Your Friends The Titanosaurs, part 26: Rinconsaurus, Rocasaurus, and Rukwatitan

And so the procession of titanosaurs continues, this time touching on three small- to medium-sized examples, only two of which are from Patagonia. All three also share relatively complete material compared to many other titanosaurs.

Rinconsaurus caudamirus

Rinconsaurus caudamirus has two claims to fame (which is two more than most titanosaurs, but I digress). First, it is the namesake genus of Rinconsauria (Calvo et al. 2007), a titanosaur clade which shockingly shows up in more than one phylogenetic analysis. Granted, it's basically Rinconsaurus and its best buddy Muyelensaurus, with a decent chance of pulling in Bonitasaura and/or aeolosaurs in any given phylogeny, but at least it's there. The other thing it's known for is the eccentric articulations of the caudal vertebrae. It feels like I've brought up the weird tail of Rinconsaurus every other post in this series, and now we finally get to it. See below:

Not the greatest reproduction, but you get the picture. Part of Plate 3 in Calvo and González Riga (2003); scale is 5 cm. CC BY-NC 4.0.

That's not a diagram illustrating different types of vertebral articulations. (Well, okay, it does do that, but that's not its primary function.) That's parts of two articulated series of R. caudamirus caudals. Apparently it couldn't decide how it wanted its tail to go together. The strangeness of this is baked into the animal's scientific name: "caudamirus" means "amazing or astonishing tail". With the genus name being a reference to the discovery locality Rincón de los Sauces, the name means something akin to "the dinosaur from Rincón with amazing tail", as the authors translate (Calvo and González Riga 2003).

R. caudamirus is among the better represented titanosaurs. It is known from fossils pertaining to two adults and a juvenile found at Cañadón Río Seco, 2 km north of Rincón de los Sauces in Neuquén, Argentina (Calvo and González Riga 2003). Calvo and González Riga (2003) attributed the locality to the Río Neuquén Formation, but as we've seen several times, Patagonian geologic units have undergone significant reassessment in the past couple of decades. The discovery site is now known to be in the Santonian-aged Bajo de la Carpa Formation (Filippi 2015), home to several other titanosaurs (most notably for our purposes Bonitasaura salgadoi, mentioned above). The holotype is MRS-Pv 26 (Rincón de los Sauces Museum), consisting of 13 articulated caudals and two ilia. Other bones from the site include teeth, a prefrontal and a couple of mandible bones, examples of cervical, dorsal, sacral, and additional caudal vertebrae, chevrons, a scapula and coracoid, sternal plates, additional ilia, pubic bones, ischia, a humerus, metacarpals, femora, and a metatarsal (Calvo and González Riga 2003). Together these give a pretty good general idea of R. caudamirus, which the authors estimated at 11 m long (36 ft). The best femur is estimated as 990 mm long (3.25 ft), and the humerus as 790 mm long (2.59 ft) (Calvo and González Riga 2003).

Apart from its tail, R. caudamirus is not particularly unusual for a smallish, slender titanosaur. Its other diagnostic features are backswept neural spines on the middle-anterior dorsals and the articulations of the middle caudal postzygapophyses. The unusual series of switching articulations doesn't kick in until the posterior caudals; until then, it's typical procoelous articulations.

Rocasaurus muniozi

Rocasaurus muniozi is the afterthought of saltasaurs. It's doing the best that it can, but Neuquensaurus and Saltasaurus got there first. (It also doesn't help that saltasaurs were not exactly the most variable of sauropods.) Consider: Saltasaurus shows up in all 29 phylogenetic results I've transcribed beginning with 2015, Neuquensaurus shows up in 21, and Rocasaurus shows up in 4 (in which it is inseparable from the other two).

R. muniozi was named in Salgado and Azpilicueta (2000) for a partial juvenile skeleton discovered in the lower member of the Allen Formation, in Salitral Moreno, Río Negro, Argentina. The genus name refers to the nearby city General Roca, and the species name honors Juan Carlos Muñoz for his support of paleontological investigations (Salgado and Azpilicueta 2000), giving us something like "Juan Carlos Munoz's lizard from General Roca". Per the most recent published assessment (Garcia and Salgado 2013), the type specimen, MPCA-Pv 46 (Paleovertebrate Collection of the Provincial Museum of Cipoletti "Carlos Ameghino"), includes a cervical neural arch and centrum, two dorsal centra and four neural arches, a couple of caudals, ischia, the left pubis, the left ilium and a fragment of the right ilium, and the left femur, with several other cervicals and caudals from the Salitral Moreno site referred to the species.

Figure 8 in Garcia and Salgado (2013), showing the femur and pelvic elements of Rocasaurus muniozi. CC-BY-4.0.

We haven't had a good mixed-up locality in a while, but Salitral Moreno comes through to fix that. Per Garcia and Salgado (2013), Salitral Moreno includes at least four small quarries in an area of 0.12 square km (0.46 square miles). The R. muniozi type locality has also yielded bones of a second distinct but unnamed gracile titanosaur known from two caudals, a humerus, a tibia, and a femur, and is perhaps a source of genuine Patagonian ankylosaur material (distinct from Salitral Moreno titanosaur osteoderms). Another quarry at Salitral Moreno produced material of Aeolosaurus sp. and no fewer than three other unnamed titanosaur taxa, known primarily from caudals (Garcia and Salgado 2013). If you're inclined to think there are too many sauropods, you should probably avoid Patagonia.

