Implications of Hamititan and Silutitan for somphospondyl macroevolution in Early-Middle Cretaceous of East Asia

Over the past two decades, numerous somphospondyl sauropod taxa have been described from the Early-Middle Cretaceous of East Asia (Dong et al. 2001; Mannion et al. 2019), augmenting the previously described taxa Euhelopus zdanskyi (Wiman, 1929), Asiatosaurus mongoliensis Osborn, 1924, Mongolosaurus haplodon Gilmore, 1933, and Asiatosaurus kwangshiensis Hou, Yeh, and Zhao, 1975, in terms of illuminating somphospondyl evolution in the Far East during the first 45 million years of the Cretaceous. However, the fossil record of East Asian titanosaurian taxa from the Berriasian-Albian has for the most part remained patchy and sparse compared to the multitude of euhelopodids from the region, and this is one special reason why titanosaur interrelationships are still a work in progress despite advances towards understanding the cladistic diversity of lithostrotian titanosaurs. Wang et al. (2021) have recently described two new somphospondylian taxa from the Early Cretaceous (early Barremian-early Aptian) Shengkinkou Formation of Xinjiang, the euhelopodid Silutitan sinensis and the titanosaur Hamititan tianshanensis, constituting one of the first instances of euhelopodids and titanosaurs being coeval in the Berriasian-Barremian of East Asia. Hence, it should be worth discussing the implications of Hamititan for the macroevolution of titanosaurs in East Asia during the early to middle Cretaceous.  

Cladogram (using Mannion et al. 2019 dataset) showing the positions of Hamititan and Silutitan (after Wang et al. 2021) 

To begin with, the cladistic analyses conducted by Wang et al. for Hamititan and Silutitan, especially with respect to the datasets from Mannion et al. (2019), offer some new insights into somphospondyl evolution in East Asia during the Early Cretaceous. The most noteworthy aspect of the strict consensus cladogram in figure 8 where Hamititan and Silutitan are treated as distinct operational taxonomic units (OTUs) is the recovery of Hamititan as a derived lithostrotian closely related to Aeolosaurus and Rapetosaurus, more derived than Nemegtosaurus, Tapuiasaurus, and Xianshanosaurus. Regardless of a spree of cladistic studies by Poropat et al. (2015), Wilson et al. (2016), and Carballido et al. (2017) whereby Nemegtosauridae sensu Zaher et al. (2011) is recovered as paraphyletic/polyphyletic in part due to the widespread distribution of putative cranial synapomophies of Nemegtosauridae among different lithostrotian lineages, the cladistic composition of derived Lithostrotia as per Wang et al. with a Nemegtosaurus+Tapuiasaurus+Xianshanosaurus sister to a larger clade formed by titanosaur taxa currently assigned to Saltasauridae and Aeolosaurini would indicate that derived titanosaurs were widespread in Eurasia by the Hauterivan-early Aptian, as indicated by the colossosaurian affinities of Tengrisaurus and Volgatitan from the Hauterivian and Barremian-early Aptian of Russia respectively (Averianov and Efimov 2018; Averianov et al. 2021) and the lithostrotian nature of NHMUK R5333, a specimen comprising a few caudal vertebrae from the Barremian-age Wessex Formation of the Isle of Wight in southern England (Upchurch et al. 2011). Given that Ninjatitan hails from the Berriasian-Valanginian Bajada Colorada Formation of Argentina and indicates a probable Gondwanan origin for Titanosauria (Gallina et al, 2021), it is possible that derived titanosaur clades spread from Gondwana into Eurasia by the Hauterivan-Barremian because Hamititan has strongly procoelous middle caudal vertebrae as in all derived titanosaurs. However, the placement of Hamititan as sister to Aeolosaurus and Rapetosaurus creates a ghost lineage within Lithostrotia, as Hamititan is much older than either  Aeolosaurus or Rapetosaurus. It is therefore possible that future study could find Hamititan to occupy a position more basal than Aeolosaurus, Isisaurus, Nemegtosaurus, Rapetosaurus, or Xianshanosaurus because Navarro (2019) classifies Tapuiasaurus and Yongjinglong with an unnamed taxon from the Adamantina Formation and (possibly) Gobititan in a new lithostrotian clade sister to Isisaurus and Rapetosaurus.  

When taking into account the phylogenetic results obtained by Wang et al. for Titanosauria, it should be noted that Andesaurus, traditionally recovered as the basalmost titanosaur, clusters with the East Asian taxa Abdarainurus and Huabeisaurus, suggesting a potential Gondwanan origin for titanosaurs with opisthocoelous and slightly procoelous anterior caudals. Since slightly procoelous anterior caudal vertebrae have been reported in Andesaurus (Mannion and Calvo 2011) and Ninjatitan (Gallina et al. 2021), it is possible that the distinctive caudal morphology of Abdarainurus noted by Averianov and Lopatin (2020) could be reflective of evolutionary reversals in the caudal morphology of some basal titanosaurs due to the strongly opisthocoelous caudals of Abdarainurus compared to slight opisthocoely in some caudal vertebrae of Huabeisaurus and Sonidosaurus (D'Emic et al. 2013; Xu et al. 2006). Of further importance is the fact that the cladistic analysis of Hamititan and Silutitan agrees with Mannion et al. (2019) and Poropat et al. (2021) in recovering Diamantinasaurus and Savannasaurus outside Lithostrotia, because Baotianmansaurus, Dongyangosaurus, Huabeisaurus and Savannasaurus have amphicoelous anterior and middle caudals (Mannion et al. 2019; Poropat et al. 2021), indicating that more than one branch of non-lithostrotian titanosaurs with non-procoelous anterior and middle caudal vertebrae evolved in East Asia by the Aptian-Albian. As noted by Wang et al., even though the taxa Daxiatitan and Dongbeititan have strongly procoelous caudal vertebrae, they differ from Hamititan in lacking ventrolateral ridges on the caudal centra. The lithostrotian nature of the titanosaur specimen NHMUK R5333 from the Isle of Wight and the presence of both euhelopodids and lithostrotians from the Barremian-early Aptian of East Asia should provide context for understanding the macroevolution of somphospondyls in Laurasia during the Berriasian-Albian interval, considering that Malawisaurus from Malawi and Normanniasaurus from northern France possess procoelous anterior caudal vertebrae and amphicoelous middle caudal vertebrae (Gomani 2005; Le Loeuff et al. 2013), and some European titanosaurs from the Early-Middle Cretaceous apparently had Gondwanan origins (Mocho et al. 2019).

