchurch et al. 2011). Given that Ninjatitan hails from the Berriasian-Valanginian Bajada Colorada Formation of Argentina and indicates a probable Gondwanan origin for Titanosauria (Gallina et al, 2021), it is possible that derived titanosaur clades spread from Gondwana into Eurasia by the Hauterivan-Barremian because Hamititan has strongly procoelous middle caudal vertebrae as in all derived titanosaurs. However, the placement of Hamititan as sister to Aeolosaurus and Rapetosaurus creates a ghost lineage within Lithostrotia, as Hamititan is much older than either Aeolosaurus or Rapetosaurus. It is therefore possible that future study could find Hamititan to occupy a position more basal than Aeolosaurus, Isisaurus, Nemegtosaurus, Rapetosaurus, or Xianshanosaurus because Navarro (2019) classifies Tapuiasaurus and Yongjinglong with an unnamed taxon from the Adamantina Formation and (possibly) Gobititan in a new lithostrotian clade sister to Isisaurus and Rapetosaurus.
When taking into account the phylogenetic results obtained by Wang et al. for Titanosauria, it should be noted that Andesaurus, traditionally recovered as the basalmost titanosaur, clusters with the East Asian taxa Abdarainurus and Huabeisaurus, suggesting a potential Gondwanan origin for titanosaurs with opisthocoelous and slightly procoelous anterior caudals. Since slightly procoelous anterior caudal vertebrae have been reported in Andesaurus (Mannion and Calvo 2011) and Ninjatitan (Gallina et al. 2021), it is possible that the distinctive caudal morphology of Abdarainurus noted by Averianov and Lopatin (2020) could be reflective of evolutionary reversals in the caudal morphology of some basal titanosaurs due to the strongly opisthocoelous caudals of Abdarainurus compared to slight opisthocoely in some caudal vertebrae of Huabeisaurus and Sonidosaurus (D'Emic et al. 2013; Xu et al. 2006). Of further importance is the fact that the cladistic analysis of Hamititan and Silutitan agrees with Mannion et al. (2019) and Poropat et al. (2021) in recovering Diamantinasaurus and Savannasaurus outside Lithostrotia, because Baotianmansaurus, Dongyangosaurus, Huabeisaurus and Savannasaurus have amphicoelous anterior and middle caudals (Mannion et al. 2019; Poropat et al. 2021), indicating that more than one branch of non-lithostrotian titanosaurs with non-procoelous anterior and middle caudal vertebrae evolved in East Asia by the Aptian-Albian. As noted by Wang et al., even though the taxa Daxiatitan and Dongbeititan have strongly procoelous caudal vertebrae, they differ from Hamititan in lacking ventrolateral ridges on the caudal centra. The lithostrotian nature of the titanosaur specimen NHMUK R5333 from the Isle of Wight and the presence of both euhelopodids and lithostrotians from the Barremian-early Aptian of East Asia should provide context for understanding the macroevolution of somphospondyls in Laurasia during the Berriasian-Albian interval, considering that Malawisaurus from Malawi and Normanniasaurus from northern France possess procoelous anterior caudal vertebrae and amphicoelous middle caudal vertebrae (Gomani 2005; Le Loeuff et al. 2013), and some European titanosaurs from the Early-Middle Cretaceous apparently had Gondwanan origins (Mocho et al. 2019).
Although knowledge of titanosaurs from the earliest Cretaceous is still a work in progress, the recovery of Hamititan as a derived lithostrotian in contrast to other Asian titanosaurs from the Berriasian-Albian interval demonstrates that almost every titanosaur clade was present in East Asia by the Barremian-Aptian, being co-eval with Silutitan and other euhelopodids. Considering that Andesaurus clusters with Abdarainurus and Huabeisaurus at the base of Titanosauria and has slightly procoelous caudals in contrast to the latter two, but also the amphicoely in the caudals of Savannasaurus, titanosaurs with procoelous caudal vertebrae may have spread to East Asia from South America and Africa during the Barremian-Aptian, beginning a gradual process of displacing non-titanosaur somphospondyls and titanosaurs with amphicoelous caudal vertebrae. As more little-known Asian somphospondyl taxa are included in phylogenetic studies, a better understanding of somphospondyl macroevolution in East Asia will emerge.
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