Here I assess sources of intraspecific morphological variability in the armoured dinosaurs (Dinosauria: Ornithischia: Ankylosauria), approached from the viewpoints of ontogenetic allometry, fossil bone histology, and individual variation. Juvenile specimens of Pinacosaurus grangeri Gilmore, 1933, demonstrate strong positive allometry in features of their forelimbs, a trend lacking in the hindlimb. This is likely related to an increase in weight-bearing of the anterior body with ontogeny. Although postcrania can provide useful taxonomic characters, the effects of ontogenetic allometry should be considered. Bone histology indicates that juvenile ankylosaurs experienced rapid growth, followed by a decline in growth rates. It is likely that this decline was more prolonged than in other dinosaurian taxa, possibly related to the mineralization of osteoderms later in ontogeny. Although it is not possible to construct a growth-dynamic curve for ankylosaurs given the current data, the rough ontogenetic stage of an individual may be assessed histologically. A specimen-based parsimony analysis is supported as a repeatable method to make falsifiable taxonomic hypotheses, including explicit a priori character weighting, and delimit species. It provides for a revision of Upper Cretaceous Nodosauridae, a group for which the degree of intraspecific morphological diversity has been difficult to distinguish from taxonomic variation. In addition to Edmontonia spp. and Panoplosaurus mirus, Denversaurus schlessmani Bakker, 1988, is likely valid. It also allows for most currently-known specimens to be referred to species. Endocranial anatomy is conservative and, although not useful at the species-level, may inform deeper evolutionary relationships within the Ankylosauria.