Although knowledge of titanosaurs from the earliest Cretaceous is still a work in progress, the recovery of Hamititan as a derived lithostrotian in contrast to other Asian titanosaurs from the Berriasian-Albian interval demonstrates that almost every titanosaur clade was present in East Asia by the Barremian-Aptian, being co-eval with Silutitan and other euhelopodids. Considering that Andesaurus clusters with Abdarainurus and Huabeisaurus at the base of Titanosauria and has slightly procoelous caudals in contrast to the latter two, but also the amphicoely in the caudals of Savannasaurus, titanosaurs with procoelous caudal vertebrae may have spread to East Asia from South America and Africa during the Barremian-Aptian, beginning a gradual process of displacing non-titanosaur somphospondyls and titanosaurs with amphicoelous caudal vertebrae. As more little-known Asian somphospondyl taxa are included in phylogenetic studies, a better understanding of somphospondyl macroevolution in East Asia will emerge.   

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.

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

Averianov, A., Sizov, A. V., and Skutschas, P. P., 2021. Gondwanan affinities of Tengrisaurus, Early Cretaceous titanosaur from Transbaikalia, Russia (Dinosauria, Sauropoda). Cretaceous Research 122:104731. doi:10.1016/j.cretres.2020.104731

Carballido, J. L., D. Pol, A. Otero, I. A. Cerda, L. Salgado, A. C. Garrido, J. Ramezani, N. R. Cúneo, and J. M. Krausem 2017. A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs. Proceedings of the Royal Society B: Biological Sciences 284(1860):20171219. doi:10.1098/rspb.2017.1219.

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.

Dong, Z.M., Paik, I.S., and Kim, H.J., 2001. A preliminary report on a sauropod from the Hasandong Formation (Lower Cretaceous), Korea. pp. 41-53. In Deng, T.; Wang, Y. (eds.). Proceedings of the Eighth Annual Meeting of the Chinese Society of Vertebrate Paleontology. Beijing: China Ocean Press.

Gallina, P. A., J. I. Canale, and J. L. Carballido, 2021. The earliest known titanosaur sauropod dinosaur. Ameghiniana 58(1):35–51. doi:10.5710/AMGH.20.08.2020.3376.

Gomani, E.M., 2005. Sauropod Dinosaurs from the Early Cretaceous of Malawi, Africa. Palaeontologia Electronica: 8.1.27A.

Le Loeuff, J.; Suteethorn, S., and Buffetaut, E., 2013. A new sauropod dinosaur from the Albian of Le Havre (Normandy, France). Oryctos 10: 23–30. 

Mannion, P. D., and J. O. Calvo, 2011. Anatomy of the basal titanosaur (Dinosauria, Sauropoda)  Andesaurus delgadoi from the mid-Cretaceous (Albian–early Cenomanian) Río Limay Formation, Neuquén Province, Argentina: implications for titanosaur systematics. Zoological Journal of the Linnaean Society 163(1):155–181.

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.

Mocho, P., A. Pérez-García, M. Martín Jiménez, and F. Ortega, 2019. New remains from the Spanish Cenomanian shed light on the Gondwanan origin of European Early Cretaceous titanosaurs. Cretaceous Research 95:164–190. doi:10.1016/j.cretres.2018.09.016.

Navarro, B. A., 2019. Postcranial osteology and phylogenetic relationships of the Early Cretaceous titanosaur Tapuiasaurus macedoi Zaher et al. 2011. Master's dissertation. Instituto de Biociências da Universidade de São Paulo, São Paulo.

Poropat, S.F., Upchurch, P., Mannion, P.D., Hocknull, S., Kear, B.P., Sloan, T., Sinapius, G.H.K., and Elliott, D.A., 2016. Revision of the sauropod dinosaur Diamantinasaurus matildae Hocknull et al. 2009 from the mid-Cretaceous of Australia: Implications for Gondwanan titanosauriform dispersal.  Gondwana Research 27 (3): 995–1033. doi: 10.1016/j.gr.2014.03.014

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  

Upchurch, P., P. D. Mannion, and P. M. Barrett, 2011. Sauropod dinosaurs. pp. 476–525. In: D. J. Batten (eds.). English Wealden fossils. The Palaeontological Association, London, United Kingdom. Field Guide to Fossils 14.

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.

Wilson, J. A., D. Pol, A. B. Carvalho, and H. Zaher. 2016. The skull of the titanosaur Tapuiasaurus macedoi (Dinosauria: Sauropoda), a basal titanosaur from the Lower Cretaceous of Brazil. Zoological Journal of the Linnean Society 178(3):611–662.

Xu X., Zhang X., Tan Q., Zhao X., and Tan L., 2006. A new titanosaurian sauropod from Late Cretaceous of Nei Mongol, China. Acta Geologica Sinica 80(1): 20–26.

Zaher, H., D. Pol, A. B. Carvalho, P. M. Nascimento, C. Riccomini, P. Larson, R. Juarez-Valieri, R. Pires-Domingues, N. J. da Silva, Jr., and D. de Almeida Campos. 2011. A complete skull of an Early Cretaceous sauropod and the evolution of advanced titanosaurians. PLoS ONE 6(2):e16663. doi:10.1371/journal.pone.0016663.

Amanzia greppini: a Swiss giant

We frequently associate Switzerland with chocolates, various types of cheese like Swiss cheese, and the Matterhorn mountain, not to mention that it hosts the headquarters of the United Nations and Red Cross. However, mainly lost in talk about Switzerland is the fact that fossils of dinosaurs and marine reptiles have been in Mesozoic deposits in Switzerland.

Right humerus NMB M.H. 260, left femur NMB M.H. 372, caudal vertebrae NMB M.H. 280 and NMB M.H. 255, and pedal ungual phalanx NMB M.H. 270, part of the syntype series of Amanzia greppini

