The phylogenetic relationships and evolutionary history of the armoured dinosaurs (Ornithischia: Thyreophora)

The armoured dinosaurs (Thyreophora) were a significant component of Mesozoic terrestrial ecosystems, appearing in the earliest Jurassic and surviving until the latest Cretaceous, and fossils of the group have been found on all continents, including Antarctica. However, a patchy fossil record and highly modified anatomy has hindered reconstruction of their evolutionary history. For example, the relationships of many early-diverging taxa are labile and the degree of convergence between the two major clades, Ankylosauria and Stegosauria, has been difficult to assess. There has never been a species-level phylogenetic analysis of the thyreophoran dinosaurs; until recently, the computational ability to analyse such a dataset did not exist and, consequently, the interrelationships of taxa within the group are debated. Here, we address these issues with a new phylogenetic dataset that includes the majority of named thyreophoran taxa (340 characters, 91 taxa). This dataset was analysed using equal- and implied-weights parsimony and Bayesian inference, and further explored using constraint trees and partitioned datasets. Stratigraphical congruence was used to identify a ‘preferred tree’ and these analyses reveal a novel hypothesis for thyreophoran relationships. The traditional ankylosaurian dichotomy is not supported: instead, four distinct ankylosaur clades are identified, with the long-standing ‘traditional’ clade Nodosauridae rendered paraphyletic. Ankylosauridae, Panoplosauridae, Polacanthidae and Struthiosauridae have distinct morphotypes, typified by Euoplocephalus, Edmontonia/Panoplosaurus, Gastonia and Struthiosaurus, respectively. Isaberrysaura is an early stegosaur and Scelidosaurus is a non-eurypodan. Many characters related to feeding and quadrupedality are coincident with the diversification of Eurypoda. Unstable taxa in the analyses are generally highly incomplete but other better-known taxa are also unstable, suggesting the need for taxonomic revisions. Partitioned datasets show a high degree of convergence in thyreophoran postcrania and that osteoderm characters do not contain a strong phylogenetic signal.