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

Traditional categories of locomotor habit in mammals are largely based on v ariables that are continuous in nature, making intermediate forms difficult to evaluate quantitatively. Interpretations of these categories have varie d 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 mu ltivariate continuum. By basing this continuum on morphological variables t hat fulfil predictions of limb design under biomechanical theory, it can be tied to limb mechanics and applied to both extant and extinct animals alik e. A series of such measurements were taken from a large sample of mammal a nd dinosaur hindlimb bones, and subjected to statistical testing. Patterns of variation in dinosaurs are similar to those seen in mammals, ranging bet ween extremes traditionally designated as 'cursorial' and 'graviportal'. An evaluation of dinosaur locomotor evolution in light of this continuum sugg ests that dinosaurs originated as small cursors, but that most lineages acq uired a more mid-grade locomotor habit. Large taxa (sauropods, armoured orn ithischians) were essentially graviportal, while smaller forms tended towar ds cursoriality; only coelurosaur theropods developed cursoriality at large body sizes. The discrepancy between large, graviportal herbivores and larg e, mid-grade to cursorial carnivores in Mesozoic communities argues against pursuit predation as a major influence in dinosaur locomotor evolution.