The history of the discovery of Amanzia begins in the 1860s, when local workers found fossil reptile remains in a limestone quarry of the Late Jurassic (early Kimmeridgian) Reuchenette Formation in the Basse Motagne, near Moutier, Bern, northwestern Switzerland. Some of these fossils were sold to collectors, but others were placed in the collections of the Naturhistorisches Museum Basil (NHB) in Basel, Switzerland at the notice of geologist Jean-Baptiste Greppin. Because these remains were found in association with the holotype tooth of the indeterminate ceratosaurian theropod (NHB M.B. 350), Greppin (1870) assumed that these remains were conspecific with 'M. meriani'. Postcranial reptile remains included in 'M. meriani' included: NMB M.H. 265 (cervical vertebra), NMB M.H. 266 (prezygapophysis of cervical vertebra), NMB M.H. 267–268 (cervical vertebrae), NMB M.H. 239 (caudal vertebra), NMB M.H. 245 (caudal vertebra), NMB M.H. 252–254 (caudal vertebrae), NMB M.H. 258 (caudal vertebra), NMB M.H. 271 (caudal vertebra), NMB M.H. 275–280 (caudal vertebrae), NMB M.H. 297 (caudal vertebra), NMB M.H. 324 (caudal vertebra), NMB M.H. 353–355 (caudal vertebrae), NMB M.H. 286 (caudal neural spine), NMB M.H. 300 (caudal neural spine), NMB M.H. 369–370 (caudal neural spines), NMB M.H. 291 (dorsal rib), NMB M.H. 306 (dorsal rib), NMB M.H. 344 (scapula), NMB M.H. 368 (scapula), NMB M.H. 284 (coracoid), NMB M.H. 260 (humerus), NMB M.H. 341 (humerus), NMB M.H. 259 (ulna), NMB M.H. 340 (ulna), NMB M.H. 264 (radius), NMB M.H. 346–347 (pubes), NMB M.H. 359 (pubis), NMB M.H. 358 (ischium), NMB M.H. 262 (femur), NMB M.H. 349 (femur), NMB M.H. 372 (femur), NMB M.H. 339 (tibia), NMB M.H. 342 (tibia), NMB M.H. 282 (fibula), NMB M.H. 373 (fibula), NMB M.H. 387 (fibula, ex NMB M.H. 374 and NMB M.H. 386), NMB M.H. 246 (metatarsal), NMB M.H. 269–270 (ungual phalanges), NMB M.H. 285 (long bone fragment), NMB M.H. 332 (bone fragment), NMB M.H. 345 (long bone fragment).  The quarry that yielded the remains of Amanzia greppini was in long-term use by the Swiss Army, and entry into the quarry was impossible until the late 1990s; at the current time of writing, the quarry near Moutier is now abandoned.

Friedrich von Huene (1875-1969), describer of Amanzia greppini

In the early 1920s, German paleontologist Werner Janensch examined the dinosaur remains unearthed at the Basse Motagne, and while he agreed that the 'Megalosaurus meriani' holotype was from a theropod (meriani was referred to the genus Labrosaurus [a junior synonym of Allosaurus] by Janensch [1920]), he noted that the postcranial remains associated with NHB M.B. 350 were actually referrable to Sauropoda. Based on this information from Janensch, von Huene (1922) described the sauropod remains as a new species of the Early Cretaceous macronarian genus Ornithopsis, O. greppini, the species name honoring the original describer of the sauropod material from the Reuchenette Formation. The syntype series of greppini constitutes as many as four individuals due to the presence of duplicate limb elements. Later, von Huene (1927) realized that O. greppini was generically distinct from the type species of Ornithopsis and referred it to the new eusauropod genus Cetiosauriscus, which is know only from the Middle Jurassic (Callovian) of southern England. Steel (1970) did not consider Cetiosauriscus to be a distinct genus from Cetiosaurus, and thus referred greppini to Cetiosaurus as Cetiosaurus greppini. McIntosh (1990), for his part, followed von Huene (1927) in referring the eusauropod material from the Reuchenette Formation to Cetiosauriscus. 

Skeletal restoration of Amanzia greppini with preserved elements in blue (from Schwarz et al. 2020)

Despite being significant as the most complete Jurassic dinosaur from Switzerland, "Ornithopsis" greppini itself received almost no attention in the literature until a review of dinosaur body and trace fossils from Switzerland was published by Meyer and Thüring (2003). They agreed with McIntosh (1990) that greppini is referable to Cetiosauriscus, noting that the two forms share anteroposteriorly short anterior caudal vertebrae and more elongate distal vertebrae, while pointing out that greppini differs from the Cetiosauriscus type species, C. stewarti, in having a more robust humerus whose shaft is proportionally more waisted anteriorly and with a more prominently developed deltopectoral crest. Schwarz et al. (2007a) described the preservation of cartilage in the syntypes of this taxon, and they later (2007b; see also Hofer 2005) indicated that "Ornithopsis" greppini constitutes a new genus of non-neosauropod eusauropod distinct from Cetiosauriscus based of preparation of "O." greppini syntypes by Antoine Heitz. A re-description of the taxon was finally published by Schwarz et al. (2020), who erected the new genus Amanzia for "O." greppini and phylogenetically recovered this taxon as a member of the eusauropod clade Turiasauria. The genus name was chosen to honor Amanz Gressly (1814-1865), who excavated the type material of the basal sauropodomorph Gresslyosaurus ingens in northern Switzerland in the late 1850s.

Although Amanzia was almost neglected by the paleontological community from the time of its initial description until the early 2000s, its journey from a referred specimen of a dubious theropod to being a sauropod, initially as a species of Ornithopsis, Cetiosauriscus, and Cetiosaurus before being finally recognized as a distinct genus in its own right is quite a superfluous one. Considering that the vast majority of sauropod taxa from Kimmeridgian-Tithonian marine deposits in Europe are known only from isolated elements, Amanzia, along with the dwarf macronarian Europasaurus from northwestern Germany, constitute the most complete Late Jurassic sauropods from central Europe. Who knows, one day a diplodocoid or a euhelopodid about as skeletally complete as Amanzia will be found in Late Jurassic marine deposits in western or central Europe.

References:

Greppin, J. P., 1870. Description géologique du Jura bernois et de quelques districts adjacents. Matériaux pour la carte géologique de la Suisse 8: 1–357.

Hofer, C., 2005. Osteologie und Taxonomie von Cetiosauriscus greppini (Huene 1927a, b) aus dem späten Jura von Moutier (Reuchenette Formation) [Osteology and taxonomy of Cetiosauriscus greppini (Huene 1927a, b) from the Late Jurassic of Moutier (Reuchenette Formation)]. Unpublished thesis, University of Basel. 70 pp.

Janensch, W., 1920. Ueber Elaphrosaurus bambergi und die Megalosaurier aus den Tendaguru-Schichten Deutsch-Ostafrikas. Sitzungsberichte der Gesellschaft Naturforschender Freunde zu Berlin 1920: 225–235.

McIntosh, J. S., 1990. Sauropoda. pp. 345-401. In: D. B. Weishampel, P. Dodson, & H. Osmolska (eds.), The Dinosauria. Berkeley: University of California Press.

Meyer, C. A., and Thüring, B., 2003. Dinosaurs of Switzerland. Comptes Rendus Palevol 2: 103–117.

Schwarz, D., Wings, O., and Meyer, C. A., 2007a. Super sizing the giants: first cartilage preservation at a sauropod limb joint. Journal of the Geological Society 164: 61–65. 

Schwarz, D., Wings, O., & Meyer, C. A., 2007b. Taxonomische und systematische Revision von Cetiosauriscus greppini (Sauropoda). p. 147. In: O. Elicki & J. W. Schneider (Eds.). Fossile Ökosysteme (Vol. 36). Wissenschaftliche Mitteilungen, Institut für Geologie: Freiberg.

