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.

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.

Tharosaurus and implications for the geographical distribution of neosauropods in the Middle Jurassic

Until the late 2010s there were no named diplodocoid species reported from Middle-Late Jurassic deposits anywhere in Asia despite the cladistic diversity of eusauropods in Asia and Europe during the Middle to Late Jurassic, raising the question of whether or not the supposed absence of diplodocoids from Asia could be a sampling artifact, but the description of the dicraeosaurid Lingwulong from the Middle Jurassic of north-central China in 2018 confirmed my suspicions that East Asia boasted diplodocoids despite the Turgai Strait separating Central and East Asia from Eastern Europe beginning in the Middle Jurassic. Recently, Bajpai et al. (2023) have described a new genus and species of dicraeosaurid from the Middle Jurassic (early-middle Bathonian) Fort Member of the Jaisalmer Formation in western India, Tharosaurus indicus, constituting not only the first diplodocoid from the Indian subcontinent but also the second record of a neosauropod from the Middle Jurassic (the other being an indeterminate basal macronarian from the Bajocian of western India; Moser et al. [2006]). Relying on paleobiogeographic considerations, Bajpai et al. view India as the epicenter of the radiation of Neosauropoda, but issue a caveat regarding the true geographical origin of diplodocoids within Gondwana given the older age of Tharosaurus and the placement of Bajadasaurus, Suuwassea and Lingwulong within Dicraeosauridae. Given enhanced knowledge of the early biogeographic evolution of eusauropods in recent years, I would like to address a number of points raised by Bajpai et al. (2023) regarding the role of Tharosaurus in tracing the paleobiogeographic origins of the radiation of neosauropods in the Middle Jurassic.

Cladistic analysis of Tharosaurus indicus (after Bajpai et al. 2023). Note that the recovery of Tharosaurus as being more derived than Lingwulong is inconsistent with its older age than Lingwulong.

Although Bajpai et al. (2023) note that Tharosaurus is older than other dicraeosaurids, they argue that the recovery of Suuwassea and Bajadasaurus basally within Dicraeosauridae raises the question of whether dicraeosaurids originated in India due to the latter two taxa being older than Tharosaurus. However, the early-diverging cladistic positions of Bajadasaurus, Suuwassea, and Lingwulong within Dicraeosauridae recovered by Bajpai et al. are at odds with the results of the phylogeny by Whitlock and Wilson Mantilla (2020) that recovers these three taxa in derived positions within Dicraeosauridae. The non-adult nature of the Suuwassea emilieae holotype was recognized by Woodruff and Fowler (2012) and thus placed Suuwassea in an artificially basal position in Dicraeosauridae in the cladistic analyses of Bajpai et al. (2023) and Whitlock and Wilson Mantilla (2020), whereas the recovery of Bajadasaurus as basal to Tharosaurus in the Bajpai et al. (2023) phylogeny is best explained by the holotype comprising only cranial remains and a few cervical vertebrae because the cladistic analysis of Smitanosaurus by Whitlock and Wilson Mantilla also includes the unnamed dicraeosaurid MOR 592, which is recovered as a derived dicraeosaurid along with Bajadasaurus and Lingwulong. As I have  mentioned elsewhere, diplodocid remains described by Rivera-Sylva and Espinosa-Arrubarrena (2020) from the Bathonian-Callovian Otlaltepec Formation in east-central Mexico constitute the oldest record of a diplodocoid from North America and show that eusauropods began migrating to western North America from South America before separation of North and South America in the Aalenian-Bajocian interval. Given that the Tharosaurus indicus holotype comprises only vertebrae, it is additional remains of this taxon or a re-assessment of a few derived eusauropods of uncertain cladistic position from the Middle Jurassic of East Asia as diplodocoids could place Tharodocus as an early-diverging taxon within Dicraeosauridae because Whitlock and Wilson Mantilla (2020) recover Bajadasaurus and Lingwulong as derived within Dicraeosauridae, which accords with the Early Cretaceous age of Bajadasaurus.   

