Implications of Averianov et al. (2023) paper for evolution of sacral vertebral count in early titanosaurs

In their paper describing the Diamantinasaurus matildae specimen AODF 0906, Poropat et al. (2023) weighed in on whether the presence of five sacral vertebrae in Diamantinasaurus is plesiomorphic for Titanosauria or an evolutionary reversal in Diamantinasaurus among basal titanosaurs given that sacral material is not known for many early-diverging titanosaurs and a five-vertebrae sacrum is common among basal members of Somphospondyli. Recently, Averianov et al. (2023) have described a new specimen comprising caudal vertebrae (KOKM A) which they refer to the somphospondyl Sibirotitan astrosacralis based on comparisons of this specimen with referred S. astrosacralis caudal vertebra KOKM 26786 from the type locality of this taxon, and they recover Sibirotitan as a non-lithrostrotian titanosaur in spite of the presence of strongly procoelous articulations of the anterior caudal being used as a synapomorphy for Lithostrotia. In light of the cladistic results in Averianov et al. (2023), it should be worth discussing what the five-vertebrae sacrum and strongly procoelous caudals of Sibirotitan mean for interpreting the evolution of the number of sacral vertebrae in early titanosaurs.

Cladistic analysis by Averianov et al. (2023) showing the phylogenetic placement of Sibirotitan as closely related to Lithostrotia within Titanosauria.

As noted by Poropat et al. (2023), the six-vertebrae sacrum common for titanosaurs (especially the most derived clades) is not exclusive to Titanosauria and has also been described for Klamelisaurus, some specimens of Camarasaurus, the neosauropod "Apatosaurus" minimus, and several somphospondyl titanosauriform taxa, while a sacrum with five vertebrae is recognized as the plesiomorphic condition for somphospondyls. Although strong procoely of the anterior caudals of referred Sibirotitan specimens KOKM A and KOKM 26786 is shared with Hamititan (Wang et al. 2021), the sacrum of referred specimen PM TGU 120/8-Sh1-1 has five vertebrae despite the strong procoely of the anterior caudals of Sibirotitan differing from slightly procoelous anterior caudals in Andesaurus and Ninjatitan as well as the amphicoelous nature of the caudal vertebrae of Diamantinasaurus (Gallina et al. 2021; Poropat et al. 2023). Given that Stephen Poropat (pers. comm. to me, Feb. 13, 2024) now doubts that the sacrum of adult Diamantinasaurus had five vertebrae and because Sibirotitan is Barremian in age, the recovery by Averianov et al. (2023) of Sibirotitan as a non-lithostrotian titanosaur closer to Lithostrotia than to any titanosaur with non-procoelous anterior caudals could suggest that irrespective of the condition of the articulations of the anterior caudal vertebrae, a five-vertebra sacrum was probably plesiomorphic for the clade Titanosauria due to homoplasticity of six sacral vertebrae within Macronaria. For instance, a complete sacrum is not preserved for some titanosaur taxa from the Early-Middle Cretaceous of East Asia, but given that Averianov et al. (2023) recover Daxiatitan as sister to Sibirotitan, it is probable that the ancestral morphological condition for basal titanosaur clades comprised not only a five-vertebra sacrum but also non-procoelous anterior caudals, and that anterior caudal articulation morphologies diversified in those clades as the Early Cretaceous progressed. Also, the placement of Huanghetitan as sister to Titanosauria in the phylogenetic analysis by Han et al. (2024) further supports my opinion that in early titanosaurs, the five-vertebra sacrum preceded the acquisition of strongly procoelous anterior caudals because the anterior caudal of the holotype of H. liujiaxiaensis is slightly procoelous.

Given the presence of five sacral vertebrae in Sibirotitan and the non-lithostrotian titanosaur placement of this taxon by Averianov et al. (2023), one question arises: why did titanosaurs eventually evolve more than five sacrals after initially retaining the plesiomorphic five-vertebra sacrum as the Middle and Late Cretaceous progressed? Frankly, an increase in body size cannot account for a slight increase in the number of sacral vertebrae in titanosaurs because the saltasaurid Neuquensaurus is a small-sized genus despite being distinguishable from other lithostrotians in having seven sacral vertebrae (Salgado et al. 2005) and small size evolved in more than one clade of derived titanosaurs (Navarro et al. 2022). Given individual variation in the sacral vertebral count of Camarasaurus by Tidwell et al. (2005), but also the fact that no sacral remains are known for the dwarf saltasaurid Ibirania, it is possible that the emergence of the six-vertebra sacrum as a synapomorphic condition for titanosaurs by the Middle Cretaceous might have had something to do with environment-related paleobiological factors, and that some dwarf titanosaurs from Europe probably had either five or six sacral vertebra.

References:

Averianov, A., Podlesnov, A., Slobodin, D., Skutschas, P., Feofanova, O., and Vladimirova, O., 2023. First sauropod dinosaur remains from the Early Cretaceous Shestakovo 3 locality, Western Siberia, Russia.  Biological Communications 68 (4): 236–252. doi:10.21638/spbu03.2023.404.

Han, F., Yang, L., Lou, F., Sullivan, C., Xu, X., Qiu, W., Liu, H., Yu, J., Wu, R., Ke, Y., Xu, M., Hu, J., and Lu, P., 2024. A new titanosaurian sauropod Gandititan cavocaudatus gen. et sp. nov., from the Late Cretaceous of southern China. Journal of Systematic Palaeontology 22 (1): 2293038. doi: https://doi.org/10.1080/14772019.2023.2293038.

Navarro, B. A., Ghilardi, A.M., Aureliano, T., Díaz, V.D., Bandeira, K.L.N., Cattaruzzi, A.G.S., Iori, F.V., Martine, A.M., Carvalho, A.B., Anelli, L.E., Fernandes, M.A., and Zaher, H., 2022. A new nanoid titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Brazil. Ameghiniana 59 (5): 317–354. doi:10.5710/AMGH.25.08.2022.3477.

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 

Salgado, L., Apesteguía, S., and Heredia, S. 2005. A new specimen of Neuquensaurus australis, a Late Cretaceous saltasaurine titanosaur from North Patagonia. Journal of Vertebrate Paleontology 25: 623634.  

Tidwell, V., Stadtman, K., and Shaw, A., 2005. Age-related characteristics found in a partial pelvis of Camarasaurus; pp. 180-186, In: Tidwell, V., and Carpenter, K. (eds.), Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press: Bloomington, IA.

Wang, X., Bandeira, K. L. N., Qiu, R., Jiang, S., Cheng, X., Ma, Y., and Kellner, A.W.A., 2021. The first dinosaurs from the Early Cretaceous Hami Pterosaur Fauna, China. Scientific Reports 11:14962. doi:10.1038/s41598-021-94273-7.