Your Friends The Titanosaurs, part 35: Part-Time Titanosaurs, or: The Next Best Thing

Following on the previous post in the series, this post takes a look at the taxa that hang out near the base of Titanosauria, usually outside but sometimes slipping under the velvet rope held by Andesaurus. Usually on these occasions they don't really get under that rope, per se, but are wrapped up in a massive polytomy that also includes Andesaurus (e.g., González Riga et al. 2018; Averianov and Efimov 2018; Hechenleitner et al. 2019; Rubilar-Rogers et al. 2021). Alternatively, they form a bunch of little clades at the base of Titanosauria within one larger clade including Andesaurus (e.g., Sallam et al. 2018; Gorscak and O'Connor 2019; some results in Mannion et al. 2019), which I guess in principle would resurrect Andesauridae, but no one has seemed excited about the proposition. I still say that Andesaurus has done about as well as possible marking the boundary of the arbitrary human construct Titanosauria, but admittedly it does sometimes indulge an appetite for polytomies.

Which sauropods end up in this awkward situation? Well, if you get a big enough polytomy, any non-titanosaurian titanosauriform is fair game. Even Brachiosaurus has joined in the fun (e,g., Hechenleitner et al. 2020). In more practical terms, we're concerned with a smaller number of titanosauriforms. We've already encountered some of these unstable species as genuine titanosaurs, some of which have since been moved out of Titanosauria in The Compact Thescelosaurus:

• Baotianmansaurus henanensis, uncertain Late Cretaceous of China (moved to Titanosauria? after entry was written)
• Borealosaurus wimani, Early Cretaceous (Albian) of China
• Daxiatitan binglingi, uncertain Early Cretaceous of China
• Dongyangosaurus sinensis, Early Cretaceous (late Albian) of China (moved to Titanosauria? after entry was written)
• Jiangshanosaurus lixianensis, Early Cretaceous (late early Albian) of China (moved to Somphospondyli after entry was written)
• Malarguesaurus florenciae, Late Cretaceous (late Turonian–early Coniacian) of Argentina
• Ruyangosaurus giganteus, Early Cretaceous (Aptian–Albian) of China
• Xianshanosaurus shijiagouensis, Early Cretaceous (Aptian–Albian) of China

That boils down to seven species from the late Early Cretaceous (more or less) of China that have historically tended to favor the titanosaurs more than "euhelopodids", and Malarguesaurus. Then there are five species of more independent affiliation:

• Angolatitan adamastor, Late Cretaceous (Coniacian) of Angola
• Chubutisaurus insignis, Late Cretaceous (early Cenomanian) of Argentina
• Ligabuesaurus leanzai, Early Cretaceous (Aptian–Albian) of Argentina
• Tastavinsaurus sanzi, Early Cretaceous (early Aptian) of Spain
• Wintonotitan wattsi, Late Cretaceous (Cenomanian–early Turonian) of Australia

The type specimen of Wintonotitan wattsi, clipped from Figure 2 of Hocknull et al. (2009). The length of the scale bar is not stated, but I'm guessing it's 1 m. CC-BY-2.5.

Finally, there are the "euhelopodids", which on a good day can include practically every East Asian sauropod of Cretaceous age. On a bad day, Euhelopus decides it's had it up to *here* with Macronaria and goes to hang out with its old pals Mamenchisaurus and Omeisaurus, dragging some (but not all) of the erstwhile titanosauriform euhelopodids with it (Moore et al. 2020). The first list above takes care of the majority of the Chinese species that are most frequently connected with Titanosauria in the literature. An additional five "euhelopodid" or "euhelopodid"-adjacent sauropods that have some occasional attraction to Titanosauria are listed below (there are a number of other "euhelopodids", but they either don't seem to have the same conflicting loyalties or never show up in phylogenies):

• Huabeisaurus allocotus, uncertain Late Cretaceous of China
  
• Huanghetitan liujiaxiensis, uncertain Early Cretaceous of China
  
• "Huanghetitan" ruyangensis, Early Cretaceous (Aptian–Albian) of China
  
• Phuwiangosaurus sirindhornae, Early Cretaceous (Barremian–Aptian) of Thailand
  
• Tangvayosaurus hoffeti, Early Cretaceous (Aptian–Albian) of Laos
  

The quarry map for the type specimen of Huabeisaurus allocotus (Figure 3 in D'Emic et al. 2013). CC-BY-2.5.

