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.

Ksepka, D. T. & Norell, M. A. 2010. The illusory evidence for Asian Brachiosauridae: new material of Erketu ellisoni and a phylogenetic reappraisal of basal Titanosauriformes. American Museum Novitates 3700: 1–27. doi:10.1206/3700.2

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

Mannion, P. D., Upchurch, P., Jin, X. & Zheng, W. 2019. New information on the Cretaceous sauropod dinosaurs of Zhejiang Province, China: impact on Laurasian titanosauriform phylogeny and biogeography. Royal Society Open Science 6: 191057. doi:10.1098/rsos.191057

Wang, X., K. L. N. Bandeira, R. Qiu, S. Jiang, X. Cheng, Y. Ma, and A. W. A. Kellner. 2021. The first dinosaurs from the Early Cretaceous Hami Pterosaur Fauna, China. Scientific Reports 11:14962. doi:10.1038/s41598-021-94273-7.