Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships

Abstract A layer of keratinous scutes encased the skull of Scelidosaurus. The neurocranium and the associated principal sensory systems of this dinosaur are described. The cranial musculature is reconstructed and a subsequent functional analysis suggests that jaw motion was orthal, allowing pulping of vegetation and some high-angle shearing between opposing teeth. Wishboning of the lower jaw was enabled by transverse displacement of the quadrates, and the long-axis mandibular torsion that occurred during the chewing cycle was permitted by flexibility at the dentary symphysis. Limb proportions and pectoral and pelvic musculature reconstructions suggest that Scelidosaurus was a facultative quadruped of ‘average’ locomotor ability. It retained some anatomical features indicative of a bipedal-cursorial ancestry. Hindlimb motion was oblique-to-parasagittal to accommodate the girth of the abdomen. Scelidosaurus used a combination of costal and abdominally driven aspiration. The hypothesis that respiration was an ‘evolutionary driver’ of opisthopuby in all dinosaurs is overly simplistic. A critical assessment of datasets used to analyse the systematics of ornithischians (and thyreophoran subclades) has led to a revised dataset that positions Scelidosaurus as a stem ankylosaur, rather than a stem thyreophoran. The value of phylogenetic definitions is reconsidered in the light of the new thyreophoran cladogram.