Shaking the tree ...

Evolutionary biologists are constantly striving to improve our knowledge of the tree of life. Although this may seem like an esoteric exercise, of interest to a select few, understanding the pattern of evolution is fundamental to everything else that we might want to know regarding evolutionary history. Schemes of relationships offer a framework for understanding rates of evolution, the geographical history of a group, the ways in which different anatomical, physiological and behavioural features develop and change, and provide insights into the dynamics of adaptive radiations and extinctions. Given the foundational nature of this work, it should be unsurprising that scientists are forever revisiting the relationships of even the most familar of animal groups, in order to check and double-check that they've got things right. An impressive recent example of this comes from work on the interrelationships of the major mammalian groups, where new analyses of primarily molecular data were able to solve many features of the mammalian tree that were previously mysterious and that created a number of new, previously unsuspected groupings, such as Afrotheria, that paved the way for a different understanding of mammal evolution.

In 1887, Harry Govier Seeley, then a Professor at Kings College London, proposed a new classification for a group of extinct reptiles whose remains were being unearthed in Europe and North America. These animals, dinosaurs, were known to share a number of features in common, but there had been little consensus over their relationships and how they should be classified. He noticed several features that differed consistently between the various animals that had been assigned to this group and proposed that they could be divided to into two great tribes - Saurischia (the 'lizard-hipped' dinosaurs) and Ornithischia (the 'bird-hipped' dinosaurs). Unsurprisingly, the most obvious feature he used was hip structure, but Seeley also noted other features that supported these groupings and that distinguished them from each other. Seeley considered that his two groups were probably not particularly closely related and that they arose from different ancestors. Indeed, his arguments were so persuasive that his scheme held sway unchallenged for almost 100 years. The first challenge came in the early 1970s when Robert Bakker and Peter Galton (1974) argued that Saurischia and Ornithischia were each descended from the same common ancestor, making Dinosauria a natural, or monophyletic, group, rather than distant relatives as Seeley and others had advocated. This challenge was debated by the scientific community and upheld by analyses of expanded datasets (and new computational methods for assessing relationships) during the 1980s. Since the late 1980s, the palaeontological community has accepted dinosaur monophyly as dogma (and expanded this dogma to include birds firmly within the dinosaurian radiation), in comparison with the previous 100+ years where dinosaur polyphyly was the accepted model. However, in spite of this radical change, Seeley's dichotomous division of dinosaurs into Saurischia, composed of Theropoda and Sauropodomorpha, and Ornithischia survived (e.g. Gauthier 1986; Sereno 1999).

Since the 1980s, various dinosaur experts have looked at these fundamental splits in the dinosaur tree and their work upheld Seeley's model, with most of the debate concentraing on the exact positions within the tree of a few early, controversial species, such as Eoraptor and Herrerasaurus (e.g. Langer & Benton 2006). However, although these analyses often examined numerous detailed features they relied on only a handful of animals to inform the shape of the dinosaur tree. The past 20 years have witnessed a rush of discoveries of early dinosaurs, from all major lineages and from many parts of the world, and have also seen the recognition of a whole new group of close dinosaur relatives, the silesaurs (e.g. Nesbitt et al. 2010). This new information was incorporated into a variety of analyses, but all of these were focused primarily on the particular relationships of these new animals rather than the overall structure of the dinosaur evolutionary tree - probably because of the broad consensus that surrounded Seeley's neat and logical scheme.

The vast amount of new data now available, in combination with new software packages that allow huge datasets to be analysed rigorously and rapidly, seemed to offer an opportunity to take another look at dinosaur relationships with fresh eyes. With this in mind, my PhD student Matt Baron together with my former PhD advisor (and Matt's other advisor) David Norman, and I decided to see if this new information might affect our knowledge of the dinosaur tree. Matt built a data matrix containing 74 species of early dinosaurs and their close relatives, using specimens from all over the world and concentrated in the Middle Triassic–Early Jurassic, the time at which these major splits took place. Over 450 separate anatomical features were checked for each of these species and the resulting data analysed. It's worth bearing in mind that all other recent analyses of this problem included no more than 12–15 examples of early dinosaurs, on which to base the entire early evolutionary history of the group. Analysis of these data resulted in the recovery of an unexpected and radical tree topology that offers a challenge to Seeley's 130 year old hypothesis. This tree found that theropods were more closely related to ornithischians than either group was to sauropods, thus removing much of the content from Seeley's Saurischia (the carnivorous herrerasaurs remained with the sauropodomorphs, however). This new grouping of Theropoda + Ornithischia has been dubbed Ornithoscelida in a paper published in Nature that presents our new hypothesis (Baron et al. 2017). Ornithoscelida is a name originally proposed by 'Darwin's bulldog' Thomas Henry Huxley in 1870 in a classification that preceded Seeley's, but that was largely ignored. The name means 'bird-limbed' and refers to the hollow, gracile and elongate leg and arm bones found in theropods and ornithishians. In our analysis this new grouping receives strong support, with 21 features that seem to unite these animals to the exclusion of the sauropodomorph dinosaurs.

