Is Gannansaurus a non-lithostrotian titanosaur?

Until 2008, no titanosauriform remains were described from the latest Cretaceous of southern China, even though non-avian dinosaurs had been recorded from this region of China since the description in 1979 of the derived therizinosaur Nanshiungosaurus brevispinus (which paradoxically was initially classified as a sauropod). Thus, the description of the lithostrotian Qingxiusaurus youjiangensis from the Dashi site in Guangxi Province in southern China by Mo et al. (2008) constituted the first record of a titanosaur from any province of southern China even though the holotype material for this taxon is pretty sparse. A second record of Titanosauriformes from southern China was added when Lü et al. (2013) described a dorsal vertebra and mid-caudal vertebra from the Nanxiong Formation as a new somphospondyl closely related to Euhelopus, Gannansaurus sinensis. Relying on updated state-of-the-art knowledge of macronarian evolution in East Asia during the Cretaceous, I started to express doubts about Gannansaurus being a very close relative of Euhelopus due to its extremely younger age, raising the possibility that morphological similarities of G. sinensis to Euhelopus are convergences. Recently, Mo et al. (2023) describe the new lithostrotian titanosaur Jiangxititan ganzhouensis from the same geologic unit as Gannansaurus sinensis, with the holotype comprising posterior cervical vertebrae, the first four dorsal vertebrae, and five articulated cervical and dorsal ribs. Since Jiangxititan is now the third somphospondyl titanosauriform to be named from Campanian-Maastrichtian deposits in southern China, it should now be possible to investigate the hypothesis raised by me that Gannansaurus is a non-lithostrotian titanosaur rather than a Euhelopus-like form as originally described.

Line drawings of the mid-dorsal vertebra of the holotype of Gannansaurus sinensis (top; after Lü et al. 2013) and the titanosaur specimen PIN 3837/P821 (bottom; after Averianov and Lopatin 2019) showing the "K"-shaped laminae pattern on the middle dorsal vertebrae initially thought to indicate affinities between Euhelopus and Gannansaurus, but now known to be also present in titanosaurs. 

As noted by Lü et al. (2013), the dorsal vertebra of the holotype of Gannansaurus sinensis shares with the mid-dorsal vertebrae of Euhelopus zdanskyi parapophyseal and diapophyseal laminae crossing to form a "K" configuration and the presence of camellate pneumaticity in the internal structure of the centrum is shared by Gannansaurus and Euhelopus with titanosaurs. However, the authors point out that even though the pleurocoel of the dorsal vertebrae is eye-shaped in Euhelopus and Gannansaurus, the pleurocoel in the dorsal vertebra of the Gannansaurus sinensis holotype differs in being inset within a large, round, deep concavity with a sharply bounded dorsal margin. Moreover, the "K" configuration formed by the crossing of the parapophyseal and diapophyseal laminae on the mid-dorsal vertebrae used by Mo et al. to ally Gannansaurus with Euhelopus is also present in the titanosaur specimen PIN 3837/P821 from the Nemegt Formation of Mongolia (Averianov and Lopatin 2019), demonstrating that the "K" laminae pattern convergently evolved in more than one somphospondyl clade and bolstering my opinion that the Maastrichtian age of Gannansaurus makes a titanosaurian placement of this genus far more parsimonous. Although a mid-caudal vertebra is preserved in GMNH F10001, the absence of any caudal vertebrae in the known specimens of Euhelopus makes it impossible to determine if the caudal morphology of Euhelopus was similar to that of Gannansaurus.

Cladistic analysis of Titanosauriformes by Mo et al. (2023) showing Jiangxititan nested within Lognkosauria and Gannansaurus in an polytomy with other somphospondylan titanosauriforms. Although Gannansaurus is found to be an unresolved position in this phylogeny, Andesaurus and Huabeisaurus are also recovered in the polytomy in the above cladogram, so the possibility of Gannansaurus being closely related to Abdarainurus and Huabeisaurus cannot be discounted.