Now, the geologic unit in which Tharosaurus was found raises an important point about the timing of neosauropod dispersal from India to other parts of the world. Because Bajpai et al. note that the Tethys Ocean was a geographical barrier to terrestrial tetrapods in the Mesozoic, they reject any notion that diplodocoids could have dispersed into India from East Asia via North America and western Gondwana, but apart from diplodocid remains in Middle Jurassic deposits in Mexico indicating that the dispersal of neosauropods into western Laurasia from South America took place at the Toarcian-Aalenian boundary, there is evidence from the trace fossil record that neosauropod dispersal from India was instantaneous. Sauropod tracks from the Middle Jurassic (Bajocian-early Bathonian) Dande Sandstone in north-central Zimbabwe by Ahmed et al. (2004) demonstrate that neosauropods dispersed into southern Africa from India during the Aalenian-Bathonian interval because the geologic unit from which Tharosaurus hails is about the same age as the Dande Sandstone even though neosauropod body fossils have not yet been recorded from Middle Jurassic deposits in sub-Saharan Africa. Given the occurrence of sauropod tracks in the Middle Jurassic of Zimbabwe, and diplodocoid classification of "Cetiosaurus" glymptonensis from the Middle Jurassic (Bathonian) of the English Midlands by Upchurch and Martin (2003) and the recovery of Atlasaurus from the Bathonian of Morocco as a brachiosaurid by Royo-Torres et al. (2021) demonstrate that the two main neosauropod clades most probably began dispersing out of sub-Saharan Africa, South America, and India into Europe and Asia via North Africa by the beginning of the Middle Jurassic. Although Lapparentosaurus is recovered as either a titanosauriform by Upchurch et al. (2004) or as a derived non-neosauropod eusauropod by Royo-Torres et al. (2021), either classification scheme for this taxon would confirm the stretches of southern Gondwana comprising South America, southern Africa, and India as the focal origin for both neosauropods and eusauropod clades closely related to Neosauropoda because the recovery of the Australian taxon Rhoetosaurus as well as Spinophorosaurus  and Cetiosauriscus as members of Mamenchisauridae by Ren et al. (2023) and Rauhut et at. (2005) demonstrate that mamenchisaurids along with neosauropods spread to Europe and later Central and East Asia from sub-Saharan Africa prior to the Oxfordian.

References:

Ahmed, A.A., Lingham-Soliar, T., and Broderick, T., 2004. Giant sauropod tracks from the Middle-Late Jurassic of Zimbabwe in close association with theropod tracks. Lethaia 37: 467–470.

Bajpai, S., Datta, D., Pandey, P., Ghosh, T., Kumar, K., and Bhattacharya, D., 2023. Fossils of the oldest diplodocoid dinosaur suggest India was a major centre for neosauropod radiation. Scientific Reports 13: 12680. doi:10.1038/s41598-023-39759-2.

Moser, M., Mathur, U.B., Fürsich, F.T., Pandey, D.K., and Mathur, N., 2006. Oldest camarasauromorph sauropod (Dinosauria) discovered in the Middle Jurassic (Bajocian) of the Khadir Island, Kachchh, western India. Paläontologische Zeitschrift 80 (1): 34-51.

Rauhut, O.W.M., Remes, K., Fechner, R., Cladera, G., and Puerta, P., 2005. Discovery of a short-necked sauropod dinosaur from the Late Jurassic period of Patagonia. Nature 435 (7042): 670–672. doi:10.1038/nature03623

Ren, X.X., Jiang, S., Wang, X.R., Peng, G.Z., Ye, Y., Jia, L., and You, H.L., 2023. Re-examination of Dashanpusaurus dongi (Sauropoda: Macronaria) supports an early Middle Jurassic global distribution of neosauropod dinosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology 610: 111318.  doi:10.1016/j.palaeo.2022.111318

Rivera-Sylva, H. E., and Espinosa-Arrubarena, L., 2020, Remains of a diplodocid (Sauropoda: Flagellicaudata) from the Otlaltepec Formation Middle Jurassic (Bathonian-Callovian) from Puebla, Mexico. Paleontologia Mexicana 9 (3): 145-150.