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

Steel, R., 1970. Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology. Part 14. Saurischia. Stuttgart: Gustav-Fischer-Verlag.

von Huene, F., 1922. Ueber einen Sauropoden im obern Malm des Berner Jura. Eclogae Geologicae Helvetiae 17: 80–94.

von Huene, F., 1927. Short review of the present knowledge of the Sauropoda. Memoirs of the Queensland Museum 9: 121–126.

Are Qijianglong and Mamenchisaurus anyuensis of Middle Cretaceous age?

In a couple of scientific papers, Liu et al. (2017) and Wang et al. (2019) considered the Suining Formation (which overlies the Shangshaximiao Formation) to be of Aptian age based on U-Pb radiometric dating of detrital zircons from this unit, rather than Late Jurassic as stated by Dong et al. (1983) and Peng et al. (2005) based on tenuous biostratigraphic correlations. This conclusion potentially threw a wrench into the evolution of the eusauropod clade Mamenchisauridae, because the ages that Liu et al. (2017) and Wang et al. (2019) obtained for the Suining Formation indicated that mamenchisaurids survived until the middle Cretaceous. As the mamenchisaurid Qijianglong guokr and some referred specimens of Mamenchisaurus anyuensis are known from the Suining Formation (the holotype of M. anyuensis hails from the overlying Penglaizhen Formation), it occurred to me that any suggestion of the Penglaizhen and Suining Formations being much younger than the Shangshaximiao Formation once more reinforced the long-overdue need for a revision of Mamenchisaurus by implying that anyuensis would have to be assigned a new genus. In the meantime, however, Huang (2019) disputed the conclusion by Liu et al. (2017) regarding the age of the Suining Formation, arguing that the dating obtained from detrital zircons was affected by metamorphism and that the Suining Formation was not as young as concluded by Liu et al. (2017), asserting instead that the Suining Formation straddles the Tithonian-Berriasian boundary.

To begin addressing the question of whether or not Mamenchisaurus anyuensis and Qijianglong are of Middle Cretaceous age, it is imperative to analyze the available biostratigraphic evidence cited by Huang (2019) to make the case that the Suining and Penglaizhen Formations are older than asserted by Liu et al. (2017) and Wang et al. (2019) as well as state-of-the-art knowledge of mamenchisaurid evolution. Although Deng et al. (2015) and Wang & Gao (2012) date the Qigu Formation to 157-167 million years (late Callovian-early Oxfordian) based on U-Pb radiometric dating of detrital zircons from that unit, Huang (2019) asserts that the zircons used to determine this age estimate were likely recycled from the Xishanyao Formation, as volcanic rocks and tuffs have yet to be found in the Xishanyao Formation. As noted by Huang (2019), fossils of the ostracods Darwinula and Timiriasevia (which are of Late Jurassic age) appear in the lower parts of the Qigu and Suining Formations, whereas the Early Cretaceous ostracod Djungarica is present in the upper parts of the Qigu and Suining Formations but also the Penglaizhen Formation, indicating a latest Kimmeridgian-early Berriasian age for the Suining Formation and a late Berriasian-early Valanginian age for the Penglaizhen Formation. As Wang et al. (2019) admit, the traditional ages assigned to mamenchisaurid taxa from the Shangshaximiao, Suining, and Penglaizhen Formations by Dong (1980), Dong et al. (1983), and Peng et al. (2005) were based on tenuous stratigraphic correlations, and the age they assign to all Mamenchisaurus species from the Shangshaximiao Formation is largely consistent with the Oxfordian-Kimmeridgian age advocated for the Shangshaximiao by Huang (2019). Since the U-Pb radiometric dates obtained from detrital zircons from the Qigu and Suining Formations by Deng et al. (2015) and Wang et al. (2019) appear to have been subject to extraneous geologic factors like metamorphosis and recycling, it is highly reasonable to not rule out the possibly that the Suining and Penglaizhen Formations are of latest Kimmeridgian to Berriasian age rather than latest Aptian-Albian because the Cangxi Formation (located in the same basin as the Suining and Penglaizhen Formations) is of Berriasian-Valanginian age based on biostratigraphy (Hou et al. 2020).

At the current time of writing, an indeterminate cervical vertebra reported by Suteethorn et al. (2013) from the latest Tithonian-early Berriasian Phu Kradung Formation of northeastern Thailand is the only record of a mamenchisaurid from the Cretaceous, and if Wang et al. (2019) are correct, there would a be vast temporal gap between the Phu Kradung material and the mamenchisaurid taxa from the Suining and Penglaizhen Formations. No mamenchisaurid fossils have been found in the Valanginian-middle Aptian interval so far, even though basal eusauropod clade Turiasauria is present in Early Cretaceous deposits. Some of the fossils of Mamenchisaurus anyuensis have been found in the upper part of the Suining Formation, so the age of the Penglaizhen and upper Suining Formations would place M. anyuensis in the Berriasian-Valanginian interval. Since the youngest turiasaur fossils are from the Barremian-age Yellow Cat Member of the Cedar Mountain of Utah and the Cangxi Formation is slightly younger than the Penglaizhen Formation, a Berriasian-Valanginian or Hauterivan-Barremian age for Mamenchisaurus anyuensis and Qijianglong cannot be discounted, because no radiometric dates have been obtained for the Penglaizhen Formation.

Based on an analysis of the biostratigraphic evidence cited by Huang (2019) for the age of the Suning and Penglaizhen Formations as well as the age of geologic units underlying and overlying both the Suning and Penglaizhen Formations, along with prior knowledge of basal eusauropod evolution during the Early Cretaceous, it can be prudent to conclude that the Middle Cretaceous age proposed for Qijianglong and Mamenchisaurus anyuensis by Wang et al. (2019) is less likely than that concluded by Huang (2019) because of the age of the Qigu Formation and the presence of the earliest Cretaceous ostracod Djungarica in the upper Suining and Penglaizhen Formations. Even if Mamenchisaurus anyuensis and Qijianglong are not as young as proposed by Wang et al. (2019), the earliest Cretaceous age of these taxa doesn't diminish their significance, because they would still be younger than other nominal Mamenchisaurus species, and thus about the same age as the Phu Kradung mamenchisaurid, in which case Mamenchisaurus anyuensis would need a new generic name.

References:

Deng, S., Wang, S., Yang, Z., Lu, Y., Li, X., Hu, Q., An, C., Xi, D., and Wan, X., 2015. Comprehensive study of the Middle-Upper Jurassic strata in the Junggar Basin, Xinjiang (in Chinese). Acta Geosciences Sinica 36: 559–574.

Dong, Z.M. 1980. The dinosaurian faunas of China and their stratigraphic distribution. Journal of Stratigraphy 4: 256-263.

