The new African titanosaur which (almost) got away: Rukwatitan bisepultus

Rukwatitan bisepultus a new titanosaurian sauropod from the Middle Cretaceous (Aptian/Cenomanian) of Tanzania. Why does this otherwise chirpy scene feature a dying Rukwatitan? Read on...

Hot on the heels of super titanosaur Dreadnoughtuscomes another new Gondwanan titanosaur, Rukwatitan bisepultus Gorscak et al. 2014 (press release restoration, above). As indicated by the publication of two new sauropods in close succession, we live in a time where our knowledge of sauropods dinosaurs is expanding rapidly. This surge in interest and activity is perhaps less conspicuous than other expanding areas of palaeontology - sauropods don't grab the headlines as much as small, feathered theropods - but it's fair to say that the landscape of sauropodomorph research has changed considerably in the last two decades. This particularly applies to our appreciation of their diversity and distribution across space and time. Once, the sauropod story could came to a near-end in the upper Jurassic once diplodocids started to decline, but we now know that titanosauriforms kept the sauropod end up throughout the Cretaceous, being abundant, widely distributed and diverse until the end of the Mesozoic. They appear particularly important in South America, where something like 39 species have been recovered (Gorscak et al. 2014).

Schematic of known elements of Rukwatitan bisepultus. From Gorscak et al. 2014.

Rukwatitan bisepultus is not South American however, but African, specifically from the 'middle' Cretaceous (Albain-Cenomanian) Galula Formation of Tanzania. Africa's Mesozoic faunas remain poorly known and, as one of only four named sauropods from 'middle' Cretaceous Africa, as well as a component of relatively poorly-known sub-Saharan Cretaceous forms, Rukwatitan is a find. Thus far, Rukwatitan is the only named sauropod from the Galula Formation, but other Galula fossils record a 'typical' Gondwana fauna of gondwanatherian mammals, notosuchian crocodyliforms (including the carnivoran-immitating Pakasuchus, below) and osteoglossomorph fish, as well as indeterminate small theropods and turtles (Roberts et al. 2010). Rukwatitan can be seen as another component of a middle Cretaceous sub-Saharan sauropod assemblage, joining the roughly contemporaneous sauropods, Malawisaurus dixeyi and Karongasaurus gittelmani of Malawi, along with scrappy fossils which hint at additional species. Represented by an incomplete skeleton (above) and a referred humerus, Rukwatitan is a relatively small titanosaur, although its exact size is difficult to gauge. It is seemingly larger than the relatively completely known (and probably closely related, see below) Malawisaurus, Rukwatitan humeri being 20 and 28% larger than those of Malawisaurus. With Malawisaurus estimated at about 9 m long (not 16 m as indicated in Paul 2010! - see comments below), this puts Rukwatitan in a rough length ballpark of 10-12 m.

Rukwatitan is not my first artistic trip to ancient Galula: in 2010 I helped Patrick O'Connor et al. restore their unusual notosuchian crocodyliform, Pakasuchus kapilimai, famous for it's cat-like slicing dentition. The word on the palaeo grapevine is that there's a lot more to come in the world of African crocodyliforms. I'd like to have another crack at rendering these guys, so I'll be waiting by the phone if anyone wants me...

Titanosauria is an increasingly big group, so leaving Rukwatitan with this label doesn't tell us much about its relationships to other sauropods. A useful phylogenetic landmark within Titanosauria is Lithostrotia, the group of derived titanosaurs which includes many famous taxa: Saltasaurus, Opisthocoelicaudia, Alamosaurus, Nemegtosaurus and Malawisaurus. This clade also contains all known armoured titanosaurs, although armour is not ubiquitous across the group (D'Emic et al. 2009). Other titanosaurs form successive offshoots from the titanosaur evolutionary line leading to Lithostrotia, and it's among these that Gorscak et al. (2014) place Rukwatitan. It only just misses inclusion within Lithostrotia however, suggesting close evolutionary ties to basal members of this group, including the geographically and stratigraphically proximal Malawisaurus. This mirrors findings that some geographically proximal, middle Cretaceous sub-Saharan reptiles - most notably Crocodyliformes - are also closely related, and substantiates ideas that sub-Saharan faunas were evolving at a relatively local, as opposed to cosmopolitan, or even continental-scale level (O'Conner et al. 2006; Gorscak et al. 2014). Possible further evidence of sub-Saharan regions being biogeographically distinct in the mid-Cretaceous stems from an apparent absence of many north African dinosaur groups. Although titanosaurs occur across the continent, evidence of large theropods (spinosaurids and carcharodontosaurids), other sauropod groups (rebacchisaurids, non-titanosaurian titanosauriforms) and ornithopods is currently lacking in Albian-Cenomanian deposits south of the Sahara. Will these animals turn up in time? Perhaps, but the continental Cretaceous beds of Tanzania and Malawi are not new localities only now being exploited, but the sites of many years, even decades of fieldwork. If north African dinosaur groups were there, their fossils are remaining well hidden.

Giving Rukwatitan a tighter address within Titanosauria helps us flesh out a rough projection of its bauplan with a little phylogenetic bracketing. The neck was probably relatively long compared to the tail, evidenced by phylogenetic neighbours and proportions of the preserved vertebrae (note that the schematic above is probably a little wimpy on the neck end of things). It's limbs were likely robust and relatively equally sized, and it's skin probably lacked osteoderms. A short, deep skull seems likely because Rukwatitan is bracketed by short-faced Titanosauriformes, but note that the bracket here is quite loose thanks to the deficit of sauropod skull material. We leant heavily on the well-known anatomy of Malawisaurus for our reconstruction (Gomani 2005), including Scott Hartman's skeletal.

Rukwatitan: river victim

Quarry map of the Rukwatitan holotype specimen, looking at the cross-section of the quarry stratigraphy rather than a 'birds eye view' of a specimen spread over a single horizon. Note the distribution of the skeleton over two layers, the mudstones (representing overbank deposits - the riverbank) and sandstones (fluvial deposits - the river channel). From Gorscak et al. 2014.

If you're moved by the story of Rukwatitan specimen RRBP 07409 and want to know how you can help dinosaurs who've suffered river attacks, please contact me for details of charities and fundraising events.

And finally, a request

Oh no, what caption to use? 'Pods of War? 'Podageddon? DinosAWESOME? Too... many... puns...

References