Tiamat valdecii Pereira, Bandeira, Vidal, Ribeiro, Candeiro & Bergqvist, 2024, sp.nov.
Creators: name: Pereira, Paulo V L G C
name: Bandeira, Kamila L N
name: Vidal, Luciano S
name: Ribeiro, Theo B
name: Candeiro, Carlos R dos A
name: Bergqvist, Lilian P
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Tiamat
Tiamat valdecii

description: Tiamat valdecii sp.nov.
ZooBank LSID for the species: urn: lsid:zoobank.org:act: 549071C9-46D4-42D0-BE6E-6111110C0B09
Etymology: The specific epithet is in honour of 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.
Holotype: UFRJ-DG 636-R, UFRJ-DG 638-R, UFRJ-DG 606-R (anterior caudal vertebrae), UFRJ-DG 635-R, UFRJ-DG 591-R, UFRJ-DG 527-R, UFRJ-DG 704-R (middle caudal vertebrae), and UFRJ-DG 574-R (partial neural arch) (Fig. 4).
Diagnosis: A titanosaur sauropod diagnosed by four diagnostic features, all in the caudal vertebrae: marked accessory tuberosity dorsoventrally developed, located above the prezygapophyses in the posteriormost and in middle caudal vertebrae; deeply medioventral excavated articulation facets of the prezygapophysis and post-zygapophysis (see fig.07); presence of hypanthrum–hyposphene articulation; and short middle caudal centra with well-marked articular facets for the haemal arch. Additionally, some of the following combinations of characters assist the placement of Tiamat valdecii as a titanosaurian, such as strong procoelous anterior caudal vertebrae with the apex of the posterior condyle positioned on the dorsal half of the centrum; wider than high centra which, progressively widen to the posterior vertebrae; pedicles of the neural arch attached to the anterior half of the centrum and presence of a ventral longitudinal hollow on the caudal vertebral series.
Description
There are three anterior caudal vertebrae, four middle caudal vertebrae, and a neural spine (possibly an anterior one). The anterior caudal vertebrae (UFRJ-DG 636-R, UFRJ-DG 638-R, UFRJ-DG 606-R) are strongly procoelous, with subcircular outline, while one of the middle caudal vertebrae (UFRJ-DG 635-R) is slightly procoelous, and the other middle caudal (UFRJ-DG 591-R, UFRJ-DG 527-R, UFRJ-DG 704-R) are amphicoelous. This condition (transition of procoelous to slightly procoelous) is observed in Mendozasaurus (González-Riga 2003, Gonzàlez Riga et al. 2018) and Patagotitan (Carballido et al. 2017); but differs from other taxa such as Andesaurus (Mannion and Calvo 2011), Malawisaurus, and Rukwatitan (Gomani 2005, Gorscak et al. 2014), since in these taxa, the anteriormost caudal vertebrae exhibits strongly procoelous condition, while the other anterior caudal vertebrae and, consequently the middle ones, turn to an amphiplatyan condition. The most anterior caudal vertebra recovered for the new Brazilian taxa also differs from Mnyamawamtuka, since the African taxon presents a ‘mildprocoely’ (Gorscak and O’Connor 2019).
In lateral view, all vertebrae centra are anteroposteriorly short, bearing a concave lateral surface, which, in turn, is developed in the anteroposterior axis. The preserved neural arches are small and placed slightly anterior to the midpoint of the vertebral body. The vertebrae vary in height and width when observed on anterior and posterior views (Table 2). The centrum of the more anterior caudal vertebra (UFRJ-DG 636-R) is taller than wide (Table 2), while it progressively dorsoventrally compresses in the middle caudal preserved (e.g.UFRJ-DG 704-R) (Fig. 4A 1–A 3, 4G 1–G 3).
As in Tastavinasaurus (Canudo et al. 2008) and in the anteriormost caudal preserved of Malarguesaurus (González Riga et al. 2009), the ventral surface of the centrum is anteroposteriorly concave, lacking an articular surface for the haemal arch at the anteriormost vertebrae. However, the other caudal vertebrae have a visible articular surface for a haemal arch with very pronounced end, in a similar way to many other titanosaurs.
No signal of the ventrolateral ridges is preserved for Tiamat valdecii. The lack of this structure is also observed in several titanosaurs, e.g. in the African titanosaurs Rukwatitan (Gorscak et al. 2014) and Mnyamawamtuka (Gorscak and O’Connor 2019), as well as in Volgatitan (Averianov and Efimov 2018). But this differs from the well-developed ventrolateral ridges observed in the basal lithostrotians Hamititan (Wang et al. 2021). Also, it differs from the less developed ventrolateral ridges in Malawisaurus (Gomani 2005) and in the derived lithostrotian Arrudatitan (Santucci and Arruda-Campos 2011). Also, there is no ventral longitudinal groove as presented in some titanosauriforms from the Lower Cretaceous, e.g. in Volgatitan (Averianov and Efimov 2018). Also ventrally, some few small vascular foramina (‘pneumatopore’) are abundant over the ventral surface of all caudal vertebrae.
