AbstractThe armoured dinosaurs (Thyreophora) are an iconic group of dinosaurs, owing to the bizarre and eponymous covering of osteoderms on the heads, backs and tails of the members of the group. They are an early-diverging clade of bird-hipped ornithischian dinosaurs and include some of the most recognisable dinosaurs such as Ankylosaurus and Stegosaurus. They were significant components of Mesozoic terrestrial ecosystems, with a geographically and temporally widespread distribution from the earliest Jurassic to the latest Cretaceous (spanning nearly 135 million years) and with fossils of the group found on all continents, including Antarctica. They are also historically significant, with the first fossils of the group being found in 1833.
A surprisingly poor understanding of the group therefore undermines the evolutionary and historical significance of Thyreophora. Due to a patchy fossil record and dramatically different osteologies compared to their ornithischian relatives, there are still outstanding questions remaining about the evolution of the group. In this thesis, I aim to correct several of these issues using a multi-faceted approach. I redescribe the morphology and taxonomy of the British Wealden Supergroup ankylosaurs Hylaeosaurus and Polacanthus. I then incorporate these key taxa into a new anatomical dataset for a species-level phylogenetic analysis of the thyreophoran dinosaurs. This is the largest dataset to date for Thyreophora, with 340 morphological characters and 91 taxa, and it was analysed using equal- and implied-weights parsimony and Bayesian inference. This reveals a novel hypothesis for thyreophoran relationships, and the hitherto accepted ankylosaurian dichotomy is not supported. Instead, four independent radiations of ankylosaurs are identified and the long-standing ‘traditional’ clade Nodosauridae is rendered paraphyletic. This phylogeny is then used as the framework for the first quantitative assessment of thyreophoran biogeography as well as for the first comprehensive assessment of thyreophoran macroevolutionary history, including diversity, disparity and evolutionary rate analyses. The biogeographic history is dominated by multiple dispersals, subset speciation and regional extinctions. The macroevolutionary analyses show a decoupling of disparity and evolutionary rate. Together, these results suggest there is potential for ankylosaurs and stegosaurs to have been in ecological competition, offering the prospect that ankylosaurs drove the stegosaurs to extinction, although further work is necessary to fully elucidate this.
This thesis offers an overview of the evolution of the thyreophoran dinosaurs, incorporating key taxonomic, phylogenetic, biogeographic and macroevolutionary analyses that are fundamental to the study of palaeontology, and it represents a step-change in our understanding of these iconic dinosaurs.
|Date of Award
|The Natural History Museum, London
|Susie Maidment (Supervisor), Paul Barrett (Supervisor) & Christopher Joyce (Supervisor)