Does R. muniozi have much of a chance to distinguish itself from its more famous relatives? The diagnostic characteristics cited by Salgado and Azpilicueta (2000) are not the sort that inspire toy companies. However, R. muniozi is (literally) hiding some interesting trivia: it is regarded as the saltasaur with the most pneumatized skeleton (Zurriaguz and Cerda 2017), which is saying something because saltasaurs are noted for their pneumatization (Cerda et al. 2012; Zurriaguz and Cerda 2017). Its axial skeleton is pneumatized into the posterior caudals (Zurriaguz and Cerda 2017).

Rukwatitan bisepultus

Rukwatitan bisepultus is one of the small but growing number of African titanosaurs. It is primarily known from the holotype, a partially articulated skeleton discovered south of the southeast end of Late Rukwa in Songwe, southwest Tanzania. This specimen was discovered in 2007 and described in Gorscak et al. (2014). The genus name, as you might suspect, refers to Lake Rukwa (actually a double reference to the lake and the Rukwa rift basin), while the species name means "twice buried", because the taphonomy of the type specimen shows that after it was initially buried, it was partially exposed and reburied (Gorscak et al. 2014). This gives us something like "twice buried Rukwa lake and basin titan". Alternatively, I suppose it could also have been described as "twice exhumed" given the modern history, whatever that works out to in Latin. Although the reburial makes for a good story, it's also frustrating, because it has added another interval of exposure, erosion, and loss of elements. Modern erosion within the Namba River system has destroyed bones from the right side of the skeleton, further complicating the story and reducing what was probably a much more complete specimen.

R. bisepultus is based on RRBP 07409 (Rukwa Rift Basin Project, Tanzanian Antiquities Unit, Dar es Salaam, Tanzania). This specimen includes three posterior cervicals, a dorsal neural arch, three anterior and six middle caudals, partial ribs, two chevrons, parts of a scapula and both coracoids, the left humerus, a partial right ulna, the left ilium, and part of the right pubis. It was found in rocks of the middle Namba Member of the Galula Formation (Gorscak et al. 2014). At the time, this unit was thought to date to around the Early–Late Cretaceous boundary, but it is now recognized as Late Cretaceous in age, albeit still not strongly constrained (Cenomanian–Campanian) (Widlansky et al. 2018). A second humerus from another locality has also been referred to R. bisepultus. This humerus is slightly larger than the humerus of the type, at 1015 mm long (39.96 in) (Gorscak et al. 2014). For reference, this is marginally longer than the humerus of Opisthocoelicaudia (Borsuk-Białynicka 1977), although of course it is nowhere near as robust.

R. bisepultus was described as a basal titanosaur (Gorscak et al. 2014), and generally turns up in that area or towards the middle of the pack, with no consensus on close relatives. So far it has not been found to be particularly closely related to other African titanosaurs, including the other two which have been named from the Galula Formation, Mnyamawamtuka and Shingopana (Gorscak and O'Connor 2019).

References

Borsuk-Białynicka, M. M. 1977. A new camarasaurid sauropod Opisthocoelicaudia skarzynskii gen. n., sp. n. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37(5):5–64.

Calvo, J. O., and B. J. Gonzalez Riga. 2003. Rinconsaurus caudamirus gen. et sp nov., a new titanosaurid (Dinosauria, Sauropoda) from the Late Cretaceous of Patagonia, Argentina. Revista Geológica de Chile 30(2):333–353.

Calvo, J. O., B. J. González Riga, and J. D. Porfiri. 2007. A new titanosaur sauropod from the Late Cretaceous of Neuquén, Patagonia, Argentina. Arquivos do Museu Nacional, Rio de Janeiro 65(4):485–504.

Cerda, I. A., L. Salgado, and J. E. Powell. 2012. Extreme postcranial pneumaticity in sauropod dinosaurs from South America. Paläontologische Zeitschrift 86:441–449.

Filippi, L. S. 2015. Los dinosaurios Sauropoda del Cretácico Superior del Norte de la Cuenca Neuquina, Patagonia Argentina. Boletín del Instituto de Fisiografía y Geología 85:19–28.

García, R. A., and L. Salgado. 2013. The titanosaur sauropods from the late Campanian-early Maastrichtian Allen Formation of Salitral Moreno, Río Negro, Argentina. Acta Palaeontologica Polonica 58(2):269–284.

Gorscak, E., and P. M. O’Connor. 2019. A new African titanosaurian sauropod dinosaur from the middle Cretaceous Galula Formation (Mtuka Member), Rukwa Rift Basin, southwestern Tanzania. PLoS ONE 14(2):e0211412. doi:10.1371/journal.pone.0211412.

Gorscak, E., P. M. O'Connor, N. J. Stevens, and E. M. Roberts. 2014. The basal titanosaurian Rukwatitan bisepultus (Dinosauria, Sauropoda) from the middle Cretaceous Galula Formation, Rukwa Rift Basin, southwestern Tanzania. Journal of Vertebrate Paleontology 34(5):1133–1154.

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

Widlansky, S. J., W. C. Clyde, P. M. O'Connor, E. M. Roberts, and N. J. Stevens. 2018. Paleomagnetism of the Cretaceous Galula Formation and implications for vertebrate evolution. Journal of African Earth Sciences 139:403–420. doi:10.1016/j.jafrearsci.2017.11.029.

Zurriaguz, V. L., and I. A. Cerda. 2017. Caudal pneumaticity in derived titanosaurs (Dinosauria: Sauropoda). Cretaceous Research 17:14–24. doi:10.1016/j.cretres.2017.01.001.