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 Dreadnoughtus comes 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.

style="font-style: italic;">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.

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...

References

• 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.
• Paul, G. S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press.
• O’Connor, P. M., Gottfried, M. D., Stevens, N. J., Roberts, E. M., Ngasala, S., Kapilima, S., & Chami, R. (2006). A new vertebrate fauna from the Cretaceous Red Sandstone Group, Rukwa Rift Basin, southwestern Tanzania. Journal of African Earth Sciences, 44(3), 277-288.
• O’Connor, P. M., Sertich, J. J., Stevens, N. J., Roberts, E. M., Gottfried, M. D., Hieronymus, T. L., Jinnah, Z. A., Ridgely, R., Ngasala, S. E. & Temba, J. (2010). The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana. Nature, 466(7307), 748-751.
• Roberts, E. M., O’Connor, P. M., Stevens, N. J., Gottfried, M. D., Jinnah, Z. A., Ngasala, S., Choh, A. M. & Armstrong, R. A. (2010). Sedimentology and depositional environments of the Red Sandstone Group, Rukwa Rift Basin, southwestern Tanzania: New insight into Cretaceous and Paleogene terrestrial ecosystems and tectonics in sub-equatorial Africa. Journal of African Earth Sciences, 57(3), 179-212.
• 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.

Hey Dreadnoughtus, not so close

I try to avoid hopping on the bandwagons following new discoveries - few internet experiences are more tiresome than seeing social media and inboxes swollen with discussions and pictures of the same new fossil species (tyrannosaurids, for some reason, do this more than anything else). Of course, some new discoveries are just too cool to pass up: Dreadnoughtus schrani Lacovara et al., 2014 is one of them. Not only does it have a fantastically marketable and charismatic name entirely befitting one of the largest land animals to ever exist (take that, naysayers), but the sheer amount of data published on it is really first class (Lacovara et al. 2014) and the fossil is truly spectacular. If you've not done so, check out the Dreadnoughtus description and supplementary material: there's everything from measurements and photographs to interactive 3D scans of every bone for you to look at in fine detail (or spin around like crazy while giggling, if you're comfortable enough with your maturity). And before you can say 'paywall', this is all freely-available, open access information. It's not just a great paper for those interested in sauropods or dinosaurs, but also an important reference point for those interested in the evolution of extreme animal anatomies and gigantism.

How the world met Dreadnoughtus schrani in palaeoart. Left, restoration by Jennifer Hall; right, Mark A. Klingler. Images from the Dreadnoughtus media release hosted at the Drexel News Blog.

I found one aspect of the very good, super-comprehensive and fittingly giant media release for Dreadnoughtus rather unusual, however: the artwork. For a media story principally being sold on the size of a dinosaur, the two 'official' pieces of Dreadnoughtus artwork by Mark A. Klingler and Jennifer Hall (above) have - what seem to me at least - some odd choices as goes composition and posture which might undermine the awesome size of Dreadnoughtus. I'm not saying the images are bad or 'wrong': there's lots of lovely detail and atmosphere in both (note the neat sauropod and titanosaur characteristics like the lack of manual claws, the concave posterior surface of the hand etc.), and this is not a dig at the artists, who have definitely earned the wide success of the Dreadnoughtus press campaign. My problem - and I hope this comes across as the constructive criticism it's intended as - is that I'm a bit underwhelmed by the sense of scale, which I'd say is pretty important for artwork of this animal. To be fair, conveying extinct animal size in art is never straightforward, but peculiar compositional choices in each image prohibit my being fooled into thinking I'm looking at truly giant animals. For example, both position the animals in the foreground, filling the canvas with as much Dreadnoughtus hide as possible. I can understand why - it says "it's so big we can barely contain it in the edges of these illustrations", but it also leaves little room for a point of size reference between us and the animals. It also forces the adoption of stooping postures and requires significant foreshortening to fit the animals into view, the former reducing their apparent size and the latter obscuring proportions we intuitively recognise as characteristic of large animals (e.g. the relatively small heads of large animals). Hall's illustration also sets the point of view at shoulder height so we're actually looking across and somewhat down at the subject animal - not necessarily what you might want to suggest this thing was bigger and taller than us. Both images feature trees immediately alongside their animals as a means of conveying scale, but I find the rest of the composition overpowers their effect. In all, while the other aspects of the images are effective, I'm just not sold on the size.

I find these decisions interesting because I think they represent a case of a modern palaeoart convention overruling 'classical' artistic approaches. Traditionally, artists use the same basic techniques for making subjects look big and important when placing them in a scene. They stress proportional extremes (including small head size - this even occurs in renditions of royal or divine human figures), use low points of view so that the the top of the subject clears the horizon line along with other elements in the composition, and place items to give an appropriate sense of scale. Positioning smaller items in the foreground can help the viewer find their position in the scene and ground their sense of size, but these need to be placed carefully: cluttered compositions tend to dwarf their subjects. A consequence of these methods is that giant subjects are often no closer than the mid-ground. An obvious exception to this are images with points of view positioned at the very base of a subject, looking up, so it looms above the viewer (below). This is a slightly different approach to the problem, though, almost treating the subject as the landscape rather than an entity within a background.

A cockroach-eye view of a titanosaur.

