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.