Increasing locomotor efficiency among North American ungulates across the Oligocene-Miocene boundary

The proportions of the limb bones of tetrapods contribute to the energetic costs of locomotion. Consequently, they can be used to classify locomotor type and can inform hypotheses about changes in community structure. We examined mechanical advantage in the limbs of North American Oligocene and Miocene ungulates (Artiodactyla and Perissodactyla), with the expectation that the distribution of limb morphologies would change with the spread of grass-dominated plant communities; that is, we expect to find morphological optimization for efficient high-speed locomotion with more open habitats. We assess changes in the occupation of locomotor ecomorphospace (LEMS) through time using measurements of the lengths of stylopods and zeugopods of North American ungulate species. Analysis of LEMS reveals significant changes in dominant ungulate locomotor morphology between Oligocene and Miocene faunas. Oligocene ungulates show less variation in LEMS than Miocene ungulates. Artiodactyls show a significant selection towards cursorial morphologies (p < 0.01) in the Miocene. A majority of artiodactyls are non-cursorial in the Oligocene, but only one family occupies the non-cursorial morphospace in the Miocene. Perissodactyls maintain similar levels of disparity in limb proportions in the Miocene as in the Oligocene (p = 0.72), but more perissodactyls occupy the cursorial morphospace in the Miocene than the Oligocene. Our results suggest that the shift from closed to relatively open habitat structure from the Oligocene to the Miocene is associated with a significant change in the proximal limb locomotor anatomy and associated limb mechanics of North American ungulate species.