The origin and early evolution of birds has been a major topic in evolutionary biology. In the 20th century, evolutionary scenarios posited either ground-based bird ancestors or tree-dwelling ancestors. This has since been recognised as a false dichotomy [1]. We suggest that part of the problem is the loose categorisation of many extant bird species as either ground or tree locomotors when considering hind-limb function [2-7]. In reality these are not mutually exclusive alternatives. Many extant birds exhibit different degrees of ground- and tree-based behaviours. We thus propose they can be better placed on a spectrum - rather than a dichotomy - according to the extent of ground and/or tree foraging they exhibit. To test this system we analysed the toe claws of 249 species of Holocene birds, revealing that claw curvature increases as tree foraging becomes more predominant. Improved claw morphometrics allow more direct comparisons between extant and extinct birds in order to infer the behaviours of the latter. In contrast to previous studies [2-6], we find that claw curvatures of Mesozoic birds and closely related non-avian theropod dinosaurs, differ significantly from Holocene arboreal birds and more closely resemble those of Holocene 'ground-foraging' birds.

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From the camel's toes to the horse's hooves, the diversity in foot morphology among mammals is striking. One distinguishing feature is the presence of fat pads, which may play a role in reducing foot pressures, or may be related to habitat specialization. The camelid family provides a useful paradigm to explore this as within this phylogenetically constrained group we see prominent (camels) and greatly reduced (alpacas) fat pads. We found similar scaling of vertical ground reaction force with body mass, but camels had larger foot contact areas, which increased with velocity, unlike alpacas, meaning camels had relatively lower foot pressures. Further, variation between specific regions under the foot was greater in alpacas than camels. Together, these results provide strong evidence for the role of fat pads in reducing relative peak locomotor foot pressures, suggesting that the fat pad role in habitat specialization remains difficult to disentangle. All mammalian feet possess a similar role-to transmit the forces associated with locomotion to the environment. However, despite this similar mechanical function, their size, structure, and composition show remarkable variation 1-8. In some terrestrial mammals, the feet have evolved fatty/fibrous pads or digital cushions that cover bony prominences within the fore and hind feet 7. However, the primary functions of these pads remain largely unclear. Previous studies have proposed that foot fat pads play various, yet not mutually exclusive roles, such as reducing peak or transient musculoskeletal stresses along the limb 9-12 , reducing localised pressures under the foot 13-17 , or their presence may be related to substrate differences among species habitats 18-20. In some terrestrial mammals, including humans, the strain-dependent and compliant properties of fat pads have been shown to play an important role in dissipating and regulating the peak transient loads transmitted to the lower limb during locomotion 7,8,21-27. This role of fat pads in dissipating and distributing the mechanical forces associated with locomotion likely becomes even more pivotal as animals increase in size or move faster. Size becomes important because scaling patterns predict that stress and pressure become disproportionally large as animals increase in body mass 2,10-12,28-31. Stress and pressure are equal to force divided by area; thus locomotor pressures and stresses could be moderated by reducing the forces applied, or by increasing the surface area over which the forces act. Geometric scaling predicts that locomotor forces increase proportionally with body mass (M 1.0), yet the cross-sectional area available to withstand these forces increases with a lower exponent (M 0.67), predicting stress to increase as M 0.33. Fat pads may decrease this stress/pressure by reducing the loading rate and therefore the peak impact forces. For example, in humans, fat pads located under the heel have been shown to attenuate 50-90% of the heel strike impact by the time the stress wave reaches the knee, and as...