Given previous definitions of various dinosaur groups, this new tree requires some new definitions for commonly used names if we are to keep all dinosaurs as dinosaurs. For example, the most common definition (the common ancestor of sparrows and Triceratops and all of its descendants), would now exclude Diplodocus and other sauropods from Dinosauria. This was a step too far for us and so we proposed a new scheme of definitions to help stabilise the use and definition of some of these names, not only to account for if the new tree is potentially right, but also that would allow the tree to change if we were wrong without altering the meaning of these names.

In addition to giving us a new dinosaur tree, if our results stand up to detailed scrutiny by other palaeontologists, then they might be used to provide many new insights into dinosaur evolution. This would include looking again at the timing and pace of dinosaur origins, the evolution of various key characters, such as feathers and carnivory, and into the areas where dinosaurs might have first appeared. As with anything in science, our tree is a hypothesis - it is there to be stretched and tested to see if it is stronger than those that have come before and if it has the power to explain more about dinosaur evolution than other competing schemes. If we're wrong and there are better alternative explanations for the patterns we see then we'll have to accept that evidence and move on - that's how science works. In the meantime we hope that people will look at this with an open mind rather than rejecting it in a knee-jerk fashion due to its challenge to a well established and embedded dogma. While Huxley might be cheering us from the shades, we're also aware that Seeley would be shaking his head. Time will tell which one of them was closer to the truth.

References

Bakker, R. T. & Galton, P. M. Dinosaur monophyly and a new class of vertebrates. Nature 248, 168–172 (1974). 

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature (2017).

Gauthier, J. Saurischian monophyly and the origin of birds. In The origin of Birds and the Evolution of Flight (ed. K.Padian). Memoir of the California Academy of Science 8 1–55 (1986).

Huxley, T. H. On the Classification of the Dinosauria with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society 26, 32–51 (1870).

Langer, M. C. & Benton, M. J. Early dinosaurs: a phylogenetic study. Journal of Systematic Palaeontology 4, 309–358 (2006).

Nesbitt, S. J., Sidor, C. A., Irmis, R. B., Angielczyk, K. D., Smith, R. M. H & Tsuji, L. A. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464 (7285): 95–98 (2010).

Seeley, H. G. On the classification of the fossil animals commonly named Dinosauria. Proceedings of the Royal Society of London 43, 165–171 (1887).

Sereno, P. C. The evolution of dinosaurs. Science 284, 2137–2147 (1999). 

Lake Kariba dinosaur expedition: Part 3, finale

On reaching island 126/127 we began our search for the original Vulcanodon site. A thick lava flow forms the top of the cliffs, capping a series of bright orange and red mudstones and sandstones that comprise the Lower Jurassic Forest Sandstone Formation. Some of these layers were swarming with the burrows of animals that had lived in the sediment, suggesting that at times this ancient environment was a little wetter than generally thought. The island is small, so we were able to narrow down our search fairly quickly. Although we're confident that we did find the area that yielded Vulcanodon, were weren't lucky enough to find any other material on this occasion. The vertical cliffs didn't offer many opportunities to prospect for fossils and we were left wondering how the original team, lead by Prof. Michael Bond in the 1960s, had accomplished the back-breaking work of getting the huge bones out of the site. The foreshore was more promising, however, and were lucky enough to find isolated fossil bones in several places, all of which were attributable to more primitive sauropodomorph dinosaurs than Vulcanodon, which were probably from a more Massospondylus-like animal. While at the site we took the opportunity to learn more about its geology, measuring detailed sections and noting the lithology and structures present. This turned out to be a valuable exercise as it helped to form a basis for correlating the Vulcanodon site with other dinosaur-bearing sites around the lake shore and it also offered us some new clues on the geological age of Vulcanodon and on some of the other sites nearby, which will be the subject of a paper that the team is currently writing.

