Is it time to utilize Antarctosauridae for square-jawed titanosaurs?

The Mesozoic era is replete with instances of distantly related terrestrial tetrapod clades convergently evolving similar morphological traits, exemplifed by the convergence of cranial Bauplan of toothless ostrich mimic dinosaurs and shuvosaurids as well as the similar carnivorous dentitions of both predatory 'rauisuchians' and large-bodied theropods. Although it is well known that the discoveries of Rapetosaurus and Tapuiasaurus led to Nemegtosaurus and Quaesitosaurus being reclassified as titanosaurs and their cranial similarities to those of flagellicaudatan diplodocoids easily recognized as a product of convergent evolution despite some morphological similarities, another instance of convergent evolution between members of both Diplodocoidea and Titanosauria in the cranial region worthy of discussion is a square-shaped mandibular symphysis being long considered diagnostic for Rebbachisauridae until the description of the lithostrotian titanosaur Bonitasaura by Apesteguía (2004) demonstrated that a number of titanosaurs convergently evolved this morphological trait, refuting suggestions that either Antarctosaurus belonged to Diplodocoidea or the jawbone of this taxon was from a rebbachisaurid rather than a titanosaur. Recently, Filippi et al. (2024) have described a new titanosaur with a square-shaped jaw on the basis of complete remains, Inawentu oslatus, from the Santonian-age Bajo de la Carpa Formation in Neuquén Province, Argentina, and their cladistic analysis recovers this form along with Antarctosaurus as part of a previously unrecognized clade also including Rinconsauria and Aeolosaurini. Due to Antarctosaurus being the type genus of the family-group name Antarctosauridae Olshevsky, 1978, I'm pondering the question of whether Antarctosauridae should be used for the titanosaur grouping "Clade A" recovered by Filippi et al. (2024).

Semi-detailed aspects of the osteology of Antarctosaurus wichmannianus have already been given by Powell (2003) and Novas (2009), but a few salient morphological features of A. wichmannianus will be elaborated here to illuminate the history of the higher-level classification of Antarctosaurus. Until the 1970s, there was no consensus on the exact position of titanosaurs within Eusauropoda, with some authors (e.g. von Huene 1929; Stromer 1932; Gilmore 1946) and others (Nopcsa 1928; Tatarinov 1964; White 1973) placing Antarctosaurus and other titanosaur genera in either Cetiosauridae, Diplodocidae, or Morosauridae (=Camarasauridae). Despite classifying Antarctosaurus as a diplodocid, White (1973) hinted that this genus and other Cretaceous peg-toothed sauropods with procoelous anterior caudals should be placed in their own family. Olshevsky (1978) erected the new family Antarctosauridae to include Antarctosaurus and some other titanosaurs, and Powell (1986, 2003) treated Antarctosauridae as a distinct subfamily within Titanosauridae, Antarctosaurinae. However, a number of similarities of the mandible of the Antarctosaurus wichmannianus holotype to the jaws of rebbachisaurids, especially the square-shaped mandibular symphysis and the proportionally small and slender teeth at the front of the jaw, led several authors (e.g. Jacobs et al. 1993; Sereno et al. 1999; Upchurch 1999) to suggest that the either the holotype is a chimera of titanosaur and rebbachisaurid remains or A. wichmannianus is a late-surviving rebbachisaurid. However, Chiappe et al. (2001) noted that Antarctosaurus differs from rebbachisaurids in having a sharp angle between the surface of the wear facet and the longitudinal axis of the tooth as well as an angle of about 90 degrees between the main axes of the mandibular symphysis and mandible. Apesteguía (2004) further refuted suggestions about the A. wichmannianus jaw being a rebbachisaurid by pointing out similarities of the mandible of Antarctosaurus with that of the newly described titanosaur Bonitasaura. Irrespective of Bonitasaura demonstrating that the square-shaped mandibular symphysis in Antarctosaurus evolved convergently with that of rebbachisaurids, Wilson (2005) raised uncertainty over whether the cranial elements of Antarctosaurus wichmannianus are associated with the postcranial remains catalogued under MACN 6904 given that the material was collected from more than one locality.

