The 'dermal armour' of the ornithopod dinosaur Hypsilophodon from the Wealden (Early Cretaceous: Barremian) of the Isle of Wight: a reappraisal

The small ornithopod dinosaur Hypsilophodon foxii, from the Early Cretaceous (Barremian) of the Isle of Wight, England, is one of the best known of all dinosaurs; however, the reported presence of dermal armour in this taxon is poorly understood. We reassess the evidence for dermal armour: in all specimens in which they can be identified the plate-like structures formerly identified as armour are associated with the lateral surfaces of the distal sections of the anterior dorsal ribs. Comparison with the ornithopods Talenkauen and Thescelosaurus suggests that these structures do not represent armour, but are instead mineralised cartilaginous intercostal plates. Such plates were probably widespread in basal ornithopods, but have not been recognised to date due to incomplete preservation or variation in the timing and degree of mineralisation.