Dong, Z., Zhou, S. & Zhang, Y. 1983. Dinosaurs from the Jurassic of Sichuan. Palaeontologica Sinica 162: 1-151.

Hou, X.W., Shi, Z.J., Sun, Z.X., Tan, Z.Y., & Tian, X.S., 2020. The sporopollen assemblages in the Early Cretaceous red sediments in Cangxi area, northern Sichuan Basin and their geological significance. Geological Review 66 (3): 727-738. https://doi.org/10.16509/j.georeview.2020.0 3.014

Huang, D., 2019. Jurassic integrative stratigraphy and timescale of China. Science China Earth Sciences 62 223–255.

Liu G, Dong S, Chen X, Cui J. 2017. Detrital zircon U-Pb dating of Suining Fm. sandstone from the Daba Mountains, northeastern Sichuan and its stratigraphic implications. Palaeoworld 26: 380–395. 

Peng, G.Z., Ye, Y., Gao, Y.H., Shu, C.K. & Jiang, S. 2005. Jurassic Dinosaur Faunas in Zigong. People’s Publishing House of Sichuan, Chengdu, China.

Suteethorn, S., Le Loeuff, J., Buffetaut, E., Suteethorn, V., and Wongko, K. 2013. First evidence of a mamenchisaurid dinosaur from the Upper Jurassic–Lower Cretaceous Phu Kradung Formation of Thailand. Acta Palaeontologica Polonica 58 (3): 459–469.

Wang, J., Norell, M. A., Pei, R., Ye, Y. and Chang, S.-C., 2019. Surprisingly young age for the mamenchisaurid sauropods in South China. Cretaceous Research 104: 104176.

Dzharatitanis and implications for the biogeography of rebbachisaurids in the Northern Hemisphere

Rebbachisaurids are a clade of diplodocoids largely known from South America and Africa, with minor occurrences in western Europe and Croatia (Upchurch et al. 2004; Fernández-Baldor et al., 2011; Fanti et al. 2015; Wilson and Allian 2015; Taylor 2018). Recently, the temporal and geographical distribution of Rebbachisauridae was extended to the Late Jurassic of western North America when Carpenter (2018) reassigned the mega-sauropod Amphicoelias fragillimus Cope, 1878 to Rebbachisauridae and renamed it Maraapunisaurus. Now, however, Averianov and Sues (2021) have described a new rebbachisaurid from the Late Cretaceous (Turonian) Bissekty Formation of Uzbekistan, Dzharatitanis kingi, on the basis of an isolated anterior caudal vertebra. The description of Dzharatitanis renders this genus the first occurrence of a rebbachisaurid from Asia, considering that diplodocoids were basically unknown from the Asian landmass until the description of the dicraeosaurid Lingwulong by Xu et al. (2018), and rebbachisaurids are known only in Laurasia from western North America, Spain, England, and Croatia.

The holotype of Dzharatitanis kingi, USNM 538217, was originally placed as Titanosauria indeterminate by Sues et al. (2015) in their discussion of titanosaur material collected from the Bissekty Formation of Uzbekistan. Subsequently, Averianov and Sues (2017) noted that USNM 538217 was most similar to the anterior caudals of Baotianmansaurus henanensis is being anteroposteriorly short but differed in being taller. They cautioned that the neural arch of the only known anterior caudal in Baotianmansaurus is incomplete, making it unclear if it had additional fossae like those present in Dongyangosaurus and USNM 538217. The decision taken by Averianov and Sues (2021) to reclassify USNM 538217 as a new taxon of rebbachisaurid thus raises a number of questions as to why it was originally classified in Titanosauria before being removed from this clade: how does Dzharatitanis differ from macronarian sauropods if it was initially interpreted as being similar to Baotianmansaurus?

In their description of USNM 538217, Sues et al. (2015) noted that this specimen possesses a transverse process (caudal rib) rising from both the centrum and the neural arch just above the mid-height of the centrum, and that it is triangular and its dorsal margin extends ventrolaterally, unlike flagellicaudatans and some rebbachisaurids. and that the neural spine of the Dzharatitanis kingi holotype has a large pneumatic cavity with a complex, internal structure comprising variously sized chambers as in the lognkosaurian titanosaur Futalognkosaurus. A prominent, deep postzygapophyseal spinodiapophyseal fossa in USNM 538217 was described as being similar to that of Dongyangosaurus in possessing four foramina on the left and three openings on the right side, with two of the foramina being situated at the base of the fossa just dorsal to the postzygodiapophyseal lamina. The paper by Averianov and Sues (2021) lists several characters to distinguish Dzharatitanis from titanosauriforms and support a referral to Diplodocoidea and specifically to Rebbachisauridae: (1) slightly convex or flat anterior articular surface of the first caudal centrum; (2) absence of the hyposphenal ridge; (3) absence of pleurocoels; (4) triangular lateral process on neural spine; and (5) composite lamina on the dorsal part of the lateral surface of the neural spine formed by the fusion of the SPRL and SPOL. It is highly probable that the authors took note of the varying presence or absence of pleurocoels on the first caudal vertebrae among rebbachisaurids and the presence of pleurocoels in many titanosauriforms to express second thoughts about USNM 538217 belonging to Titanosauria, because they identify the holotype as having slight opisthocoely and lacking chevron facets, which they note as being present among rebbachisaurids. The presence of opisthocoely in non-titanosauriform neosauropods is noteworthy because anterior caudal vertebrae with procoely have been documented for both titanosaurs and non-titanosaur sauropods (Wilson and Upchurch 2003).

When placing the age and location of Dzharatitanis into a broader contextual pattern of rebbachisaurid biogeography, it is important to note that almost all other rebbachisaurid taxa except Demandasaurus, Maraapunisaurus, and Xenoposeidon are present in Gondwana only, and that Katepensaurus is about the same geologic age as Dzharatitanis, hailing from the Cenomanian-Turonian Bajo Barreal Formation of Chubut Province, Argentina. All Early Cretaceous rebbachisaurids from Europe are of Valanginian to early Aptian age, and no rebbachisaurid has yet been reported from the Berriasian to Cenomanian of East Asia. Since Dzharatitanis is younger than other Cretaceous rebbachisaurids found in Laurasia, it is possible that a number of rebbachisaurids entered central Asia from Europe, or that Dzharatitanis was descended from an as-yet-undiscovered Early Cretaceous rebbachisaurid from East Asia, because a land connection existed between Europe and Asia during the Berriasian-Cenomanian (Baraboshkin et al. 2003) and the record of Laurasian rebbachisaurids is very scanty.