Royo-Torres, R., Cobos, A., Mocho, P., and Alcalá, L., 2021. Origin and evolution of turiasaur dinosaurs set by means of a new 'rosetta' specimen from Spain. Zoological Journal of the Linnean Society 191 (1): 201–227. doi:10.1093/zoolinnean/zlaa091. 

Upchurch, P., and Martin, J., 2003. The anatomy and taxonomy of Cetiosaurus (Saurischia, Sauropoda) from the Middle Jurassic of England. Journal of Vertebrate Paleontology. 23 (1): 208–231. doi:10.1671/0272-4634(2003)23[208:TAATOC]2.0.CO;2

Upchurch, P., Barrett, P.M. and Dodson, P. 2004. Sauropoda. pp. 259-322. In: Weishampel, D., Dodson, P., and Osmólska, H. (eds.), The Dinosauria, 2nd edition. University of California Press, Berkeley.

Whitlock, C., and Wilson Mantilla, J., 2020. The Late Jurassic sauropod dinosaur 'Morosaurus' agilis  Marsh, 1889 reexamined and reinterpreted as a dicraeosaurid. Journal of Vertebrate Paleontology

DOI: 10.1080/02724634.2020.1780600

Woodruff, C. & Fowler, D. W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): A critical phylogenetic character. Journal of Morphology 273: 754–764. 

Bruhathkayosaurus: a genuine giant titanosaur after all

In the late 1980s fragmentary postcranial remains from the Kallamedu Formation in Tamil Nadu, southern India, were described by Indian paleontologists P. Yadagiri and Krishnan Ayyasami as a new genus and species of carnosaurian theropod, Bruhathkayosaurus matleyi, in a paper published in 1987. However, in the 1990s the theropod placement of Bruhathkayosaurus was disputed and this taxon was soon recognized as being a titanosaurian sauropod. Due to the paucity of line drawings and photographs of the Bruhathkayosaurus material as it lay on the ground, there was online speculation that this taxon was based on fossilized tree trunks rather than genuine dinosaur bones, and varying size estimates for Bruhathkayosaurus based on the published description by Yadagiri and Ayyasami (1987) posted on internet forums precluded Bruhathkayosaurus from being touted in record books as the biggest dinosaur that ever lived. Nevertheless, various authors (e.g. Krause et al. 2006; Hone et al. 2016) have come to agree with Chatterjee (1995) that Bruhathkayosaurus matleyi is a sauropod, although the questionable validity of this taxon was further compounded by the type material falling victim to the effects of local monsoons (Galton and Ayyasami 2017). Thanks to additional published photographs of the B. matleyi type material and new morphological information on this poorly known dinosaur, it is apparent that the material of Bruhathkayosaurus is indeed from a giant titanosaur and that past hints about this dinosaur being based on non-vertebrate material do not hold water.

Photographs of the holotype ilium and paratype tibia of Bruhathkayosaurus matleyi (left) and line drawings of the B. matleyi ilium and tibia (right). From Yadagiri and Ayyasami (1987).