Phylogenetic analysis of Titanosauria showing the cladistic position of Antarctosaurus, Bonitasaura, and Inawentu (after Filippi et al. 2024). Under ICZN rules, Antarctosauridae hypothetically should be applied to "Clade A" recovered in the phylogeny. 

While the description of Bonitasaura put to rest any suggestions that the jaw belonging to the holotype of Antarctosaurus wichmannianus could have come from a rebbachisaurid, Apesteguía (2004) did not carry out a cladistic analysis to test whether the stark similarities of the jaw of Antarctosaurus with that  of Bonitasaura could translate into a sister relationship between the two taxa within Titanosauria. A paper by Gallina and Apesteguía (2011) detailing the cranial osteology of Bonitasaura carried out the first cladistic analysis of Antarctosaurus, recovering it and Bonitasaura as members of the titanosaur clade Colossosauria. The cladistic analyses of Dongyangosaurus and Jiangshanosaurus by Mannion et al. (2019) also found varying placements for Antarctosaurus within Colossosauria although they did not include Bonitasaura, and thus Santucci and Filippi (2022) assigned Antarctosaurus to Colossosauria incertae sedis. In their cladistic analysis of Inawentu, Filippi et al. (2024) confirm that the squared-shaped mandibular symphysis of Antarctosaurus and Bonitasaura places these taxa as more closely related to each other than to members of Lognkosauria, Rinconsauria, and Aeolosaurini, yet at the same time also find Antarctosaurus, Baalsaurus, Brasilotitan, Bonitasaura, Inawentu, Narambuenatitan, and Uberabatitan to form a distinctive clade within Eutitanosauria, which they term "Clade A".

The cladistic analysis by Filippi et al. (2024) raises a critical question about the interrelationships of non-lithostrotian eutitanosaurs: should be the name Antarctosauridae be utilized for "Clade A" recovered by Filippi et al. (2024)? Since Brasilotitan has the square-shaped jaw of Antarctosaurus,  Baalsaurus, Bonitasaura, and Inawentu despite being more closely related to both Aeolosaurini and Rinconsauria, it could be tempting to use the name Antarctosauridae for the titanosaur clade which Filippi et al. term "Clade A" because Antarctosaurus is the type genus of Antarctosauridae and the cranial material of the Inawentu oslatus holotype (MAU-Pv-LI-595) overlaps with that known for Antarctosaurus, Bonitasaura, Brasilotitan, Muyelensaurus, Narambuenatitan, and Rinconsaurus as well as the holotype specimen of Baalsaurus mansillai. However, no mandibular material is known for colossosaurian titanosaurs which are not recovered within "Clade A" by Filippi et al., and Carballido et al. (2022) note that the varying positions of Aeolosaurini depending on different cladistic analyses conducted over the past decade might result from a lack of a detailed osteology for Aeolosaurus rionegrinus. Therefore, it may be premature for future papers to apply Antarctosauridae to Filippi et al.'s "Clade A"  and phylogenetically define it as "all taxa closer to Antarctosaurus wichmannianus von Huene, 1929 than to Saltasaurus loricatus Bonaparte and Powell, 1980". Nevertheless, Filippi et al. (2024) briefly mention before the end of their paper that cranial material of the Diamantinasaurus matildae specimen described by Poropat et al. (2023) is morphologically distinct from that of the taxa recovered as part of "Clade A" within Eutitanosauria. Given that members of Diamantinasauria have a distinct cranial morphology that that of more derived titanosaurs, the possibility that the square-jawed eutitanosaur clade recovered by Filippi et al. (2024) could hold water in future phylogenies, albeit with a slightly restricted content (Antarctosaurus, Baalsaurus, Brasilotitan, Bonitasaura, and Inawentu), should not be ruled out, in which case Antarctosauridae Olshevsky, 1978 would be applied to this clade in a phylogenetic context. 

Apesteguía, S., 2004. Bonitasaura salgadoi gen. et sp. nov.: a beaked sauropod from the Late Cretaceous of Patagonia. Naturwissenschaften 91(10):493–497.

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. 

Chiappe, L.M., Salgado, L., and Coria, R.A., 2001. Embryonic Skulls of Titanosaur Sauropod Dinosaurs. Science 293: 2444-2446.