Cervical vertebra of a Massospondylus-like dinosaur found close to the Vulcanodon type locality on island 126/127 (photo: Pia Viglietti)

The nearby islands had very similar geology and we prospected several of these hoping to find new material. All were marked by the same characteristic dark basaltic lava flows and orange-red Forest Sandstone sediments and some showed evidence of major faults. These faults created some confusion when we were trying to work out where we were in each section, but after visiting lots of sites we began to work out how all of these features were related. We found bone on most of the islands, but they were usually small, isolated pieces, rather than associated or complete specimens. Nevertheless, in those cases where we could identify the bones, they were all of sauropodomorph dinosaurs and we identified many new potential sites that we hope to revisit in the future.

After several days of prospecting the islands around 126/127, we decided to move to sites closer to Kariba town. Our captain moved Musankwa eastward to moor off of Musango Island, where Steve Edwards runs his safari camp. Steve has been prospecting the area for many years and has amassed an important collection of material. Some of these specimens, including more evidence of sauropodomorph dinosaurs, come from the Forest Sandstone, but the majority of them come from an older unit that is probably of Late Triassic age, called the Tashinga Formation. The Tashinga Formation also consists of mudstones and sandstones, but parts of it were deposited under much wetter conditions than the Forest Sandstone. This is shown by the abundance of fossil wood on Musango Island and the nearby shore - fragments of wood are scattered everywhere in the soil and some spots has masses of large tree trunks that showed the region had been densely wooded at this time. In addition, other fossils also suggest the presence of water bodies (at least on a seasonal basis) as shown by finds of numerous large lungfish tooth plates.

Some huge Late Triassic tree trunks with palaeontologists for scale (photo: Lucy Broderick)

Among Steve's collection, and among the fossils we spotted while walking, we found some sauropodomorphs , but the remains of lungfish and other reptiles were much more common. Some of these fossils represent an interesting reptile that is the first of its kind to be reported from this area of Africa and the team is currently working on a description of these specimens. Over the next few days we walked though some of the areas around the island so that we could also study the geology, with the aim of trying to pin down a narrower age estimate and to work out how the different fossil-bearing units of the area were related to each other.

 The team poring over some of Steve Edward's fossils while at Musango Safari Camp (photo: Pia Viglietti)

After long hot days of walking through the bush, usually with at least one eye peeled for nearby elephants or hippos, our evenings around Musango were spent trying to supplement dinner with the excellent fish that could be caught in the lake (with varying amounts of success), as well as spotting some of the abundant bird life. While at Musango we also witnessed brutal tropical thunderstorms, which would come rolling in during the early hours of the morning while we were asleep. As many of us slept on deck (where it was nice and cool), we'd often be awoken by the crack of thunder, some amazing celestial pyrotechnics, and the drip of cold rain onto our faces through the mosquito nets as it found a way through the canopy.

A rare example of fishing success, with a triumphant Pia showing off her skills (photo: Lucy Broderick)

Following several pleasant and productive days at Musango, the final part of the trip involved another move to the east, closer yet to Kariba town, and a stay moored off of Spurwing Island, where we walked the shorelines looking for more places that might yield fossils. Again, assisted by the local rangers, we were lucky to find sites that yielded large sauropodomorph bones. Although none of these were particularly complete, they were of high quality, well preserved and some were partially articulated, suggesting that exploratory excavations in these areas might reveal interesting new material.

Steve Edwards checking out some of the sauropodomorph bones on Spurwing Island (photo: Lucy Broderick)

By the end of our trip we'd prospected a transect over 50 km in length and had visited numerous outcrops of both Late Triassic and Early Jurassic age. Our specimens have been deposited in the National Museum in Bulawayo and the information we've obtained on new sites, many of which were previously unknown, and on the geology of the area, is currently being compiled for publication. Our trip back down the lake was marked by more interesting weather, notably an amazing waterspout that we watched cross the lake surface followed by a torrential downpour that dampened our spirits somewhat as we unloaded Musankwa back in harbour. Nevertheless, the amazing scenery of Lake Kariba, its stunning natural beauty, and the new material we'd seen all combined to make this an exceptional and productive trip. The team is already planning to return to some of the more promising locations in the hope of finding some really spectacular material in future.