Update: A new paper by Lerzo et al. (2021) questions the rebbachisaurid placement of Dzharatitanis and instead re-assigns this genus to the titanosaur clade Lognkosauria based on comparisons with the caudal vertebrae of derived titanosaurs, including members of Colossosauria. In tandem with the recent paper by Averianov et al. (2021) classifying the genera Normanniasaurus and Volgatitan in the clade containing members of Colossosauria, the formal reclassification of  Dzharatitanis as a titanosaur sheds new light on the early biogeography of Colossosauria, given that this clade was long restricted to Gondwana.  

References:

Averianov, A.O., and Sues, H-D., 2017. Review of Cretaceous sauropod dinosaurs from Central Asia. Cretaceous Research 69:184–97.

Averianov, A., and Sues, H-D., 2021. First rebbachisaurid sauropod dinosaur from Asia. PLoS ONE 16(2): e0246620. 

Averianov, A. O., Sizov, A. V., and Skutschas, P. P., 2021. Gondwanan affinities of Tengrisaurus, Early Cretaceous titanosaur from Transbaikalia, Russia (Dinosauria, Sauropoda). Cretaceous Research 122: 104731. doi:10.1016/j.cretres.2020.104731.

Baraboshkin, E.Y., Alekseev, A.S., and Kopaevich, L.F., 2003. Cretaceous palaeogeography of the North-Eastern Peri-Tethys. Palaeogeography, Palaeoclimatology, Palaeoecology 196(1–2):177–208.

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. doi:10.31711/giw.v5i0.28.

Fanti, F., Cau, A., Cantelli, L., Hassine, M., and Auditore, M., 2015. New Information on Tataouinea hannibalis from the Early Cretaceous of Tunisia and Implications for the Tempo and Mode of Rebbachisaurid Sauropod Evolution. PLoS ONE 10(4): e0123475. 

Lerzo, N.L., Carballido, J.L., and Gallina, P.A., 2021. Rebbachisaurid sauropods in Asia? A re-evaluation of the phylogenetic position of Dzharatitanis kingi from the Late Cretaceous of Uzbekistan. Publicación Electrónica de la Asociación Paleontológica Argentina. 21 (1): 18–27. doi:10.5710//PEAPA.24.03.2021.389

Sues, H-D,, Averianov, A.O., Ridgely, R.C., and Witmer, L.M., 2015. Titanosauria (Dinosauria: Sauropoda) from the Upper Cretaceous (Turonian) Bissekty Formation of Uzbekistan. Journal of Vertebrate Paleontology 35(1):e889145.

Taylor, M.P., 2018. Xenoposeidon is the earliest known rebbachisaurid sauropod dinosaur. PeerJ 6:e5212 https://doi.org/10.7717/peerj.5212

Torcida Fernández−Baldor, F., Canudo, J.I., Huerta, P., Montero, D., Pereda Suberbiola, X., and Salgado, L. 2011. Demandasaurus darwini, a new rebbachisaurid sauropod from the Early Cretaceous of the Iberian Peninsula. Acta Palaeontologica Polonica 56 (3): 535–552.

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

Wilson, J.A. and Upchurch, P., 2003. A revision of Titanosaurus Lydekker (Dinosauria – Sauropoda), the first dinosaur genus with a “Gondwanan” distribution. Journal of Systematic Palaeontology 1(3): 125-160.

Wilson, J.A., Allain, R., 2015. Osteology of Rebbachisaurus garasbae Lavocat, 1954, a diplodocoid (Dinosauria, Sauropoda) from the early Late Cretaceous–aged Kem Kem beds of southeastern Morocco. Journal of Vertebrate Paleontology 35(4):e1000701.

Xu, X., Upchurch, P., Mannion, P. D., Barrett, P. M., Regalado-Fernandez, O.R., Mo, J., Ma, J. and Liu, H., 2018. A new Middle Jurassic diplodocoid suggests an earlier dispersal and diversification of sauropod dinosaurs. Nature Communications 9, 2700. doi:10.1038/s41467-018-05128-1

anterior caudal in Baotianmansaurus henanensis is poorly preserved

and thus it is unclear if it had additional fossae like those pres

Reflections on the Whitlock and Wilson Mantilla (2020) paper on Smitanosaurus

Whitlock and Wilson Mantilla (2020) recently published a redescription of the diplodocoid sauropod "Morosaurus" agilis Marsh, 1889, erecting the new genus Smitanosaurus for it and assigning it to the family Dicraeosauridae based on comparisons with other flagellicaudatan diplodocoids from the Morrison Formation and phylogenetic analysis. The holotype of Smitanosaurus agilis, USNM 5384, was originally considered referable to Diplodocus longus by Othniel Charles Marsh judging from archival records, but was later described as a new species of the genus Morosaurus by Marsh (1889). Later, Gilmore (1907) believed it to be allied with either Brachiosauridae or Haplocanthosaurus; Osborn and Mook (1919, 1921) formally synonymized Morosaurus with Camarasaurus, and Gilmore (1925:352) clearly stated that M. agilis was not congeneric with other Camarasaurus species. McIntosh (1990) followed the suggestion by Gilmore (1907) that "M." agilis could be either a brachiosaurid or Haplocanthosaurus. Subsequently, an abstract by Tidwell et al. (2005) concluding that "Morosaurus" agilis represented a Diplodocus specimen as initially proposed by Marsh, and that it was a junior synonym of Diplodocus longus based on comparisons with skulls of diplodocid taxa from the Morrison Formation, including skulls referred to Diplodocus (AMNH 969, CM 3452, CM 11161, USNM 2672, USNM 2673). However, AMNH 969 and USNM 2673 were referred to Galeamopus hayi and G. pabsti respectively by Tschopp et al. (2015) and Tschopp & Mateus (2017), raising doubts about whether or not "Morosaurus" agilis is synonymous with Diplodocus. Suuwassea has been assigned to Dicraeosauridae by Whitlock (2011), but the placement of Smitanosaurus as a dicraeosaurid by Whitlock and Wilson Mantilla reveals a hitherto hidden diversity of dicraeosauids from the Morrison. The genus name Smitanosaurus is quite tongue-twisting, for it pays homage to not only J. August Smith, the discoverer of the holotype of S. agilis (USNM 5384), but also the Smithsonian Institution in which USNM 5384 was reposited.

