I've been remiss in getting to our latest titanosaur, in part because I was busy with other projects and in part because it took a while to get a copy of the paper. A side visit to the University of Michigan library system after conference hours solved the second issue, so without further ado I present Tiamat valdecii.
Genus and Species: Tiamat valdecii; "Tiamat" is derived from the Mesopotamian deity of the same name (Pereira et al. 2024). Tiamat the deity is identified as female but is otherwise complicated to pin down, incorporating aspects of a force of nature, a human, and a non-human animal. The specific allusion here is to Tiamat's role bringing forth dragons and the Mesopotamian pantheon versus Tiamat's position near the base of Titanosauria. The species name honors "Prof. Dr Valdeci dos Santos Júnior of Universidade Estadual do Rio Grande do Norte, for the discovery of the fossil site and for his fundamental support during the fieldworks" (Pereira et al. 2024).
Citation: Pereira, P. V. L. G. C., K. L. N. Bandeira, L. S. Vidal, T. B. Ribeiro, C. R. A. Candeiro, and L. P. Bergqvist. 2024. A new sauropod species from north-western Brazil: biomechanics and the radiation of Titanosauria (Sauropoda: Somphospondyli). Zoological Journal of the Linnean Society zlae054 (advance online publication). doi: https://doi.org/10.1093/zoolinnean/zlae054.
Geography and Stratigraphy: The type material of T. valdecii comes from the Açu Formation, dating to the "middle" Cretaceous (Albian–Cenomanian). It was found in the Potiguar Basin of Ceará State in northeastern Brazil (Pereira et al. 2024).
Holotype: The holotype is given as a group of eight associated specimens from the tail (seems like a syntype series, except for the implication that one individual is represented). They are: anterior caudal vertebrae UFRJ-DG 636-R, 638-R, and 606-R; middle caudals UFRJ-DG 635-R, 591-R, 527-R, and 704-R (which actually also includes a little sliver of the preceding caudal as well); and partial caudal neural arch UFRJ-DG 574-R. The acronyms correspond to Universidade Federal do Rio de Janeiro (UFRJ) and its Fossil Reptile Collection of the Departamento de Geologia (DG) (Pereira et al. 2024).
So, what are we dealing with? The anterior caudals are strongly procoelous,
with the apex of the ball more than halfway up the surface, and the neural
arches are cheated anteriorly. The middle caudals are slightly procoelous to
amphicoelous, but still feature anteriorly positioned neural arches. These are
all typically titanosaurian features (Pereira et al. 2024). The caudals are
definitely not elongate, and interestingly feature accessory
hyposphene-hypantrum articulation
(Pereira et al. 2024). It's rare for titanosaurs to have this; the only other
species I know of to have it in caudals are
Bustingorrytitan shiva
and
Epachthosaurus sciuttoi. The neural arches are fused to the centra, so we're not dealing with an
unusually young individual, but the processes are often broken off; the exact
anatomy of the neural spines is not known, for example. Although not a juvenile, the type individual
was definitely on the small side for a titanosaur (Pereira et al. 2024). As the authors note, this would also put it in the same size category as contemporaneous rebbachisaurids, which, as sauropod fans know, bowed out early in the Late Cretaceous, leaving only titanosaurs to carry on. Further comments on niches and competition will have to wait for a more complete sample of mid-Cretaceous sauropods (including their heads).
As noted, UFRJ-DG 704-R includes a fragment of the preceding vertebra, and Pereira et al. (2024) interpret the distance between the two as the soft tissue between them, approximately 15%–20% of the length of UFRJ-DG 704-R's centrum. This range is in turn plugged into a discussion of the tail's biomechanics. Functionally, the strongly procoelous anterior caudals would have behaved differently than the middle caudals; the anterior caudals would have been optimized for rotation and resistant to translation (sliding off-center), improving strength and flexibility, whereas the middle caudals with their flatter articulations would have had the reverse. This is further complicated by the hyposphene-hypantrum articulations and the interesting anatomy of the zygapophyes (the paired tags articulating with the neighboring vertebrae), which had heterocoelous (saddle-shaped) surfaces (resistant to twisting). The heterocoelous surfaces would have functioned as an alternative means to promote flexibility and stability without having strongly procoelous centra (Pereira et al. 2024).
Phylogenetically, T. valdecii comes out of Pereira et al. (2024)'s analysis as Andesaurus delgadoi's
new best friend. Is this the return of Andesauridae? A close
relationship is certainly appealing on general time and place; another
skeleton or two would be nice for A. delgadoi, though!
Pereira et al. (2024) also tentatively attribute an osteoderm previously reported from the Açu Formation (UFRJ-DG 549-R; Pereira et al. 2018) to T. valdecii on the grounds of compatible size, site location, and phylogenetic position, but noted that it could have come from a different titanosaur species. This would be of note as one of the earliest titanosaurian osteoderms, and if T. valdecii is really closely related to A. delgadoi, would put the origination of titanosaur osteoderms at the base of the group (at least).
References
Pereira, P. V. G. d. C., T. d. S. Marinho, C. R. d. A. Candeiro, and L. P. Bergqvist. 2018. A new titanosaurian (Sauropoda, Dinosauria) osteoderm from the Cretaceous of Brazil and its significance. Ameghiniana 55(6): 644–650. doi: https://doi.org/10.5710/AMGH.26.08.2018.3168.
Pereira, P. V. L. G. C., K. L. N. Bandeira, L. S. Vidal, T. B. Ribeiro, C. R. A. Candeiro, and L. P. Bergqvist. 2024. A new sauropod species from north-western Brazil: biomechanics and the radiation of Titanosauria (Sauropoda: Somphospondyli). Zoological Journal of the Linnean Society zlae054 (advance online publication). doi: https://doi.org/10.1093/zoolinnean/zlae054.
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