The Titanosaurus (in?)validity project Part 3 - Rocasaurus

Next up in my series of comparisons with Titanosaurus, to determine its validity, is Rocasaurus muniozi. Named by Salgado and Azpilicueta (2000), it's from the Allen Formation of Argentina. The holotype is a partial skeleton (MPCA-Pv 46) that includes three mid caudals, with paratype vertebrae including two mid caudals (MPCA-Pv 49 and 58). The type material was later described in more detail by Garcia and Salgado (2013), while Fernandez et al. (2025 online) recently published on its caudal histology, using some paratype specimens as well as MPCA-Pv 161 that they call an anterior caudal but is of comparable position to Titanosaurus, and three more fragmentary mid caudals (MPCA-Pv 311-313).

Titanosaurus indicus lectotype GSI 2191 in ventral (top), anterior (left), left lateral (center) (all after Lydekker, 1879) and left lateral (right) views (after Mohabey et al., 2013; scale = 50 mm).

So again, as apparently typical for saltasaurines, we have much wider vertebrae with transverse processes (also apparent in dorsal view of MPCA-Pv 161 not shown below). The first three vertebrae figured below seem to be distal to Titanosaurus' type based on central proportions and neural spine development. Another obvious difference is that most have dual pleurocoels arranged vertically, with Garcia and Salgado noting they are truly lacking in the holotype mid caudal, but that may be positional as it only has slight bumps for transverse processes (as seen in posterior view; Garcia and Salgado call it a posterior caudal). As with the previous two genera, the morphology of the ventral face is especially distinctive, with Rocasaurus having deep and sharply defined oval fossae with a thin median ridge subdividing it in at least its anterior portion. A subtle difference that seems constant in Rocasaurus (also in MPCA-Pv 57) is the double concavity of the dorsal centrum edge posteriorly, unlike Titanosaurus. As in the previous two saltasaurines, the neural peduncle is placed more anteriorly (0-8% of central length vs. 16%) than in Titanosaurus.

Rocasaurus muniozi holotype MPCA-Pv 46 (left; after Salgado and Azpilicueta, 2000), paratype MPCA-Pv 58 (center left; after Garcia and Salgado, 2013), paratype MPCA-Pv 49 (center right; after Fernandez et al., 2025 online) and referred MPCA-Pv 161 (right; after Fernandez et al., 2025 online) in ventral (top) and left lateral (bottom) views.

Salgado and Azpilicueta propose the short ventral concavity (in lateral view) in both preserved caudals of the holotype may be a species-level difference with paratypes MPCA-Pv 57 and 58. Yet the former looks closer to the holotype in my opinion, and all of the specimens figured by Fernandez et al. have the paratype condition where determinable. I would just ascribe this to individual variation. Similarly, while they say the neural arch is placed more anteriorly in these two paratypes, the ratio of centrum posterior to the neural peduncle is <41% and 57% respectively, versus 49% in the holotype. Paratype MPCA-Pv 49's ratio is 49% and referred MPCA-Pv 161 is 50%, so I don't see a correlation with the ventral concavity length. 

The original diagnosis of Rocasaurus muniozi includes "caudal vertebrae with a deep ventral cavity divided by a longitudinal partition", noted above as different than Titanosaurus, and "posterior articulation notably depressed and extended ventrally forward" which is partly the wide shape noted above but also the proposed interspecific difference with paratypes that I think could be individual variation.

So Rocasaurus isn't Titanosaurus. Next up, Bonatitan.

References- Lydekker, 1879. Indian pre-Tertiary Vertebrata. Part 3. Fossil Reptilia and Batrachia. Palaeontologica Indica (series 4). 1, 20-33.

Salgado and Azpilicueta, 2000. Un nuevo saltasaurino (Sauropoda, Titanosauridae) de la provincia de Río Negro (Formacíon Allen, Cretácico Superior), Patagonia, Argentina. Ameghiniana. 37(3):259-264.

Garcia and Salgado, 2013 (online 2011). The titanosaur sauropods from the Late Campanian-Early Maastrichtian Allen Formation of Salitral Moreno, Río Negro, Argentina. Acta Palaeontologica Polonica. 58(2), 269-284.

Mohabey, Sen and Wilson, 2013. India’s first dinosaur, rediscovered. Current Science. 104(1), 34-37.

Fernández, Windholz and Zurriaguz, 2025 onine. Palaeohistological characterisation of the caudal pneumaticity of Rocasaurus muniozi (Sauropoda: Titanosauria). Historical Biology. DOI: 10.1080/08912963.2025.2481526