Photograph and lateral view drawing of the holotype of Smitanosaurus agilis (after Gilmore 1907)

One anatomical aspect of Smitanosaurus agilis noted by Whitlock and Wilson Mantilla that deserves  attention is the presence of a postparietal foramen on the skull roof. Woodruff et al. (2017) considered the presence or absence of this feature in diplodocoids to be a influenced by ontogeny, with juvenile specimens possessing the postparietal foramen and adults lacking it. However, as Whitlock and Wilson Mantilla point out, the diplodocine specimen CM 3452 as well as CM 11255 (probably Barosaurus; Melstrom et al. 2016) lack a postparietal foramen despite being juvenile, while the adult apatosaurine specimen BYU 17096 has this feature, as does juvenile specimen SMM.P.84.15.3. Therefore, the presence or absence of a postparietal foramen is a non-ontogenetic trait among Morrison diplodocoid specimens. As detailed in a previous post, I did not rule out the possibility that the dicraeosaurid taxon Suuwassea could be a distinct taxon from other Morrison diplodocoids despite its holotype being a juvenile (Woodruff and Fowler 2012) because a postparietal foramen is absence in the diplodocine skull CM 11255 in spite of the juvenile nature of that specimen. The presence of a postparietal foramen in the Smitanosaurus agilis holotype and recovery of Suuwassea as derived within Dicraeosauridae by Whitlock and Wilson Mantilla (2020) confirms that despite being a juvenile, the holotype of Suuwassea emilieae (ANS 21122) is a distinct taxon in its own right rather than a Diplodocus juvenile as proposed by Woodruff and Fowler (2012). 

Cladistic analysis of Smitanosaurus from Whitlock and Wilson Mantilla (2020). Although not a diplodocid as concluded by Tidwell et al. (2005), the diplodocoid placement of Smitanosaurus as hinted as by Tidwell et al. is vindicated by the above cladogram. 

The recovery of Kaatedocus as a dicraeosaurid rather than a diplodocine diplodocid is quite surprising. This genus has been recovered as either closer to Barosaurus than to other diplodocines (Tschopp et al., 2015) or a basal diplodocine (Tschopp and Mateus 2017). Characters cited by Tschopp et al. (2015) as placing Kaatedocus in Diplodocinae include: (1) broad maxilla-quadratojugal contact (2) antorbital fenestra with concave dorsal margin; (3) ‘hooked’ prefrontal; (4) mandible without strong coronoid eminence; (5) direct crown–crown occlusion absent; (6) ‘box-like basal tubera;’ (7) 14–15 cervical vertebrae; and (8) posterior cervical neural spines with elongate lateral spine cavities. The first four of these characters are dismissed by Whitlock and Wilson Mantilla due to either the state of preservation in the holotype of Kaatedocus siberi (SMA 0004) or the fact that SMA 0004 does not preserve enough vertebrae. The concave dorsal margin of the antorbital fenestra is noted as present in the rebbachisaurid Nigersaurus, the ‘box-like’ basal tubera character is omitted by the authors from the cladistic analysis of Smitanosaurus "because the codified states conflate too many distinct morphologies in the current taxonomic scope", while a ‘hooked’ prefrontal is noted as absent in Kaatedocus and the presence of elongate cavities on the posterior cervical neural spines is inconsistently present among diplodocoid taxa. Whitlock and Wilson Mantilla (2020) assign Kaatedocus to Dicraeosauridae based on several characters, namely the presence of a postparietal foramen, an expanded crista prootica, a shelf overhanging the opening for cranial nerve V, and a flat medial margin of the prefrontal. The authors' assertion about the absence of dorsal vertebrae in the Kaatedocus siberi holotype that could provide a definite cervical/dorsal vertebral count is borne out by the fact that Apatosaurus louisae is the only apatosaurine diplocid for which 15 cervicals are preserved, while Diplodocus carnegii and Galeamopus pabsti are the only diplodocines known from specimens that preserve a complete cervical vertebral series (Tschopp et al. 2015). Moreover, given assignment of Suuwassea to Dicraeosauridae by Whitlock (2011), it is not hard to imagine four dicraeosaurid taxa in the Morrison Formation (Kaatedocus, Smitanosaurus, Suuwassea and MOR 592) because the Puesto Antigual Member of the La Amarga Formation has yielded two dicraeosaurids, Amargasaurus and Amargatitanis.

In retrospect, Othniel Charles Marsh was initially right in the first place about Smitanosaurus being a diplodocoid before changing his mind about the classification of USNM 5384, and although Tidwell et al. (2005) agree that USNM 5384 is a diplodocoid, doubts about their referral of Smitanosaurus agilis to Diplodocus longus due to the re-assignment of some putative skulls of D. longus to Galeamopus and the discovery of dicraeosaurid characters in USNM 5384 make clear that the diversity of Morrison non-diplodocid flagellicaudatans is greater than previously thought and that some traditional diplodocine synapomorphies are also present in some dicraeosaurids. 

References:

Gilmore, C.W., 1907. The type of the Jurassic reptile Morosaurus agilis redescribed, with a note on Camptosaurus. Proceedings of the United States National Museum 32: 151–165.

Gilmore, C.W., 1925.  A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument, Utah. Memoirs of the Carnegie Museum 10:347-384.

Marsh, O.C., 1889. Notice of new American Dinosauria. The American Journal of Science and Arts, series 3 38:331-336.

McIntosh, J.S., 1990. Sauropoda. pp. 345-401. In: Weishampel DB, Dodson P, Osmólska H, eds. The Dinosauria. Berkeley: University of California Press.

Melstrom, K.M., D’Emic, M.D., Chure, D.J., and Wilson, J.A., 2016. A juvenile sauropod dinosaur from the Late Jurassic of Utah, USA, presents further evidence of an avian style air-sac system. Journal of Vertebrate Paleontology e1111898.

Osborn, H. F., and Mook, C. C., 1919. Characters and Restoration of the sauropod genus Camarasaurus Cope from type material in the Cope Collection in the American Museum of Natural History. Proceedings of the American Philosophical Society 58 (6): 386-396.

Osborn, H. F., and Mook, C. C., 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History, New Series 3 (3): 247-387.

Tidwell, V., K. Carpenter, and C. Miles. 2005. A reexamination of Morosaurus agilis (Sauropoda) from Garden Park, Colorado. Journal of Vertebrate Paleontology 25 (supplement to 3):122A.

Tschopp, E., Mateus, O., and Benson, R.B.J., 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda) PeerJ 3:e857 https://doi.org/10.7717/peerj.857 

Tschopp, E., and Mateus, O., 2017. Osteology of Galeamopus pabsti sp. nov. (Sauropoda: Diplodocidae), with implications for neurocentral closure timing, and the cervico-dorsal transition in diplodocids. PeerJ 5:e3179 https://doi.org/10.7717/peerj.3179

Whitlock, J. A. 2011. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society 161: 872–915.

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. 

 

Woodruff, D. C., Fowler, D. W. and Horner, J. R., 2017. A new multi-faceted framework for deciphering diplodocid ontogeny. Palaeontologia Electronica 20.3.43A: 1–53.