When Bruhhathkyosaurus was first described by Yadagiri and Ayyasami (1987), it was considered by the authors to be a very large theropod possibly comparable to or exceeding Tyrannosaurus rex in size (radius and pubis of Bruhathkayosaurus matleyi were not described or illustrated). The first author to question the theropod classification of this taxon was Olshevsky (1994), who noted that the proportions of the ilium and tibia were unusually large for a huge theropod judging from the illustrations in Yadagiri and Ayyasami (1987), therefore suggesting that Bruhathkayosaurus might not be a theropod. In a 1994 post on Dinosaur Mailing list, John Schneiderman regarded the tibia of Bruhathkayosaurus as either a sauropod or petrified wood, while Thomas Holtz Jr. in a 1995 post on the DML suggested that the type material of B. matleyi could be fossilized tree trunks due to the poor quality of the line drawings of the ilium and tibia by Yadagiri and Ayyasami (1987) Chatterjee (1995) showed that B. matleyi represented a titanosaurian sauropod by noting that the high iliac crest of the holotype ilium (GSI PAL/SR/20) and proportions of the distal femur and tibia were consistent with that of titanosaurs. Upchurch et al. (2004) tabulated Bruhathkayosaurus matleyi as a nomen dubium without any further comment, most probably aware of Chatterjee's (1995) re-assessment of Bruhathkayosaurus because that paper is not included in the bibliography for the second edition of the book The Dinosauria. Krause et al. (2006) also classified Bruhathkayosaurus as a sauropod, and Hone et al. (2016) likewise agreed with Chatterjee's (1995) identification of Bruhathkayosaurus as a titanosaur. Pal and Ayyasami (2022) provide photographs of the tibia of Bruhathkayosaurus matleyi during excavation from the Cauvery Basin and after being wrapped in a plaster jacket, and they clinch the titanosaur identification of the tibia by noting that the cnemial crest of the tibia is identical to that of titanosaurs in being smaller and less prominent than in abelisaurids, while debunking online suggestions that the B. matleyi type material is merely fossilized wood.

Now all this brings me to discussions online and published scientific literature regarding the possible size of Bruhathkayosaurus.  Mickey Mortimer wrote in a June 2001 post on the Dinosaur Mailing List that Bruhathkayosaurus most likely measured 145 feet (44.1 meters) based on measurements provided by Yadagiri and Ayyasami (1987), but in later posts on the DML he revised to size estimate for this taxon to 92-154 feet (28-47 meters). In a May 2008 post on his blog Sauropoda Vertebra of the Week, Mike Taylor suggested that Bruhathkayosaurus was 20 percent bigger than Argentinosaurus based on the length of the tibia. Although Pal and Ayyasami (2022) declined to provide a precise size estimate for B. matleyi given the limited material, they did confirm the gigantic nature of Bruhathkayosaurus by showing that its tibia was bigger than that of Argentinosaurus and Dreadnoughtus and that the width of the distal femur was bigger than that of Patagotitan. The authors also noted that the holotype ilium and paratype tibia are unlikely to belong the same individual due to size differences (the length of the ilium of B. matleyi is shorter than the ilium of the Dreadnoughtus schrani holotype), supporting Olshevshy's (1994) suggestion that the Bruhathkayosaurus material might be chimeric, but the ilium itself would still be comparable in size to those known for some large-sized titanosaurs, namely Futalognkosaurus, because its length is much greater than the longest ilia of any abelisaurid. Paul and Larramendi (2023) provide a narrower size estimate for Bruhathkayosaurus ranging from 115-148 feet (35-45 meters) with a mean length of 131 feet (40 meters). In my opinion, Bruhathkayosaurus is probably close to the lower end of the size range deduced by Paul and Larramendi (2023) for this taxon, at 120 feet (36 meters) in length, because Diplodocus hallorum and Maraapunisaurus were once considered to attain lengths of 170 feet (52 meters) and 190 feet (58 meters) respectively, only to have their size estimates reduced drastically to within 100-115 feet (30-35 meters) by later research. Regardless of the fact that the holotype ilium and paratype tibia of Bruhathkayosaurus matleyi do not appear to come from the same individual, the the tibia indicates that Bruhathkayosaurus is indeed a taxon of gigantic titanosaur as surmised online by Mickey Mortimer and Mike Taylor.

