|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...|
|Schematic of known elements of Rukwatitan bisepultus. From Gorscak et al. 2014.|
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.
style="font-style: italic;">Rukwatitan: river victim
The Rukwatitan type specimen has a story to tell beyond representing a new species and carving up African dinosaur biogeography: it has an unusual taphonomic history. The taphonomic agents removing bones from ancient carcasses destined to fossilise are largely anonymous: scavenging, decay and physical processes all have their part to play, but which processes affect specific specimens is often anyone's guess. This is not so with the Rukwatitan holotype: taphonomy, foul destroyer of data and frustrater of palaeontologists everywhere, has been caught with it's pants down.
Unusually for a fossil of any kind, the Rukwatitan holotype is spread over two sedimentary horizons: a layer of fine clays and muds, which represent an ancient overbank deposit (the fine sediments laid down by floodwaters in the area alongside a river), and an irregularly bedded sandstone horizon (an erosive river channel deposited over the hardened muds). We can interpret this story as beginning with a Rukwatitan carcass lying alongside a river, having finally come to rest on it's left side, indicated by the left elements of the skeleton being preserved lowest in the sequence. Clearly, the left side of the animal was buried first. The semi-articulated nature of the remains indicate that the carcass was in reasonable shape while this was happening: there was probably still soft-tissues holding it together. How completely it was buried is not clear, but it was left long enough for those soft-tissues to at least rot and weaken, if not disappear entirely. We know this because the carcass was not left buried indefinitely: a river channel scoured through the muds burying the Rukwatitan and began removing pieces of the carcass either wholesale, or by breaking the bones to pieces. The Galula Formation is essentially a large river braidplain where large (hundred of metres wide, and c. 10 m deep), relatively straight rivers would frequently change course to rework their environment (Roberts et al. 2010). Even though preserved soils and root-systems indicate that the riverbanks were bound together by plants (presumably doing well in the sub-tropical climate - Roberts et al. 2010), it seems that they were no match for these large, ephemeral rivers, and the remains of ancient bank collapses were visible alongside the in situ Rukwatitan remains. Now exposed to a torrent of water, the carcass lost many smaller bones (these are absent in the holotype) and larger bones were being disassembled. If left unabated, this Rukwatitan would have probably been eroded completely, but the river channel was particularly short lived and rapidly filled with sand. Indeed, the high energy phase of the channel incision didn't last too long at all, as many larger bones were only transported metres downstream, and their broken margins still fit the elements left in the mudstones, indicating limited exposure on the newly formed riverbed. This left us with a good chunk of titanosaur to find, but Gorscak et al. (2014) think another 'river attack' - this time the River Namba - scoured more material away in recent years. The Rukwatitan species name, bisepultus, means 'twice buried', a reference to the holotype being a veteran of erosive and re-burial processes.
|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.|
We wanted to include a nod to this taphonomic story in our press artwork, which is why there's a dying or recently dead Rukwatitan at the base of the image. The cause of death for the Rukwatitan holotype is unknown, but we wanted to include some live sauropods, so it seemed sensible to attribute the death to 'natural causes' rather than an environmental catastrophe or predatory species. For fun, I included a few lesions around the mouth of the dying individual as hints of a trichomonosis-like infection, the same protozoan known to infect birds and other theropod dinosaurs to erode their bones and inflame their upper digestive tracts, leading to death from starvation (Wolff et al. 2009). Would sauropods be vulnerable to this infection? Possibly: trichomonosis leaves lesions in the lower jaw of it's victims which, to a pathologist, are quite characteristic. These lesions haven't been found in any sauropods to my knowledge, but similar ones have been found in other ornithodirans - pterosaurs (Wolff, pers. comm. in Witton 2013) - suggesting many members of this group were vulnerable to this protozoan. It's speculative, sure, but I figured it was a fun nod to other recent dinosaur research.
And finally, a request
We're just about done here, but one last point to make. Between this post, the last, and featuring more new sauropod art over at Palaeontology Online, I've developed a real hankering for a good sauropod book. You know, a readable, fully referenced overview of their history of study, anatomy, palaeoecology, biomechanics, evolutionary history and diversity (so, nothing major then). I'm quite serious here: they're an awesome, popular group of animals, fully deserved of their own semi-technical overview, ideally with lots of images to showcase their anatomy and habits. I'm sure this idea has sufficient legs to interest a major publisher. I lack the expertise to write it, so this is my attempt to plant a seed in the minds of those who can. For what's it's worth, I'd gladly help illustrate it: sauropods are fantastic fun to draw, and it'd be terrific to bring the diversity of this group to life in artwork.
I leave you with this image, which was drafted in response to Eric Gorscak's comments about the Rukwatitan press image: "Other than the lack of laser beams, I think it is looking fantastic!" Not wanting to disappoint, I duly complied...
|Oh no, what caption to use? 'Pods of War? 'Podageddon? DinosAWESOME? Too... many... puns...|
- D'Emic, M. D., Wilson, J. A., & Chatterjee, S. (2009). The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India. Journal of Vertebrate Paleontology, 29(1), 165-177.
- Gomani, E. M. (2005). Sauropod Dinosaurs from the Early Cretaceous of Malawi, Africa, Palaeontologia Electronica Vol. 8, Issue 1, 27A: 37p.
- Gorscak, E., O'Connor, P. M., Stevens, N. J. & Roberts, E. M. (2014). The basal titanosaurian Rukwatitan bisepultus (Dinosauria, Sauropoda) from the middle Cretaceous Galula Formation, Rukwa Rift Basin, southwestern Tanzania. Journal of Vertebrate Paleontology. In press.
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- Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.
- Wolff, E. D., Salisbury, S. W., Horner, J. R., & Varricchio, D. J. (2009). Common avian infection plagued the tyrant dinosaurs. PloS one, 4(9), e7288.