Takeaways from the Averianov and Lopatin (2020) paper on Abdarainurus barsboldi

In a new paper, Averianov and Lopatin (2020) describe a new sauropod, Abdarainurus barsboldi, for a series of caudal vertebrae (including one middle caudal) from the Alagteeg Formation of Abdrant Nuru in the northern Gobi Desert, Mongolia. Titanosauriforms have been recorded from the Late Cretaceous of Mongolia, including the lithostrotian titanosaurs Nemegtosaurus, Opisthocoelicauda, and Quaesitosaurus as well as the euhelopodid somphospondylan Erketu. However, Averianov and Lopatin note that the holotype of   Abdarainurus barsboldi (PIN 5669/1) is quite distinct from other Mongolian titanosaurs in having opisthocoely present in the anterior and middle caudals, longitudinal ridges on the neural canal walls, postprezygapophyseal processes, a hypertrophied postspinal fossa, mediolaterally constricted neural spines, and a deep pocket-like spinodiapophyseal fossa covered laterally by high postzygodiapophyseal lamina. Bearing in mind comparisons of the Abdarainurus caudal vertebrae with those of other Asian titanosaurs, Averianov and Lopatin (2020) conduct a cladistic analysis that places Abdarainurus as a basal titanosaur in a clade with the basal titanosaurs Andesaurus and the somphospondylan Huabeisaurus (the latter recovered as a non-titanosaur somphospondylan by Mannion et al. 2019). Based on the results of the cladistic analysis as well as comparative anatomy, Averianov and Lopatin conclude that Abdarainurus could belong to an unknown clade of basal titanosaurs.

In comparing Abdarainurus with titanosaurs and derived non-titanosaur somphospondylans, Averianov and Lopatin note that the holotype is similar to the lithostrotian titanosaur Normanniasaurus in having the dorsoventral curvature of the transverse processes and the presence of a large spinodiapophyseal fossa at the base of the neural spine perforating the spine (the latter character listed as autapomorphic for Normanniasaurus by Le Loeuff et al. 2013), but differs in lacking strongly procoelous anterior caudal vertebrae. The authors then compare Abdarainurus to the taxon Sonidosaurus from the Iren Dabasu Formation of Inner Mongolia, China, and find the two forms similar to each other in having opisthocoelous centra, possibly widely separated postzygapophyses, and ridges on the proximal ramus of the anterior chevrons. Averianov and Lopatin (2020) acknowledge that Opisthocoelicauda and Borealosaurus both possess opisthocoely in the anterior and middle caudals respectively, but they note that opisthocoely is restricted to the anterior caudals in Opisthocoelicauda and that the middle caudals of Borealosaurus have more pronounced opisthocoely, a greatly convex ventral centrum margin in lateral view, shorter prezygapophyses, and an anteroposteriorly longer neural spine. 

Although Averianov and Lopatin (2020) assert that the phylogenetic position of Sonidosaurus is unclear despite the genus being similar to Abdarainurus in its opisthocoelus centra and other caudal features, they overlook the fact that Sonidosaurus is recovered as a titanosaur more derived than Andesaurus but outside Lithostrotia (which includes Opisthocoelicauda) by Ksepka and Norell (2010; figure 4). When recovering Abdarainurus as a basal titanosaur in a clade with Andesaurus and Huabeisaurus, the authors state that this result could be attributable to the dearth of existing knowledge about the early evolution of titanosaurs in general. This conclusion appears to hold water in several respects, in terms of morphology. For instance, postzygodiapophyseal lamina on the anterior caudal vertebrae remain unknown for Andesaurus because most of the anterior caudal vertebrae for this taxon are incomplete, so more complete material of Andesaurus would be essential to determine of the anterior caudals of Andesaurus had postzygodiapophyseal lamina. Additionally, the recovery of Huabeisaurus by Mannion et al. (2019) of Huabeisaurus as a somphospondylan closer to Titanosauria than to Euhelopodidae (contra D'Emic et al. 2013, who considered it a probable euhelopodid) raises the possibility that the characters cited by Averianov and Lopatin as supporting a clade formed by Abdarainurus, Andesaurus, and Huabeisaurus could end up being reflective of convergent evolution in more than one high-ranking somphospondylan clade, considering that Abdarainurus and Huabeisaurus are much younger than Andesaurus. The Hauterivan age of the titanosaur genus Volgatitan also reinforces the possibility that future study could find Abdarainurus to be distantly related to Andesaurus within Titanosauria, as Volgatitan is closely related to the colossosaurian clade Lognkosauria (Averianov and Efimov 2018).

Judging from the available morphological evidence, but also greater similarities to the titanosaur Sonidosaurus, it could be parsimonous to conclude that instead of belonging to an unknown high-ranking somphospondylan clade, Abdarainurus could belong to an unnamed clade of basal Asian titanosaurs outside Lithostrotia along with Baotianmansaurus, Dongyangosaurus, Sonidosaurus, and an unnamed titanosaur from the Bissekty Formation of Uzbekistan, as suggested by Averianov and Sues (2017). Although Abdarainurus is indeed unusual in the tail morphology, the lack of substantial material in the holotype leaves open the possibility that this genus could be closer to Lithostrotia (like Sonidosaurus) than to Andesaurus. 

Note: The cladistic analysis of Hamititan and Siluotitan by Wang et al. (2021) demonstrates that Abdarainurus and Huabeisaurus are sister taxa to the basalmost titanosaur Andesaurus, while Dongyangosaurus and Jiangshanosaurus (both about the same age as Andesaurus) are recovered as basal titanosaurs more primitive than Diamantinasauria and Lithostrotia. Therefore, the Wang et al. paper has made me revise my earlier take on the conclusions by Averianov and Lopatin (2020) regarding the phylogenetic placement of Abdarainurus within Titanosauria by suggesting that at least some very primitive titanosaurs with slightly procoelous anterior caudal vertebrae survived into the Campanian-Maastrichtian due to the older age of Andesaurus and the earliest Cretaceous basal titanosaur Ninjatitan (Gallina et al. 2021). Moreover, the Early Cretaceous age of Hamititan makes clear than titanosaurs with strongly procoelous anterior caudals diverged from basal titanosaurs with slightly procoelous anterior caudals in the early Cretaceous, so future study could confirm that some Late Cretaceous titanosaurs from Asia with slightly procoely in the anterior caudals are closely related to Andesaurus or Ninjatitan.     

References:

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

Averianov, A.O., and Lopatin, A.V., 2020. An unusual new sauropod dinosaur from the Late Cretaceous of Mongolia. Journal of Systematic Palaeontology. DOI: 10.1080/14772019.2020.1716402

Averianov, A., and Sues, H.-D., 2017. Review of Cretaceous sauropod dinosaurs from Central Asia. Cretaceous Research 69:184-197. DOI: 10.1016/j.cretres.2016.09.006

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

Gallina, P. A., J. I. Canale, and J. L. Carballido, 2021. The earliest known titanosaur sauropod dinosaur. Ameghiniana 58(1):35–51. doi:10.5710/AMGH.20.08.2020.3376.

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