One of the features of the new species, the short middle caudal centra (Table 1) with well-marked articular facets for the haemal arch, is also distinctive from other titanosaurian taxa. This is not observed in other titanosauriforms from the Lower Cretaceous, for example, and even in other taxa from the Upper Cretaceous.
The transverse processes are relatively short (Table 2) and dorsolaterally projected from the dorsal margin of the vertebral centra, as in many titanosaurs (e.g. Gomani 2005, Santucci and Arruda-Campos 2011, Averianov and Efimov 2018, Gonzàlez Riga et al. 2018, Gorscak and O’Connor 2019, Wang et al. 2021). This is indicative of the positioning of the caudal vertebrae, due to the development of the transverse processes, especially in the middle caudal vertebrae, where they progressively diminish in size until becoming vestigial (e.g. Ibiricu et al. 2017). This reduction in size progresses in the transverse processes, to the point where these structures transform into heavy blunt knobs, regarded here as the beginning of the middle caudal series, especially in UFRJ-DG 527-R, UFRJ-DG 591-R, UFRJ-DG 635-R, and UFRJ-DG 704-R (Fig. 4D–G). The transverse processes are triangular and curved posteriorly, similarly to Tastavinasaurus and Malarguesaurus, being supported by a massive and robust PRDL, especially on the most anterior caudal vertebrae (UFRJ-DG 636-R, UFRJ-DG 638-R, and UFRJ-DG 606-R). The neural canal is subcircular being delimited by thick intraprezygapophyseal laminae (TPRL, on anterior view) and intrapost-zygapophyseal lamina (TPOL, on posterior view). The thick TPRL and TPOL of Tiamat differ from the thin laminae presented by Rukwatitan (Gorsack et al. 2014) and other titanosaurs.
From the middle caudal vertebrae preserved, UFRJ-DG 635-R shows prezygapophyses dorsomedially positioned, supported ventrally by sharp CPRL. Above the prezygapophyses and the post-zygapophyses, Tiamat valdecii shows a marked accessory tuberosity. The SPRL observed in the most anterior caudal vertebra and in the middle caudal are weakly developed, like the weakly developed SPRL of Andesaurus (Mannion and Calvo 2011) and Epachthosaurus (Martínez et al. 2004) but differing from the thick SPRL presented by Patagotitan (Carballido et al. 2017).
Concerning one of the diagnostic features proposed, the marked accessory tuberosity dorsoventrally developed located above the prezygapophyses; is an unusual feature. To our knowledge, no titanosaur presents this tuberosity. The post-zygapophyses are slightly displaced laterally and have concave articular facets, a condition also observed on more anterior caudal vertebrae of Malawisaurus (Gomani 2005). However, those concave articular facets, being deeply excavated, in the prezygapophyses and the post-zygapophyses of Tiamat are a unique feature among titanosaurs, especially with the contemporaneous (or temporally close) taxa compared here, as Rukwatitan, Malawisaurus, Mnyamawamtuka, Volgatitan, Patagotitan, among others.
The new species also presents a hyposphene–hypantrum articulation Epachthosaurus (Powell 1990, Martínez et al. 2004). However, in Epachthosaurus, the hyposphene–hypantrum is observable in some of the anterior caudal vertebrae, but not in caudal vertebrae sequence, which differs from the new species proposed here, where in almost all vertebrae this feature was observed. Both prespinal and postspinal fossae are dorsoventrally elongated but relatively shallow in the anteriormost caudal vertebrae (Fig. 4A–A 3) and in the isolated neural spine associated (Fig. 4H). There is no sign of the prespinal or postspinal laminae as in some neosauropods (Mannion et al. 2013).
Lastly, as in Malarguesaurus, the neural spine of Tiamat is rhomboid shaped in dorsal view but is proportionally lower, more elongated, and relatively more robust than the Argentinean species. The neural spine preserved lacks the incipient bifurcation with anteriorly directed tips presented by Patagotitan (Carballido et al. 2017). The isolated neural spine presents a weakly spinoprezygapophyseal lamina (SPRL) while the spinopost-zygapophyseal lamina (SPOL) is comparatively more robust than the SPRL, but the SPOL merges with the rough structure presented on this surface. To our knowledge, this rough structure is unique for titanosaurs, and here is interpreted as an autapomorphy of Tiamat. Lastly, the overall SPRL and SPOL morphology looks like those presented in some derived titanosaurs, such as Lohuecotitan. However, a marked difference is that in Tiamat, these laminae are more robust, while those of Lohuecotitan present a more complex lamination (Díez Díaz et al. 2016).
The anterior vertebrae of Tiamat do not have well-preserved zygapophyses; however, the middle caudal vertebrae have characteristics that warrant biomechanical observations.
description: Published as part of Pereira, Paulo V L G C, Bandeira, Kamila L N, Vidal, Luciano S, Ribeiro, Theo B, Candeiro, Carlos R dos A & Bergqvist, Lilian P, 2024, A new sauropod species from north-western Brazil: biomechanics and the radiation of Titanosauria (Sauropoda: Somphospondyli), pp. 1-21 in Zoological Journal of the Linnean Society 202 (4) on pages 5-7, DOI: 10.1093/zoolinnean/zlae054, http://zenodo.org/record/14764381