These lists are not exhaustive, and I'm sure someone will write in to the comments about some other claimant. I think we've got enough to get the flavor, though, certainly all of the most frequent part-time titanosaurs. What do these eighteen species have in common? In terms of age, the Aptian and Albian are very popular, which makes intuitive sense: with titanosaurs being the main sauropod show of the Late Cretaceous, it's not surprising to have their closest cousins flourishing toward the end of the Early Cretaceous. Most of them are from China, with smaller clusters from Argentina and southeast Asia. This suggests, if nothing else, that somphospondylans took firm hold in the Early Cretaceous of Asia compared to other areas with rocks of similar age. (This is somewhat muddled if Moore et al. [2020] are correct about some or most "euhelopodids" being related to Mamenchisaurus and Omeisaurus rather than titanosaurs. You've still got a few in Asia, such as Phuwiangosaurus, Tangvayosaurus, and some of the Chinese species, but it's not quite as impressive an assortment.) Also, near-titanosaurs had an impressively wide geographic distribution for being relative latecomers.

Almost all of these species have been named since 2005, with Chubutisaurus the one major outlier (1975). They aren't exactly household names. Sure, a few of them have some notice for size (e.g., Daxiatitan, Huanghetitan, Huabeisaurus, Ruyangosaurus), Wintonotitan has had some good press, and Chubutisaurus can be found in '80s and '90s dinosaur books as a mysterious sauropod, but that's not quite A-list or even B-list material. Many of them are better-represented by fossils than you might expect from their instability and obscurity; they aren't just a random dorsal vertebra or something of that ilk. There aren't any complete skeletons, though. (That would be too convenient.) It's a two-fold problem: there's just enough of most of them to know they're distinct while still having significant gaps, and we're still in the phase where more are being found and described than we have time to fully assess. (Come to think of it, that's basically the situation with titanosaur classification as well.) It would not be surprising if some from the second and third lists turned out to be titanosaurs, nor would it be surprising if more of the first list got bumped.

Finally, before we wrap up this post, an addendum to part 34. In hindsight, there were two other species that would have made sense as "one-time titanosaurs": Triunfosaurus leonardii and Yongjinglong datangi.

Triunfosaurus leonardii: This species is another among the group contending to be the earliest titanosaur. It was named by Carvalho et al. (2017) for a few bones (three middle-posterior caudals, the right ischium, three chevrons, and three neural spines) from the Rio Piranhas Formation of Paraíba, northeastern Brazil. The Rio Piranhas Formation is dated to the Berriasian–early Hauterivian, which would potentially make T. leonardii the earliest titanosaur. However, Poropat et al. (2017) found the evidence of a titanosaurian identity to be weak, and found it to be a problematic probable somphospondylan. It has not been reviewed in any detail since then.

Yongjinglong datangi: Y. datangi comes from the Lower Cretaceous Hekou Group of Gansu, China. It was named for a partial skeleton including three teeth, a posterior cervical, seven dorsals, a dorsal rib, the left scapulocoracoid, and the right ulna and radius (Li et al. 2014). This specimen is striking for the comically long and skinny shoulder girdle (a little shy of 2 m long), paired with a chunky radius and ulna that could have been flown in from any small stocky titanosaur. Meanwhile, the vertebrae and spoon-shaped teeth are more like those of Euhelopus than a titanosaur (Li et al. 2014). Saegusa and Ikeda (2014) raised the obvious question: could this be a chimera? Although Li et al. (2014) found Y. datangi to group with Opisthocoelicaudia, Mannion et al. (2019) consistently found it nested within Euhelopodidae (Euhelopodidae sometimes turned up near the base of Titanosauria, but that's another story).

Yongjinglong datangi: go ahead, cry on its shoulder; it has room. The scale bar is 600 mm (24 in), R is right, and L is left. Figure 17 in Li et al. (2014). CC-BY-4.0.

References

Averianov, A., and V. Efimov. 2018. The oldest titanosaurian sauropod of the Northern Hemisphere. Biological Communications 63(6):145–162. doi:10.21638/spbu03.2018.301.

