New paper by EMB: The evolution of body shape in sauropod dinosaurs

This week on EMBlog is Dr. Karl Bates, discussing his latest paper on sauropods!.

‘Temporal and phylogenetic evolution of the sauropod dinosaur body plan’, published by Royal Society Open Science and can be found here http://rsos.royalsocietypublishing.org/content/3/3/150636

 Hi guys. I’m excited to post about a paper that my colleagues and I published last month in the journal Royal Society Open Science. A link to the paper can be found above and as the paper is open access anyone should be able to read and download it. The first thing you might notice about the paper is the large number of authors – 10 in all! The paper was a real team effort, we drew together different expertise and techniques in an attempt to better understand how body shape and size in sauropod dinosaurs evolved over more than 150 million years of earth history. Colleagues Phil Mannion (Imperial College London), Peter Falkingham (Liverpool John Moores University) and myself hatched the idea for the study way back in 2010 over drinks at the annual meeting of the Symposium of Vertebrate in Cambridge. Without the funding resources to travel the world digitising sauropod skeletons we had to come up with a creative solution. Thanks to the generosity of colleagues and museum curators giving up their precious time to take hundreds of photographs of sauropod skeletons mounted in their institutions we were able to generate 3D models using photogrammetry. Other collaborators came on board along the way as we found ourselves in need of further expertise (notably Viv Allen’s skills in phylogenetic statistics!) and six years on we’re proud to see the work published. Before I tell you about what we found, let me introduce the dinosaurs this paper is all about.

sauropods
Some examples of the 3D models used in this study (source: Karl Bates)

Sauropod dinosaurs, such as the well-known Diplodocus, Apatosaurus and Brontosaurus, include the largest land animals to ever have lived. As such large organisms, research surrounding them pushes our understanding of biomechanics to the absolute limits. They are renowned for their unusual body plan: an extremely long neck and tail, with four thick pillar-like legs and a small head. But how did their unique body shape and gigantic body sizes evolve? Were changes in body shape and size connected? We used three-dimensional computer models to reconstruct the bodies of sauropod dinosaurs to analyse how their size, shape and weight-distribution changed over evolutionary time. Models of three of the largest sauropods we studied can be seen the image below. We found good support for changes in body shape that coincided with major events in sauropod evolutionary history. The early ancestors that sauropods evolved from were small dinosaurs that walked on two legs, with long tails, small chests and small forelimbs. We found that this body shape concentrated their weight close to the hip joint, which would have helped them balance while walking bipedally on their hind legs. As sauropods evolved they gradually altered both their size and shape from this ancestral template, becoming not only significantly larger and heavier, but also gaining a proportionally larger chest, forelimbs and in particular a dramatically larger neck. Thus we found that these changes altered sauropods’ weight distribution as they grew in size, gradually shifting from being tail-heavy, two-legged animals to being front-heavy, four-legged animals, such as the large, fully quadrupedal Jurassic sauropods Diplodocus and Apatosaurus.

However, we suggest that these linked trends in size, body shape and weight distribution did not end with the evolution of fully quadrupedal sauropods. In the Cretaceous period, the last of the three ages of the dinosaurs, many earlier sauropod groups dwindled. In their place, a new and extremely large type of sauropod known as titanosaurs evolved, including the truly massive Argentinosaurus and Dreadnoughtus, among the largest known animals ever to have lived. These animals have recently captured the public’s imagination, being the stars of one of David Attenborough’s recent documentaries (Attenborough and the Giant Dinosaur). Our computer models suggest that in addition to their size, the titanosaurs evolved the most extreme ‘front heavy’ body shape of all sauropods, as a result of their extremely long necks. These innovations in body shape might have been key to the success of titanosaurs, which were the only sauropod dinosaurs to survive until the end-Cretaceous mass extinction, 66 million years ago.

com-vs-time-768x541
An estimate of patterns in the evolution of whole-body centre of mass change along the head-tail axis in sauropod dinosaurs (from Bates et al. 2016).

What’s important to remember about studies like this is that there is a very high degree of uncertainty about exactly how these animals were put together and what shape they were exactly. While we have good skeletons for many of them, it’s difficult to be sure how much soft tissue there was around each of the bones. We have built this uncertainly into our models, ranging each body part from emaciated to borderline obesity, and even using these extremes we still find reasonably good support for the trending changes in body proportions over sauropod evolution as described above.

So what’s next? Well, we’ve made all our models and data freely available so others can repeat and modify our analysis, and perhaps build on what we’ve done. In my mind the key to improving analyses like this one is getting better body shape and size data from living animals, which is the research focus of EMB PhD Sophie Macaulay. Basically, I’m hoping Sophie will solve all the problems we encountered in this current paper – no pressure Sophie!