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.

Bearing of Han et al. (2024) paper on the inclusion of Andesaurus in titanosaur phylogenetic definitions

The Patagonian titanosaur Andesaurus delgadoi has been universally recognized as a basal titanosaur from the time of its description in 1991 because it has anterior caudal vertebrae with slight procoely in marked contrast to the strongly procoelous anterior caudal vertebrae of derived titanosaurs, hence its inclusion in phylogenetic definitions for Titanosauria following Salgado et al. (1997) and Wilson & Upchurch (2003). However, the holotype of A. delgadoi is rather incomplete, comprising only caudal vertebrae, four dorsal vertebrae, a few limb bones, pelvic elements, and rib fragments. Carballido et al. (2022) noted that a few recent cladistic analyses have found Andesaurus to occupy a rather unstable phylogenetic position within Somphospondyli or basal Titanosauria, stressing the need for further testing of the cladistic instability of Andesaurus to see whether a redefinition of Titanosauria is required. Han et al. (2024) have recently described a new titanosaur from the Cenomanian-Turonian of southern China, Gandititan cavocaudatus, recovering this form and Andesaurus in a basal titanosaur clade which also includes Abdarainurus, Baotianmansaurus, Dongyangosaurus, and Huabeisaurus. Given the results of the cladistic analysis by Han et al., it is imperative to discuss the impact of the basal titanosaur placement of Gandititan on continued use of Andesaurus in phylogenetic definitions employed for Titanosauria.

As I have mentioned previously, Ninjatitan not only is currently the oldest titanosaur genus described so far but is also similar to Andesaurus in having slightly procoelous anterior caudal vertebrae, and the phylogenetic results in Wang et al. (2021) indicate that titanosaurs found in East Asia evolved a diverse array of morphologies of the articular surfaces of the anterior caudal vertebrae during the Barremian-Albian interval given that Hamititan has strongly procoelous anterior caudals unlike Andesaurus and Ninjatitan. Although Ninjatitan is recovered within Titanosauria as either a basal form or a member of Lognkosauria in the different topologies obtained by Gallina et al. (2021), a basal position for this genus is most likely because the anterior caudals of this taxon have slight procoely as in Andesaurus and Ninjatitan is far older than known lognkosaurian taxa. However, Han et al. (2024) did not include Ninjatitan in their cladistic analysis despite its Berriasian-Valanginian age because of the paucity of known material for the N. zapatai holotype even though doing so would have tested the phylogenetic placement of Ninjatitan within the basal titanosaur grouping formed by Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, Gandititan, and Huabeisaurus. The strict consensus cladogram in Mannion et al. (2013) applies the name Andesauroidea to the basalmost titanosaur clade that includes Andesaurus, but the cladistic analysis by Han et al. places some of the taxa included in that clade by Mannion et al. outside Titanosauria while keeping Andesaurus and Baotianmansaurus in Titanosauria, so even if Andesaurus is younger than Ninjatitan, a redefinition of Titanosauria to exclude Andesaurus is unwise because both taxa exhibit discrete titanosaur synapomorphies despite being known from sparse axial and appendicular material.

A time-calibrated cladistic analysis of Titanosauria showing Gandititan in a basal titanosaur clade also comprising Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, and Huabeisaurus (after Han et al. 2024)

When compartmentalizing the results of the cladistic analysis by Han et al. (2024) with the tendency by many cladistic analyses to root titanosaur phylogenetic trees with Andesaurus, a number of important things ought to be emphasized when it comes to continuing to include Andesaurus in a phylogenetic definition for Titanosauria. First, the opisthocoelous nature of the caudal vertebrae in Gandititan (which is convergent in the saltasaurid Opisthocoelicauda) differs from the slightly procoelous anterior caudals of Andesaurus and occurs in Abdarainurus, while the caudals of Huabeisaurus, Baotianmansaurus, and Dongyangosaurus are amphicoelous. Abdarainurus and Huabeisaurus are recovered as late-surviving basal titanosaurs by Wang et al. (2021) and Poropat et al. (2023), so it is prudent to surmise that if the clade formed by Abdarainurus, Andesaurus, Baotianmansaurus, Dongyangosaurus, Gandititan, and  Huabeisaurus in the Han et al. (2024) cladistic analysis is supported by future papers, then Andesaurus-like titanosaurs evolved different caudal articulation morphologies and Andesauroidea could be used for this clade. Second, the recovery of the two nominal Huanghetitan species and Diamantinasauria just outside Titanosauria by Han et al. (2024) might further preclude omitting Andesaurus from future phylogenetic definitions of Titanosauria because it is unclear if Andesaurus and Baotianmansaurus had six sacral vertebrae as in Dongyangosaurus, Gandititan, and Huabeisaurus, or if they possessed the five sacral vertebral count noted by Poropat et al. (2023) for Diamantinasaurus and Huanghetitan due to  Andesaurus delgadoi preserving no sacral remains and the Baotianmansaurus henanensis holotype preserving one and a half sacrals. Irrespective of discussion in Poropat et al. (2023) as to whether the sacral vertebral count for Diamantinasaurus places Diamantinasauria just outside Titanosauria or merely reaffirms the position of this clade within Titanosauria by virtue of being plesiomorphic for titanosaurs, the systematic position of Andesaurus inside Titanosauria still appears secure enough for continued inclusion of this genus in a phylogenetic definition for Titanosauria.

References:

Carballido, J.L., Otero, A., Mannion, P.D., Salgado, L., and Moreno, A.P., 2022. Titanosauria: A Critical Reappraisal of Its Systematics and the Relevance of the South American Record. pp. 269-298. In Otero, A.; Carballido, J.L.; Pol, D. (eds.). South American Sauropodomorph Dinosaurs. Record, Diversity and Evolution. Cham, Switzerland: Springer. doi:10.1007/978-3-030-95959-3. 

Gallina, P. A., Canale, J. I., and Carballido, J. L., 2021. The Earliest Known Titanosaur Sauropod Dinosaur. Ameghiniana 58 (1): 35–51. doi:10.5710/AMGH.20.08.2020.3376.

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.

Mannion, P. D., Upchurch P., Barnes R. N., and Mateus O., 2013. Osteology of the Late Jurassic Portuguese sauropod dinosaur Lusotitan atalaiensis (Macronaria) and the evolutionary history of basal titanosauriforms. Zoological Journal of the Linnean Society 168: 98–206.

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., Coria, R.A., & Calvo, J.O. 1997. Evolution of titanosaurid Sauropods. I: Phylogenetic analysis based on the postcranial evidence. Ameghiniana 34: 3-32.

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.

Wilson, J.A., and Upchurch, P. 2003. A revision of Titanosaurus Lydekker (Dinosauria – Sauropoda), the first dinosaur genus with a ‘Gondwanan’ distribution. Journal of Systematic Palaeontology 1(3): 125–160.