The Titanosaurus (in?)validity project Part 2 - Neuquensaurus

Continuing our project, the next taxon is Neuquensaurus australis, first named Titanosaurus australis by Lydekker (1893). It was then separated as "Neuquensaurus" in Powell's (1986) thesis, becoming official in Powel''s (1992) osteology of Saltasaurus*. Neuquensaurus is based on six holotype caudals (MLP Ly 2-6), to which a sacrum (MLP Ly 1, 7) and two more caudals (MLP Ly 48, 66; now lost) were later added (Powell, 2003; D'Emic and Wilson, 2011). Huene (1929) and Powell referred a lot of material to the species, but of that only three unfigured mid caudals and three distal ones were accepted by D'Emic and Wilson. Importantly however, those authors also accept the referral of a partial skeleton described by (Salgado et al., 2005) that includes a "probably" continuous series of proximal to mid caudals (MCS-5/2-MCS-5/15). Both the type and MCS-5 come from the Anacleto Formation of Argentina.

* Or was it? That's how Powell (2003) and D'Emic and Wilson (2011) cite it, but ICZN Article 13.3 states "To be available, every new genus-group name published after 1930 (except those proposed for collective groups or ichnotaxa) must, in addition to satisfying the provisions of Article 13.1, be accompanied by the fixation of a type species in the original publication", and Powell only ever says "Neuquensaurus australis (see Powell, 1986)", "Neuquensaurus australis (Huene, 1929)", "Neuquensaurus australis (Huene, 1929: plate 9)", "Neuquensaurus (= “Titanosaurus”) australis", AND importantly "Neuquensaurus robustus as described by Huene (1929)" and "Neuquensaurus (= "Titanosaurus" robustus, Huene 1929; plate 19: 1)" (all translated). Powell makes australis the type species in his 1986 thesis, but as a thesis that's not recognized by the ICZN. In 2003 he gives australis as the type species, but also credits his 1992 paper for the genus, which is a problem because it's from after 1999, so Article 16.1 kicks in ("Every new name published after 1999, including new replacement names (nomina nova), must be explicitly indicated as intentionally new").

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).

In any case, MCS-5 is convenient because it lets us determine more precisely what caudal the Titanosaurus type belongs to. It seems closest to caudals 6-11 in Neuquensaurus in elongation, prezygapophyseal shape and neural spine development. As in Saltasaurus, all these caudals in Neuquensaurus have transverse processes unlike Titanosaurus, with even the last preserved in MCS-5 (fourteenth) showing some development of one. The centra are so wide I don't think taphonomy can explain the difference this time and further differ in having a concave dorsal cotyle edge. The ventral edge of all Neuquensaurus centra are more concave than Titanosaurus. Ventrally the centra show a broad fossa with no lateral ridges, unlike Titanosaurus that has anterior and posterior grooves delimited laterally with ridges, and a broad and deep median keel in the anteroposterior middle. Like Saltasaurus, the neural peduncle ends more anteriorly in Neuquensaurus (10-11% vs. 16% in Titanosaurus), which is consistent in all comparable vertebrae. The prezygapophyses are low in anterior view as in Saltasaurus.

Neuquensaurus australis holotype MLP Ly 3 (upper left) in ventral (top), anterior (left) and left lateral (right) views (after D'Emic and Wilson, 2011); holotype MLP Ly 5 (upper right)  in ventral (top), anterior (left) and left lateral (right) views (after D'Emic and Wilson, 2011); referred MCS-5 (bottom) caudals 6-11 in left lateral view (after Salgado et al., 2005).

MLP Ly 5b has comparable round and pronounced posterior chevron facets to Titanosaurus, unlike MLP Ly 3, so that difference noted for Saltasaurus could be positional variation. MLP Ly 5b also has a deep concavity to the dorsal centrum posteriorly like Titanosaurus, but unlike other vertebrae of either the type or MCS-5, so this could be individual variation. The shape of the concavity between the neural arch and centrum posteriorly is highly variable, with caudals 8 and 11 of MCS-5 pretty close to Titanosaurus, and caudals 9 and 10 having completely different shapes from one another (acutely pointed vs. rounded), showing this is positional/random variation. While the neural spine is angled more highly compared to the prezygapophysis in Titanosaurus than MLP Ly 5 and the ninth-eleventh caudals of MCS-5, the fifth caudal is comparable, so perhaps the dorsally broken spines of the sixth, seventh and/or eighth were too.

The only caudal character in Powell's (2003) diagnosis is "caudal centra rather short and with a very concave ventral face bounded by rounded edges", which was also mentioned in his 1992 paper that said in Saltasaurus mid caudals "The ventral side has a longitudinal groove much narrower than that seen in Neuquensaurus". As noted above, this is quite unlike Titanosaurus. Salgado et al. add a character from the 1986 thesis "lateral walls of caudal vertebral centra little exposed in ventral view", but this seems the same as in Titanosaurus and was said by D'Emic and Wilson to be present in several other titanosaurs as well. The latter also list "longitudinal ridge below transverse processes" of the mid caudals as diagnostic, which is absent in Titanosaurus. Finally, they list "[postzygodiapophyseal lamina] present and elongate" which is just a gentle convexity in Titanosaurus (seemingly similar to the eighth caudal of MCS-5, but maybe it's just illustrated inaccurately).

Note no caudal material was ever assigned to Neuquensaurus? robustus, so we won't be dealing with it.

So Neuquensaurus isn't Titanosaurus, which even Lydekker recognized was quite possible- "the reference of the latter to Titanosaurus must be regarded as a more or less provisional measure, rendered necessary by our very incomplete knowledge of the type species."  Next up, Rocasaurus.

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

Lydekker, 1893. The dinosaurs of Patagonia. Anales del Museo de La Plata. 2, 1-14.

Huene, 1929. Los saurisquios y ornitisquios del Cretáceo Argentino. Anales del Museo de la Plata (series 3). 3, 1-196.

Powell, 1986. Revisión de los titanosaurios de América del Sur. PhD Thesis. Universidad Nacional de Tucumán. 340 pp.

Powell, 1992. Osteologia de Saltasaurus loricatus (Sauropoda - Titanosauridae) del Cretácico Superior del noroeste Argentino. In Sanz and Buscalioni (eds.). Los Dinosaurios y Su Entorno Biotico: Actas del Segundo Curso de Paleontologia in Cuenca. Institutio "Juan de Valdes", Cuenca, Argentina. 165-230.

Powell, 2003. Revision of South American titanosaurid dinosaurs: Palaeobiological, palaeobiogeographical and phylogenetic aspects. Records of the Queen Victoria Museum. 111, 173 pp.

Salgado, Apesteguía and Heredia, 2005. A new specimen of Neuquensaurus australis, a Late Cretaceous saltasaurine titanosaur from North Patagonia. Journal of Vertebrate Paleontology. 25(3), 623-634.

D'Emic and Wilson, 2011 (online 2010). New remains attributable to the holotype of the sauropod dinosaur Neuquensaurus australis, with implications for saltasaurine systematics. Acta Palaeontologica Polonica. 56(1), 61-73.

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