In their paper describing Jiangxititan, Mo et al. (2023) feel confident that Jiangxititan is a different taxon from Gannansaurus based on the phylogenetic positions of these taxa recovered by the authors in the data matrix of the cladistic analysis by Poropat et al. (2023), although they take note of the lack of overlap between the holotypes of Gannansaurus sinensis and Jiangxititan ganzhouensis. The recovery of Gannansaurus in a polytomy with other somphospondyl taxa and the placement of Jiangxititan as the sister taxon of the middle Cretaceous taxon Mongolosaurus within the lithostrotian titanosaur clade Lognkosauria leaves open the possibility that Jiangxititan had a caudal morphology different from that of Gannansaurus. For instance, strongly procoelous anterior caudals are prevalent in lithostrotians and the lithostrotian placement of Jiangxititan would suggest that the anterior caudals of this genus had  strong procoely, and Lü et al. (2013) note that the middle caudal vertebra of Gannansaurus is distinct from that of lithostrotian titanosaurs in having wide, deep ventral fossae with a wide, shallow concavity. Therefore, it is probable that because the "K" laminae pattern on the middle dorsal vertebrae used by Lü et al. (2013) used to infer affinities between Euhelopus and Gannansaurus is now known to be present in titanosaurs, Gannansaurus could be a basal titanosaur with either mild procoely or amphicoely on the anterior caudals. In particular, the titanosaur Qinlingosaurus from the Maastrichtian-age Shanyang Formation of Shaanxi Province in north-central China has mildly procoelous anterior caudals (Xue et al. 1996), whereas the basal titanosaur Abdarainurus has anterior and middle caudal vertebrae with opisthocoely and the lithostrotian Sonidosaurus has a slightly opisthocoelous first caudal vertebra (Averianov and Lopatin 2020). Therefore, it is highly conceivable that basal titanosaurs co-existed with lithostrotian titanosaurs in the Nanxiong Formation.

In summary, the initial interpretation of Gannansaurus as a relative of Euhelopus by Lü et al. (2013) based on the "K"-shaped laminae configuration formed by the crossing of the parapophyseal and diapophyseal laminae on the mid-dorsal vertebrae is untenable because this morphological pattern is also present in lithostrotian titanosaurs. In this way, due to its much younger age compared to that of Euhelopus, Gannansaurus can be best considered a basal titanosaur because morphological features of the sole middle caudal vertebra known for this taxon discriminate it from those seen in the middle caudals of lithostrotian titanosaurs. Despite the absence of overlapping material for Gannansaurus and Jiangxititan, the presumed basal placement of Gannansaurus within Titanosauria and lognkosaurian affinities for Jiangxititan indicated by the phylogenetic analysis of Mo et al. (2023) give me no reason to rule out the existence of more than titanosaur clade in southern China during the Maastrichtian because the Nanxiong Formation has yielded a multitude of oviraptorid taxa belonging to more than one clade of Oviraptoridae. Hopefully additional material anatomically overlapping with the holotypes of Gannansaurus sinensis and Jiangxititan ganzhouensis will be discovered and described that could shed additional light on some critical anatomical regions for these taxa, including the caudal vertebrae.

References:

Averianov, A.O., and Lopatin, A.V., 2019. Sauropod diversity in the Upper Cretaceous Nemegt Formation of Mongolia— a possible new specimen of Nemegtosaurus. Acta Palaeontologica Polonica 64 (2): 313–321.

Averianov, A.O., and Lopatin, A.V., 2020. An unusual new sauropod dinosaur from the Late Cretaceous of Mongolia. Journal of Systematic Palaeontology 18 (12): 1009–1032.  doi:10.1080/14772019.2020.1716402.

Lü, J.C., Yi, L.P., Zhong, H., and Wei, X.F., 2013. A new Somphospondylan sauropod (Dinosauria, Titanosauriformes) from the Late Cretaceous of Ganzhou, Jiangxi Province of southern China. Acta Geologica Sinica 87 (3): 678–685. doi: 10.1111/1755-6724.12079. 

Mo, J.-Y., Hua, C.-L., Zhao, Z.-R., Wang, W., and Xu, X., 2008. A new titanosaur (Dinosauria: Sauropoda) from the Late Cretaceous of Guangxi, China. Vertebrata Palaeontologica Asiatica 46 (2): 147-156.

Mo, J.-Y., Fu, Q.-Y., Yu, Y.-L., and Xu, X., 2023. A New Titanosaurian Sauropod from the Upper Cretaceous of Jiangxi Province, Southern China. Historical Biology: in press. doi:10.1080/08912963.2023.2259413.

Poropat, S. F., Mannion, P. D., Rigby, S. L., Duncan, R. J., Pentland, A. H., Bevitt, J. J., Sloan, T., and Elliott, D. A., 2023. A nearly complete skull of the sauropod dinosaur Diamantinasaurus matildae from the Upper Cretaceous Winton Formation of Australia and implications for the early evolution of titanosaurs. Royal Society Open Science 10(4): 221618. https://doi.org/10.1098/rsos.221618

Xue, X.X., Zhang, Y.X., Bi, Y., and Chen, D.L., 1996. The development and environmental changes of the intermontane basins in the eastern part of Qinling Mountains. Beijing: Geological Publishing House.