Although it is unfortunate that monsoons ended up causing the type material of Bruhathkayosaurus matleyi to disintegrate before being taken to its respective paleontological institution and there were rumors online that Bruhathkayosaurus was based on petrified wood rather than genuine dinosaur fossils even if it was clear by the mid-1990s that the original theropod classification of this taxon did not hold water, it is heartening to see that additional photographic evidence has vindicated the conclusion by Chatterjee (1995) that Bruhathkayosaurus itself is not just a titanosaur but also roughly comparable to Argentinosaurus and Diplodocus hallorum in size. Considering that Matley (1929) was the first author to report giant titanosaur remains from the Kallamedu Formation, Bruhathkayosaurus constitutes the second instance of a super-size titanosaur from this geologic unit, suggesting that giant titanosaurs had a widespread distribution across Gondwana by the Maastrichtian.

References:

Chatterjee, S., 1995. The last dinosaurs of India. The Dinosaur Report (Fall 1995): 12-18.

Galton, P.M., and Ayyasami, K., 2017. Purported latest bone of a plated dinosaur (Ornithischia: Stegosauria), a "dermal plate" from the Maastrichtian (Upper Cretaceous) of southern India. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 285 (1): 91–96.  doi:10.1127/njgpa/2017/0671.

Hone, D.W. E., Farke, A.A., and Wedel, M.J., 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 12 (2): 20150947. doi:10.1098/rsbl.2015.0947.

Krause, D.W., O'Connor, P.M., Rogers, K.C., Sampson, S.D., Buckley, G.A., and Rogers, R.R., 2006. Late Cretaceous terrestrial vertebrates from Madagascar: Implications for Latin American biogeography. Annals of the Missouri Botanical Garden 93 (2): 178–208. doi:10.3417/0026-6493(2006)93[178:LCTVFM]2.0.CO;2.

Matley, C.A., 1929. The Cretaceous dinosaurs of the Trichinopoly District, and the rocks associated with them. Records of the Geological Survey of India 61: 337–349.

Olshevsky, G., 1994. Bruhathkayosaurus: Bigger Than T. rex? The Dinosaur Report (Winter 1994): 12–13.

Pal, S., and Ayyasami, K., 2022. The lost titan of Cauvery. Geology Today 39: 112–116.

Paul, G.S., and Larramendi, A., 2023. Body mass estimate of Bruhathkayosaurus and other fragmentary sauropod remains suggest the largest land animals were about as big as the greatest whales. Lethaia 56 (2): 1–11. doi:10.18261/let.56.2.5.

Upchurch, P., Barrett, P.M., and Dodson, P. 2004. Sauropoda. pp. 259-322. In: Weishampel, D.B., Dodson, P., and Osmólska, A.H. (eds). The Dinosauria, second edition. University of California Press: Berkeley, CA.

Yadagiri, P., and Ayyasami, K., 1987. A carnosaurian dinosaur from the Kallamedu Formation (Maastrichtian horizon), Tamil Nadu. Geological Survey of India, Special Publications 11: 523–528.

Thoughts on paper by D'Emic (2023) regarding evolution of huge body size in sauropods

For generations, sauropod dinosaurs have starred in the popular imagination as the biggest land animals to ever walk the Earth, with interest in giant sauropods going back to the 1970s and 1980s with the discovery of the derived titanosaur Argentinosaurus and the diplodocids Diplodocus (=Seismosaurus) hallorum and Supersaurus (the giant rebbachisaurid Maraapunisaurus was seen in some publications as possibly the biggest-ever sauropod, although the missing nature of the holotype dorsal vertebra made size estimates of 190 feet guesswork, but Carpenter [2018] revised the size estimate of this genus to 105 feet, which makes sense because the initial size estimates of 170 feet for Diplodocus  hallorum was later revised to 110 feet). While children and adults alike marvel at the immense size of sauropods compared to elephants, rhinoceroses, and the extinct paraceratheres, I have not forgotten the fact that some sauropods living on island arcs in southern Europe during the Late Cretaceous which today are part of France, Spain, and Romania exhibited insular dwarfism, and that not all sauropods approached the body sizes attained by Apatosaurus, Brachiosaurus, Brontosaurus, Diplodocus, Galeamopus, Giraffatitan, and Supersaurus. Recently, D'Emic (2023) published a new paper illuminating the evolution of body size among sauropod taxa, relying on measurements of the circumferences of sauropod limb bones to propose that massive body size evolved convergently in multiple sauropod clades, while noting that select taxa of sauropods showed decreasing trends in body size. Having had the chance to read this paper, there are some noteworthy observations from the study that I wish to emphasize in detail. 