Palaeoart produced before the 1970s/1980s stuck to the classic rules of depicting giant animals: Zallinger, Knight, Burian et al. rarely deviated from 'standard' methods of conveying large size when drawing sauropods and other big extinct animals. The scientific transformation of dinosaurs into dynamic, active animals in the late 20th century also brought on a artistic shift where some artists abandoned 'classic' compositions in favour of more exciting, convention-defying and 'extreme' images. One consequence of this was some artists moving (frequently giant) animals closer to the foreground, turning them to face viewers and sometimes, through their body language, 'interacting' with those looking at them. The first seeds of this were probably sown by by the likes of Robert Bakker who, in many of his illustrations, fills every possible square inch with his animals to the point of using extreme postures - particularly arching backs and curving tails - to do so (e.g. illustrations in Bakker 1986). Bakker's works frequently lack the context of backgrounds however, leaving other artists to bring dynamically posed, big extinct animals closer and closer in landscaped works. I think Mark Hallett may have be particularly instrumental here, with works such as his famous 1984 'Dawn of a New Day', and the 1985 paintings 'Awakening of Hunger' and 'Ancient One' leaning towards, or perhaps even pioneering, an 'in your face' style of palaeoart where the subjects are looking at, sometimes menacing, their viewers (if anyone did this earlier, please let me know). Such artworks would become common in the 1990s, with Luis Rey famously combining these compositions with extremes of colour, perspective and pose to produce a style which has since been widely imitated. It's from such imagery that 'slasher' palaeoart arose, those images were animals are rushing, teeth and claws bared, at the viewer from within the painting.

Attitudes towards these foreground-emphasised, perspective heavy images are often divisive among palaeoart aficionados - some love them, others hate them. Fans of such works point out their utility for outreach, in that they're relatively novel, different, fun and striking, while detractors note their distortion of proportion, not to mention that many look, well, silly (I've argued elsewhere that this may have negatively skewed public perception of feathered dinosaurs). The most relevant common complaint to our discussion is that they lose all sense of scale, essentially for all the reasons listed above: unfamiliar proportions, a lack of foreground space to place 'scaling' elements, and often the loss of height associated with moving the anatomy into a position where it can all be seen behind the head (for many infamous examples, see Brusatte and Benton's enormous book Dinosaurs (2008)). Whatever your opinion, we can't deny their success and influence. such images are now a standard palaeoart convention, particularly in children's books, and have been used to showcase virtually any prehistoric animal you can think of. In this respect, the arching, frame-filling Dreadnoughtus images released last week are just following this now familiar palaeoart convention.

Thing is, I'm not sure if this practise works for all palaeoart, and especially in images where conveying size and anatomical details are important. Of course, the ultimate success of a composition is a matter of taste, and there is no actual 'right' or 'wrong' to palaeoart so long as it obeys basic laws of anatomy. But here's the beef: palaeoartworks often have a purpose - very commonly to convey the anatomy and size of a new species - but 'full frame' animal compositions are probably the worst composition to demonstrate these attributes, for reasons discussed above. Moreover, and fundamentally related to the goal of palaeoart being realistic portraiture of extinct species - how do we rationalise the adoption of the contorted postures required to fit the animals into frame? Why would these animals be condensing themselves into such weird shapes? And what do these poses look like from other angles? Wouldn't they look, at best, a bit odd? For me, seeing a restored animal in an unconventional, maybe even biomechanically implausible pose so it can take up more of the canvas is jarring, a reminder than I'm looking at an reconstructed animal rather than one an artist saw with their own eyes.

For art where proportions and a sense of scale is important, pushing our subjects back to the tried and tested middle distance would alleviate these problems, without jeopardising their excitement. Palaeoart was just as inspirational and exciting to audiences before we started rendering animals right under our viewer's noses, after all. Ultimately, while there's nothing inherently 'wrong' with any composition in palaeoart, some compositions suit certain scenes and animals more than others, and some are definitely more informative and educational than others. 'Full frame' compositions certainly have their place within palaeoart, but they're probably more limiting artistically and educationally than the alternatives.

I'll leave you with my own take on Dreadnoughtus, a quick painting done as the end result of my spate of fanboyism on Thursday night. And if you like sauropods, stay tuned, because there's more on the way...

The mighty Late Cretaceous titanosaur Dreadnoughtus schrani, making a mockery of two abelisaurids just by existing. Abelisaurids aren't known from the same formation as Dreadnoughtus, but are the most likely theropods to have occurred there given their abundance in the other Late Cretaceous South America. These are loosely based on Aucasaurus.

Update: 07/09/2014, well past bedtime

Not many moments after posting this, arty chum Jon Davies (@SovanJedi) responded with an image on Twitter which sums up the few thousand words above into one image:

The evolution of the dinosaur book cover: pic.twitter.com/ehszJQjS3J
— Jon Davies (@SovanJedi) September 6, 2014

It's funny because it's true.

References

• Bakker, R. T. (1986). The Dinosaur Heresies. London, Penguin.
• Brusatte, S. and Benton, M. J. (2008). Dinosaurs. Quercus.
• Lacovara, K. J., Lamanna, M. C, Ibiricu, L. M., Poole, J. C., Schroeter, E. R., Ullmann, P. V., Voegele, K. K., Boles, Z. M., Carter, A. M., Fowler, E. K., Egerton, V. M., Moyer, A. E., Coughenour, C. L., Schein, J. P., Harris, J. D., Martínez, R. D., and Novas, F. E. (2014). A gigantic, exceptionally complete titanosaurian sauropod dinosaur from Southern Patagonia, Argentina. Scientific Reports. 4, 6196; DOI:10.1038/srep06196.