Did early titanosaurs have non-procoelous anterior caudals and broad-crowned teeth? My thoughts on Poropat et al. 2023 paper

I remember that the description of  Rapetosaurus from Madagascar in the early 2000s demonstrated that the supposed late-surviving diplodocoids Nemegtosaurus and Quaesitosaurus are derived titanosaurs, the morphological similarities of the skulls of these taxa to diplodocids stemming from convergent evolution. Ironically, very few non-lithostrotian somphospondyl taxa preserve cranial remains, namely Diamantinasaurus, Euhelopus, Liaoningotitan, Ligabuesaurus, Mongolosaurus,  Phuwiangosaurus, Sarmientosaurus, and Tambatitanis, and the dearth of cranial remains for both basal titanosaurs and basal somphospondyls has been an obstacle to better understanding the ancestral cranial Bauplan of the most primitive titanosaurs, although Sarmientosaurus has a skull morphologically intermediate between that of basal titanosauriforms and derived titanosaurs. Recently, Poropat et al. (2023) have described a new specimen of Diamantinasaurus matildae (recovered by Poropat et al. [2021] as the sister taxon of Sarmientosaurus and Savannasaurus in the new titanosaur clade Diamantinasauria) preserving a nearly complete skull, AODF 906, illuminating aspects of the skull anatomy of this genus not preserved in referred Diamantinasaurus matildae specimen AODF 836 (described in detail by Poropat et al. 2021). Since AODF 906 reveals that Diamantinasaurus had a Sarmientosaurus-like skull and non-procoelous anterior caudal vertebrae, I have an opportunity to investigate whether the most primitive titanosaurs had broad, spatulate teeth and non-procoelous anterior caudals.

Equal-weights cladistic analysis of Diamantinasaurus matildae based on new morphological information from referred specimen AODF 906 (after Poropat et al. 2023). The figure drawings of Diamantinasaurus and Sarmientosaurus in the cladogram highlight the greater height of the rear skull relative to the snout distinguishing diamantinasaurians from lithrostrotian taxa for which skull material is known.  

In their paper erecting the name Diamantinasauria for the non-lithostrotian titanosaur clade formed by Diamantinasaurus, Sarmientosaurus, and Savannasaurus, Poropat et al. (2021) listed amphicoelous anterior caudal vertebrae as one of the distinguishing features of Diamantinasauria, but stressed that amphicoelous anterior caudals could only be assessed in Savannasaurus in contrast to the holotypes of Diamantinasaurus matildae and Sarmientosaurus musacchioi as well as referred D. matildae specimen AODF 836 lacking caudal remains even though Andesaurus has been consistently recovered as a basal titanosaur, raising the question of whether the presence of slight procoely in the anterior caudals of Andesaurus represents an independent morphological acquisition from lithostrotian titanosaurs. As Carballido et al. (2022) point out, Andesaurus has been consistently recovered as a basal titanosaur in nearly all titanosaur phylogenies, but its phylogenetic instability in some recent cladistic studies renders its role as a phylogenetic exemplar for basal Titanosauria labile, and it should be noted that the holotype of Andesaurus delgadoi is incomplete, which raises the possibility that future discoveries could render Andesaurus less basal within Titanosauria but still outside Lithostrotia. Indeed, the cladistic analysis of the titanosaur Ruixinia by Mo et al. (2023) places Andesaurus in a basal titanosaur clade that also includes Dongyangosaurus, Huabeisaurus, and Tambatitanis in a basal clade of Titan, and because Baotianmansaurus, Dongyangosaurus, Huabeisaurus and Tambatitanis share with Diamantinasaurus and Savannasaurus the presence of amphicoelous anterior caudals, it is possible that slightly procoelous in the anterior caudals of Andesaurus constitutes an autapomorphy among non-lithostrotian titanosaurs, since Poropat et al. (2023) recover Baotianmansaurus, Dongyangosaurus, and Huabeisaurus basally within Titanosauria outside Lithostrotia, like the clade Diamantinasauria. Although Hamititan from the Early Cretaceous of Xinjiang (Wang et al. 2021) is much older than Andesaurus and Diamantinasaurus, it differs from diamantinasaurians in having strongly procoelous caudal vertebrae, suggesting that some early titanosaurs had strongly procoelous anterior caudals and that others had either amphicoelous or opisthocoelous anterior caudal vertebrae because the anterior caudal vertebrae of Ninjatitan from the earliest Cretaceous of Argentina is slightly procoelous like Andesaurus (Gallina at al. 2021).