As admitted by D'Emic (2023), the patchy sauropod fossil record has constrained the focus on sauropod body size evolution to less than half of the approximately 250 described sauropod taxa, but the fact that Franz Nopcsa's interpretation of Magyarosaurus as a dwarf taxon has been vindicated by several studies (e.g. Benton et al. 2010; Stein et al. 2010) along with the small size of the short-necked dicraeosaurid Brachytrachelopan and the dwarf brachiosaurid Europasaurus shows that diminutive body size was present in more than one eusauropod clade. Although Cope's rule says that tetrapod lineages tend to increase in body size over time, it should be noted that when Edward Drinker Cope and Othniel Charles Marsh described to science the sauropod fauna of the Morrison Formation in western North America, they did not foresee that a few sauropod taxa of diminutive body size would be named in a handful of publications long after their deaths, and Magyarosaurus would be the first sauropod to be described that deviates from Cope's rule when it comes to body size in sauropods. With respect to the argument by D'Emic that the evolutionary cascade hypothesis put forward by Sander (2013) to interpret gigantism in some sauropods as being driven by anatomical and physiological innovations stemming from a nested array of historical prerequisites is best explained by ecological and life-history factors rather than any shared morphological traits, I should emphasize that the co-existence of dwarf and gigantic titanosaurs in the Late Cretaceous of Romania, Spain, and France (Stein et al. 2010; Vila et al. 2022) lends support to D'Emic's conclusion that ecological and life-history factors influenced the degree of dwarfism or gigantism among sauropod clades by showing that the insular nature of the ecosystems of the Ibero-Armorican island and Hațeg Island didn't necessarily translate into uniform dwarfism among titanosaur taxa from the Late Ceretaceous of Europe. For instance, Vila et al. (2022) note that the holotype of Abditosaurus kuehnei is substantially larger than specimens of other titanosaur taxa from the Ibero-Armorican Island as well as titanosaur specimens from the Hațeg Island, indicating that Cope's rule might apply to the evolution of body size in titanosaurs over the course of the Maastrichtian in southern Europe.

Phenogram of sauropod body size evolution over the course of the Mesozoic (from D'Emic 2023). Note that the lineages of the six eusauropod lineages that surpassed the maximum mammalian body mass are denoted by longitudinal lines highlighted in different colors (light green=non-neosauropod eusauropods; dark green=turiasaurians; blue=diplodocoids; orange=basal macronarians and non-titanosaur somphospondyls; purple=brachiosaurids; red=titanosaurs).