Filippi, L.S., Juárez Valieri, R.D., Gallina, P.A., Méndez, A.H., Gianechini, F.A., and Garrido, A.C., 2024. A rebbachisaurid-mimicking titanosaur and evidence of a Late Cretaceous faunal disturbance event in South-West Gondwana. Cretaceous Research 154: 105754. doi:10.1016/j.cretres.2023.105754.

Gallina, P.A. and Apesteguía, S. 2011. Cranial anatomy and phylogenetic position of the titanosaurian sauropod Bonitasaura salgadoi. Acta Palaeontologica Polonica 56 (1): 45–60.

Gilmore, C. W., 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey Professional Paper 210-C: 29-51.

Jacobs, L., Winkler, D.A., Downs, W.R., and Gomani, E.M., 1993. New material of an Early Cretaceous titanosaurid sauropod dinosaur from Malawi. Palaeontology 36:523–534.

Mannion, P. D., Upchurch, P., Jin, X., and Zheng, W., 2019. New information on the Cretaceous sauropods of Zhejiang Province, China: impact on Laurasian titanosauriform phylogeny and biogeography. Royal Society Open Science 6(8):191057. doi:10.1098/rsos.191057.

Nopcsa, F., 1928. Paleontological Notes on Reptiles. Geologica Hungarica, Series Palaeontologica 1  (1): 1-102

Novas, F. E. 2009. The Age of Dinosaurs in South America. Indiana University Press: Bloomington and Indianapolis, IN.

Olshevsky, G., 1978. The archosaurian taxa (excluding the Crocodylia). Mesozoic Meanderings 1(1): 1-50.

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 

Powell, J.E., 1986. Revision de los Titanosauridos de America del Sur. Ph.D. dissertation, Universidad Nacional de Tucuman, Argentina, 493 pp.

Powell, J. E. 2003. Revision of South American titanosaurid dinosaurs: palaeobiological, palaeobiogeographical, and phylogenetic aspects. Records of the Queen Victoria Museum 111: 1-173.

Santucci, R.M., and Filippi, L.S., 2022. Last Titans: Titanosaurs From the Campanian–Maastrichtian Age. pp. 341-391. 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

Sereno, P.C., Beck, A.L., Dutheil, D.B., Larsson, H.C.E, Lyon, G.H., Moussa, B., Sadleir, R.W., Sidor, C.A., Varricchio, D.J., Wilson, G.P., and Wilson, J.A., 1999. Cretaceous sauropods from the Sahara and the uneven rate of skeletal evolution among dinosaurs. Science 286 (5443): 1342–1347.  doi:10.1126/science.286.5443.1342.

Stromer, E., 1932. Ergebnisse der Forschungsreisen Prof. E. Stromers in den Wüsten Ägyptens. II. Wirbeltier-Reste der Baharîjestufe (unterstes Cenoman). 11. Sauropoda. Abhandlungen der Bayerischen Akademie der Wissenschaften, Mathematisch-naturwissenschaftliche Abteilung, Neue Folge 10:1–20.

Tatarinov, L.P., 1964. Nadotryad Dinosauria. Dinozavry [Superorder Dinosauria. Dinosaurs] pp. 523-589. In: Rozhdestvensky, A.K., and Tatarinov, L.P.. (eds.) Fundamentals of Paleontology: Amphibians, Reptiles and Birds. Nauka: Moscow.

Upchurch, P., 1999. The phylogenetic relationships of the Nemegtosauridae (Saurischia, Sauropoda). Journal of Vertebrate Paleontology 19(1):106-125.

von Huene, F., 1929. Los Saurisquios y Ornitisquios del Cretáceo Argentino. Anales del Museo de La Plata 3:1–196.

White, T.E., 1973. Catalogue of the genera of dinosaurs. Annals of the Carnegie Museum. 44: 117–155.

Wilson, J. A., 2005. Redescription of the Mongolian sauropod Nemegtosaurus mongoliensis Nowinski (Dinosauria: Saurischia) and comments on Late Cretaceous sauropod diversity. Journal of Systematic Palaeontology 3(3):283–318.