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.
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| 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.
These paleobiogeographic analyses have annoyed me for decades because 99% of Mesozoic formations are terribly sampled, with maybe only the Morrison and Dinosaur Park Formations showing us something similar to the real diversity of what dinosaur groups lived there. And if you're trying to map it onto a cladogram you're missing the thousands of unnamed and usually indeterminate examples of clades from other places. And that's not even getting into the uncertainty of the cladograms all these hypotheses are using as their base data, as exemplified in this post by the lack of consensus for dicraeosaurid topology.
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