As far as I'm concerned, 2021 has been relatively quiet for new dinosaurs (great year for ophthalmosaurid ichthyosaurs, though; I might even learn to spell "ophthalmosaurid" correctly the first time through). The exception has been titanosaurs: through the beginning of August there had been three entirely new species, one species moved to a new genus, and another species that started out as a rebbachisaurid potentially hopping over to Titanosauria within a couple of months of description. Hamititan xinjiangensis makes another new addition. It was published this week (Wang et al. 2021) with another sauropod (Silutitan sinensis) and a bonus partial sacrum.
Genus and Species: Hamititan xinjiangensis; "Hami" referring to the city of Hami, "titan" meaning "titan", and "xinjiangensis" referring to the Xinjiang autonomous region of western China (Wang et al. 2021). Together they mean something akin to "Hami titan from Xinjiang".
Citation: 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.
Stratigraphy and Geography: H. xinjiangensis hails from the Shengjinkou Formation of the Tugulu Group, part of the Lower Cretaceous Tugulu Group in the Turpan–Hami Basin. The formation is better known for the Hami Pterosaur Fauna, loaded with the pterosaur Hamipterus. The holotype and only known specimen of H. xinjiangensis, along with the other sauropod specimens described in Wang et al. (2021), came from lacustrine sandstone. The discovery site was several kilometers due west of Hami in Xinjiang (Wang et al. 2021).
Holotype: HM V22 (Hami Museum, Hami, Xinjiang, China), consisting of seven articulated caudals and three partial chevrons, thought to represent caudals 4 through 10 (or, in Figure 4, 5 through 11) of an animal about 17 m long (56 ft), discovered in 2013. A small theropod shed tooth was found nearby (Wang et al. 2021).
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| Figure 4 in Wang et al. (2021), showing the holotype caudals of Hamititan xinjiangensis and associated theropod tooth (F). Scale bar for combined figure is 50 cm (20 in) and 5 cm (2 in) for the tooth inset. See here for full caption. CC BY 4.0. |
Is H. xinjiangensis indeed a titanosaur? It's a fair question, given both the historical difficulties surrounding Early Cretaceous titanosaurs and the particular difficulties classifying East Asian Early Cretaceous sauropods, which seem to be doing their own thing. First things first: H. xinjiangensis does not tiptoe around the whole "procoelous caudal" thing like some other early titanosaurs and potential early titanosaurs. It is boldly, proudly procoelous. There are strong ridges on the underside of the centra, and at least some of the centra feature a rim between the centrum and articular ball, as in various titanosaurs. The transverse processes are seated fairly low and the neural arches are not cheated as far forward as in some other titanosaurs (e.g., aeolosaurs). The bones do not feature spongy texture (Wang et al. 2021). Despite some quibbles, it's certainly got more going for it than some other putative early titanosaurs (although I certainly would not be surprised if within a few years someone argued it was not a titanosaur, just another East Asian Early Cretaceous sauropod with a titanosaur-like tail).
Is it Silutitan? Well, we can be reasonably certain that the holotype of H. xinjiangensis is not from the same individual as the holotype of S. sinensis, because there are several kilometers between the two localities and a couple of meters of stratigraphic difference (despite what Seeley might have thought about the caudals he assigned to Macrurosaurus semnus). To look at this phylogenetically, Wang et al. (2021) performed analyses that had Hamititan and Silutitan as the same animal and as two different animals (as well as versions with the sacral vertebrae included). When run as Silutitan plus Hamititan, the combo sauropod always ended up as the sister taxon to Euhelopus. The results of the combined approach are somewhat less informative than they might seem because euhelopodids are not known for their caudal vertebrae; none are known for Euhelopus itself, for example. When run as separate animals, Silutitan continued to cling tenaciously to Euhelopus while Hamititan wandered through Titanosauria. Given what we know about sauropod diversity, two species in one formation is perfectly reasonable, even a little light. (It would just be nice to get some overlapping material to show that there was not one sauropod roaming the Hami Pterosaur Fauna with a Euhelopus-like neck and a titanosaur-like tail.)
References
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.
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