According to D'Emic, a total of 36 sauropod lineages independently evolved huge body sizes greater than those of other terrestrial tetrapods from the Mesozoic and Cenozoic, 32 of them belonging to the clade Neosauropoda. The discovery of isolated postcranial remains from the Bajocian-Callovian age Dongdaqiao Formation of Tibet belonging to a eusauropod over 66 feet (20 meters) long (Wei et al. 2023), two gigantic eusauropod cervical vertebrae from the Shishugou Formation of Xinjiang indicating a eusauropod 115 feet (35 meters) in length (which are referred to Mamenchisaurus sinocanadorum by Paul [2019] but considered to be of indeterminate taxonomic placement by Moore et al. [2023]), and a maximum size estimate of 105 feet (32 meters) for the Middle Jurassic mamenchisaurid Xinjiangtitan given by Wu et al. (2013), indicate that gigantic body size evolved in more than one clade of non-turiasaurian, non-neosauropod eusauropods during the Middle Jurassic, although the limited amount of material known for Mamenchisaurus sinocanadorum renders the exact size of this taxon unclear. The gigantic size of Turiasaurus relative to that of other turiasaurs makes me feel tempted to suggest that the rate of body size evolution in Turiasauria is inconsistent with Cope's rule because of the medium body size of the Early Cretaceous turiasaurs Mierasaurus and Moabosaurus and small body size of the Late Jurassic form Amanzia from Switzerland (Schwarz et al. 2020), the minor age difference between  Turiasaurus and the Yellow Cat Member of the Cedar Mountain Formation yielding Mierasaurus and Moabosaurus could suggest large-bodied turiasaurs might survived into the Valanginian-Barremian interval and that the small size of Amanzia can be best explained as a result of insular dwarfism given that much of western and central Europe was covered by seas during the Late Jurassic.

References:

Benton, M.J., Csiki, Z., Grigorescu, D., Redelstorff, R., Sander, P.M., Stein, K., and Weishampel, D.B., 2010. Dinosaurs and the island rule: The dwarfed dinosaurs from Haţeg Island. Palaeogeography, Palaeoclimatology, Palaeoecology 293 (3): 438–454. doi:10.1016/j.palaeo.2010.01.026

Carpenter, K, 2018. Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado. Geology of the Intermountain West 5: 227–244.

D'Emic, M. D., 2023. The evolution of maximum terrestrial body mass in sauropod dinosaurs. Current Biology 33 (9): R349–R350. doi:10.1016/j.cub.2023.02.067

Paul, G.S., 2019. Determining the largest known land animal: A critical comparison of differing methods for restoring the volume and mass of extinct animals. Annals of the Carnegie Museum 85 (4): 335–358. doi:10.2992/007.085.0403

Sander, P.M., 2013. An Evolutionary Cascade Model for Sauropod Dinosaur Gigantism - Overview, Update and Tests. PLoS ONE 8(10): e78573. https://doi.org/10.1371/journal.pone.0078573

Schwarz, D., Mannion, P.D., Wings, O., and Meyer, C.A., 2020. Re-description of the sauropod dinosaur Amanzia (‘Ornithopsis/Cetiosauriscus’) greppini n. gen. and other vertebrate remains from the Kimmeridgian (Late Jurassic) Reuchenette Formation of Moutier, Switzerland. Swiss Journal of Geosciences 113: 2. https://doi.org/10.1186/s00015-020-00355-5

Stein, K., Csiki, Z., Curry Rogers, K., Weishampel, D.B., Redelstorff, R., Carballidoa, J.L., and Sander, P.M., 2010. Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria). Proceedings of the National Academy of Sciences of the United States of America 107 (20): 9258–9263. doi:10.1073/pnas.1000781107

Vila, B., Sellés, A., Moreno-Azanza, M., Razzolini, N.L., Gil-Delgado, A., Canudo, J.I., and Galobart, A., 2022. A titanosaurian sauropod with Gondwanan affinities in the latest Cretaceous of Europe. Nature Ecology & Evolution 6: 288-296. doi:10.1038/s41559-021-01651-5.

Wei, X.-F.; Wang, Q.-Y., An, X.-Y., Wang, B.-D., Zhang, Y.-J., Mou, C.-L., Li, Y., Wang, D.-B., Ma, W., and Kundrát, M., 2023. New sauropod remains from the Middle Jurassic Dongdaqiao Formation of Qamdo, eastern Tibet. Palaeoworld: in press. doi:10.1016/j.palwor.2023.02.002

Wu, W.H., Zhou, C.F., Wings, O., Sekiya, T., and Dong, Z.M., 2013. A new gigantic sauropod dinosaur from the Middle Jurassic of Shanshan, Xinjiang. Global Geology 32: 437–446.

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.