As noted by Poropat et al. (2021), the cranial material of the Diamantinasaurus matildae specimen AODF 836 is similar to the holotype of Sarmientosaurus musacchioi in having a quadratojugal with a posterior tongue-like process, a braincase with more than one ossified exit for cranial nerve V, and compressed and conical chisel-shaped teeth, but the new D. matildae specimen described by Poropat et al. (2023) demonstrates that the skull of diamantinasaurian titanosaurs was taller and acutely elevated relative to the snout, and that members of Diamantinasauria have robust dentigerous elements. I earlier mentioned that Euhelopus, Liaoningotitan, Mongolosaurus, Phuwiangosaurus, and Tambatitanis are the only non-lithostrotian somphospondyls besides diamantinasaurians that preserve extensive cranial remains, but the skulls of Diamantinasaurus and Sarmientosaurus are similar in every respect to those of brachiosaurids and differ from Euhelopus in having narrower tooth crowns, although they are not as slender as that of the basal somphospondylan Phuwiangosaurus (for which one referred specimen containing cranial and dental elements is known) or derived titanosaurs (Poropat et al. 2022, 2023). The occurrence of a relatively broad-crowned titanosauriform tooth in Santonian-age deposits in Hungary (Ősi et al. 2017) and the presence of broad-crowned teeth in the lithostrotian titanosaur Ampelosaurus (Le Loeuff 2005), in tandem with the spatulate teeth of Euhelopus and the fairly narrow tooth crowns of Diamantinasaurus, Huabeisaurus, Sarmientosaurus, and Tambatitanis, could indicate that the tooth crowns of the earliest and most primitive titanosaurs were more similar to those of Camarasaurus and Euhelopus, especially considering that the teeth of Diamantinasaurus, Huabeisaurus, Sarmientosaurus, and Tambatitanis are not as slender as those of lithostrotians such as Nemegtosaurus, Quaesitosaurus, Rapetosaurus, and Tapuiasaurus. Although the holotype of the basal somphospondyl Yongjinglong datangi does not preserve any cranial remains, it does include three teeth, all of which are somewhat similar to Euhelopus, and the basal somphospondyl Sibirotitan also had broad tooth crowns (Poropat et al. 2022). Therefore, broad-crowned teeth are most probably symplesiomorphic for the most primitive titanosaurs and a few derived titanosaurs, with conical chisel-shaped teeth and slender pencil-shaped teeth being derived states 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., and Pol, D. (eds.). South American Sauropodomorph Dinosaurs. Record, Diversity and Evolution. Cham, Switzerland: Springer. ISBN 978-3-030-95958-6

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.

Le Loeuff, J., 2005. Osteology of Ampelosaurus atacis (Titanosauria) from Southern France. pp. 115–137. In: Tidwell, V., and Carpenter, K. (eds.). Thunder-Lizards: The Sauropodomorph Dinosaurs. Bloomington: Indiana University Press. ISBN 978-0-253-34542-4.

Mo, J., Ma, F., Yu, Y., and Xu, X., 2023. A new titanosauriform sauropod with an unusual tail from the Lower Cretaceous of northeastern China. Cretaceous Research 144: 105449.  doi:10.1016/j.cretres.2022.105449.

Ősi, A., Csiki-Sava, Z., and Prondvai, E., 2017. A sauropod tooth from the Santonian of Hungary and the European Late Cretaceous 'Sauropod Hiatus.' Scientific Reports 7: 3261. https://doi.org/10.1038/s41598-017-03602-2 

Poropat, S. F., Kundrát, M., Mannion, P. D., Upchurch, P., Tischler, T. R., and Elliott, D. A., 2021. Second specimen of the Late Cretaceous Australian sauropod dinosaur Diamantinasaurus matildae  provides new anatomical information on the skull and neck of early titanosaurs. Zoological Journal of the Linnean Society 192 (2): 610-674. doi:10.1093/zoolinnean/zlaa173 

Poropat, S.F., Frauenfelder, T.G., Mannion, P.D., Rigby, S.L., Pentland, A.H., Sloan, T. and Elliott, D.A., 2022. Sauropod dinosaur teeth from the lower Upper Cretaceous Winton Formation of Queensland, Australia and the global record of early titanosauriforms. Royal Society Open Science 9: 220381.

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   

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.