Carvalho, I. S., L. Salgado, R. M. Lindoso, H. I. de Araújo-Júnior, F. C. Costa Nogueir, and J. A. Soares. 2017. A new basal titanosaur (Dinosauria, Sauropoda) from the Lower Cretaceous of Brazil. Journal of South American Earth Sciences 75:74–84.

D'Emic, M. D., P. D. Mannion, P. Upchurch, R. B. J. Benson, Q. Pang, and Z. Cheng. 2013. Osteology of Huabeisaurus allocotus (Sauropoda: Titanosauriformes) from the Upper Cretaceous of China. PLoS ONE 8(8):e69375. doi:10.1371/journal.pone.0069375.

González Riga, B. J., P. D. Mannion, S. F. Poropat, L. D. Ortiz David, and J. P. Coria. 2018. Osteology of the Late Cretaceous Argentinean sauropod dinosaur Mendozasaurus neguyelap: implications for basal titanosaur relationships. Zoological Journal of the Linnean Society 184(1):136–181. doi:10.1093/zoolinnean/zlx103.

Gorscak, E., and P. M. O’Connor. 2019. A new African titanosaurian sauropod dinosaur from the middle Cretaceous Galula Formation (Mtuka Member), Rukwa Rift Basin, southwestern Tanzania. PLoS ONE 14(2):e0211412. doi:10.1371/journal.pone.0211412.

Hechenleitner, E. M., L. Leuzinger, A. G. Martinelli, S. Rocher, L. E. Fiorelli, J. R. A. Taborda, and L. Salgado. 2020. Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America. Communications Biology 3:article number 622. doi:10.1038/s42003-020-01338-w. 

Hocknull, S. A., M. A. White, T. R. Tischler, A. G. Cook, N. D. Calleja, T. Sloan, and D. A. Elliott. 2009. New mid-Cretaceous (latest Albian) dinosaurs from Winton, Queensland, Australia. PLoS ONE 4 (7):e6190. doi:10.1371/journal.pone.0006190.

Li, L. G., D. Q. Li, H. L. You, and P. Dodson. 2014. A new titanosaurian sauropod from the Hekou Group (Lower Cretaceous) of the Lanzhou-Minhe Basin, Gansu Province, China. PLoS ONE 9:e85979. doi:10.1371/journal.pone.0085979.

Mannion, P. D., P. Upchurch, X. Jin, and W. Zheng. 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.

Moore, A. J., P. Upchurch, P. M. Barrett, J. M. Clark, and X. Xing. 2020. Osteology of Klamelisaurus gobiensis (Dinosauria, Eusauropoda) and the evolutionary history of Middle–Late Jurassic Chinese sauropods. Journal of Systematic Palaeontology 18(16):1299–1393. doi:10.1080/14772019.2020.1759706.

Poropat, S. F., J. P. Nair, C. E. Syme, P. D. Mannion, P. Upchurch, S. A. Hocknull, A. G. Cook, R. T. Tischler, and T. Holland. 2017. Reappraisal of Austrosaurus mckillopi Longman, 1933 from the Allaru Mudstone of Queensland, Australia's first named Cretaceous sauropod dinosaur. Alcheringa 41(4):543–580. doi:10.1080/03115518.2017.1334826.

Rubilar-Rogers, D., A. O.Vargas, B. Gonzalez Riga, S. Soto-Acuña, J. Alarcón-Muñoz, J. Iriarte-Díaz, C. Arévalo, and C. S. Gutstein. 2021. Arackar licanantay gen. et sp. nov. a new lithostrotian (Dinosauria, Sauropoda) from the Upper Cretaceous of the Atacama Region, northern Chile. Cretaceous Research. doi:10.1016/j.cretres.2021.104802.

Saegusa, H., and T. Ikeda. 2014. A new titanosauriform sauropod (Dinosauria: Saurischia) from the Lower Cretaceous of Hyogo, Japan. Zootaxa 3848(1):1–66.

Sallam, H. M., E. Gorscak, P. M. O’Connor, I. A. El-Dawoudi, S. El-Sayed, S. Saber, M. A. Kora, J. J. W. Sertich, E. R. Seiffert, and M. C. Lamanna. 2018. New Egyptian sauropod reveals Late Cretaceous dinosaur dispersal between Europe and Africa. Nature Ecology & Evolution 2:445–451. doi:10.1038/s41559-017-0455-5. Supplementary information.