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Tuesday, 9 September 2014

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.

27 comments:

  1. Dr. Evil: You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads! Now evidently my cycloptic colleague informs me that that cannot be done. Ah, would you remind me what I pay you people for, honestly? Throw me a bone here! What do we have?
    Number Two: Sauropods.
    Dr. Evil: [pause] Right.
    Number Two: They're mutated sauropods.
    Dr. Evil: Are they ill tempered?
    Number Two: Absolutely.
    Dr. Evil: Oh well, that's a start.

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  2. Nice! Sauropod book: do you know about the Wedel and Hallett project?

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    1. I do, but I didn't realise there was actually something going ahead. Given the talent involved, it'll be worth the wait.

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  3. A sauropod book would be fantastic! There would be a lot of interest I feel. Have you ever considered crowdfunding, a kickstarter perhaps?

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  4. I would contribute to a kickstarter campaign for a new sauropod book, especially featuring illustrations by Mr. Witton. Hell, even one just about titanosaurs.

    Also, because I ain't too prod to a beg, any chance for a future blog post about the possible return of brontosaurus in light of Mossbrucker's and Bakker's recent work?

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    1. Steve and Chad: I reckon you'd be best taking a sauropod book to a publisher (and maybe someone has - see comment from Darren, above) before heading down the crowd-sourcing route. Not that crowd-sourcing is a bad idea, but there are many advantages to working with a professional publishing body, not least that the product may be less expensive for everyone in the end.

      On 'Brontosaurus': As far as I know, neither Bakker or Mossbrucker have published their ideas on this, so any comment - and I'm probably not the most qualified person to evaluate them anyway - should wait until then. I must admit to not seeing the fascination with the 'Brontosaurus' name. It's a neat name, sure, and will forever have a place in palaeontological culture, but the synonymy with Apatosaurus is decades old now, and a lot of neat work has been done on this animal under it's correct name. For me, and I suspect others, Apatosaurus has stepped out from the shadow of Brontosaurus and, unless there's a very compelling reason to bring the latter back into use (which would be surprising, after all this time), it might be best to move on.

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    2. Thanks for the quick reply Mark. I confess, I've long been one of those annoying people that corrected people about the bronto/apato nomenclature since I was 8 years old and never thought bronto would come back until I heard an interview Mossbrucker gave on a podcast about it. Evidently he has uncovered a nearly complete apatosaurus ajax snout that sheds new light on the relationship. I'm very much looking forward to his publishing the findings. (Of course this doesn't mean that bronto will come back but he sounded confident that the new skull will differentiate the two.) I guess I'll just have to do what comes difficult for me and wait!

      Thanks for this blog by the way. I'm not competent enough to do anything but read and lurk, but I always look forward to a new entry.

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    3. very good points, I'll defer to your greater knowledge on publishing! I'm very new to this all.

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  5. Malawisaurus was nowhere near 16 meters long! Hartman's skeletal puts it at roughly 9 meters in length. The paper for Rukwatitan also depicts a ~9 meter animal. Awesome post though! Great to see all these new sauropods coming out of the ground, although a few brachiosaurs wouldn't hurt!

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    1. Indeed, you are correct - thanks for the heads up, and I've modified the post. Wonder where Greg Paul got 16 m from?

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    2. Perhaps he was extrapolating an adult size? Is the holotype considered juvenile?

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    3. I went back to Gomani (2005) to look into this. Unless there's new material not covered in there, there are no remains indicating animals exceeding the 9-10 m ballpark. I wonder if it's just a mistake in Paul's book.

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  6. Hi Mark,

    Great post, and a very, very nice image.

    As for the sauropod book you requested--Mark Hallett and I are making an attempt at that, which should be out next year from Johns Hopkins University Press. Stay tuned!

    [Blogger does not seem to know who I am--this is Matt Wedel]

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    1. Hey Matt,

      This is awesome news: there are three names which came to mind as authors for my hypothetical Big Book of Sauropods, and yours was one of them. You and Mark can have my money now.

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  7. David Marjanović9 September 2014 at 08:45

    Lying on its left side? Because the one in the illustration is lying on its right side.

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    1. Yeah, it's inconsistent, but the image is meant to be a nod to the taphonomic history of the specimen, not necessarily a reconstruction of a specific events leading to its burial, so I'm not too worried. I did wonder about changing it (a lazy way would just be flipping the image!) but liked the composition as it was. In any case, a get out clause is that the reclined animal is meant to be dying or only freshly dead, so there's still potential for movement of the carcass, and based on what's known of the specimen, it's not clear that the entire carcass was buried, either.

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  8. What's that sauropod in the background of the Pakasuchus image?

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    1. It's not meant to be anything in particular. Sauropod remains had been recovered from the Galula Formation by 2010 but nothing concrete was known about them, so I rendered generic Titanosauriformes as reasonable guess for what they may be. They're only in there as PR fodder to let people know they're looking at the Mesozoic, so it didn't seem an unreasonable move.

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  9. Really great post.
    I personally think that a sauropod book written in your pterosaurs book "bauplan" will be fantastic. I really hope some of the experts in the field (the SV-POW guys!!!) are working on something right now.

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    1. Scroll up the comment feed to see what Matt Wedel has to say: it will put you in a very good mood.

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  10. Also recently described from the Galula Formation is the peirosaurid Rukwasuchus. Just thought I'd mention it...a sauropod book would be wonderful, especially if it was full of illustrations by Mark Witton.

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  11. "indeterminate small theropods and turtles" - how indeterminate are the latter? Might they be pturtles?

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    1. Couldn't tell you, but I certainly hope not! Turtles and pterosaurs don't get mixed up too much: it takes a dogged tenaciousness to look past the obvious differences, which most of us lack.

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  12. I wonder if a taxon by taxon overview would be possible for sauropoda. How many known species are out there anyway? Id's guess about 80 species?

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  13. You're quickly solidifying your place on the list of my favorite palaeo-illustrators. The nonrealist style and focus on the desired scientific message are inspirational!

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  14. Do we actually have any clear arguments AGAINST Rukwatitans being able to shoot laser beams out of their eyes? The way I see it:

    1. Sauropods' long necks seem like an obvious adaptation for being able to shoot deadly laser beams from high altitude and across a wide area
    2. Sauropod size could facilitate the collection of sunlight for the purposes of charging up the lasers
    3. Science (fiction) provides us with plenty of examples where large size and height correspond to the ability of shooting laser beams (c.f. War of the Worlds, StarCraft II)
    4. The absence of a fossilized skull makes it impossible to rule out that Rukwatitan's eye sockets weren't apertures for shooting lasers

    In light of the above (no pun intended), I believe the burden of proof should be on the opponents of the Sauropod Eye Lazurs theory to convincingly demonstrate the ABSENCE of any such eye-beam weapon in Rukwatitan.

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