What, if anything, is a cursor? Categories versus continua for determining locomotor habit in mammals and dinosaurs

Traditional categories of locomotor habit in mammals are largely based on variables that are continuous in nature, making intermediate forms difficult to evaluate quantitatively. Interpretations of these categories have varied greatly among authors, mainly owing to the inconsistent meanings ascribed to these essentially morphological variables. As a result, it is not clear whether these categories reflect any true locomotor influence, or if they can be applied in any form to non-mammalian taxa. In order to rectify these two difficulties, locomotor categories are rejected here in favour of a multivariate continuum. By basing this continuum on morphological variables that fulfil predictions of limb design under biomechanical theory, it can be tied to limb mechanics and applied to both extant and extinct animals alike. A series of such measurements were taken from a large sample of mammal and dinosaur hindlimb bones, and subjected to statistical testing. Patterns of variation in dinosaurs are similar to those seen in mammals, ranging between extremes traditionally designated as 'cursorial' and 'graviportal'. An evaluation of dinosaur locomotor evolution in light of this continuum suggests that dinosaurs originated as small cursors, but that most lineages acquired a more mid-grade locomotor habit. Large taxa (sauropods, armoured ornithischians) were essentially graviportal, while smaller forms tended towards cursoriality; only coelurosaur theropods developed cursoriality at large body sizes. The discrepancy between large, graviportal herbivores and large, mid-grade to cursorial carnivores in Mesozoic communities argues against pursuit predation as a major influence in dinosaur locomotor evolution.