Theodor, J. M.*; Janis, C. M.; Boisvert, B.: Camelid foot morphology and the evolution of the pacing gait

Among wild mammals, camels are unique in their use of a running pacing gait instead of a trot. Extant camelids are characterized by features unusual among artiodactyls: their feet are secondarily digitigrade, with a broad, flat pad in place of hooves; and the distal metapodials are widely splayed. These features have been assumed to represent an adaptation for pacing, to increase lateral stability in this rolling gait. Morphometric analysis of 16 measurements of the metapodials and phalanges of extant camelids and ruminants revealed distinct differences. Extant camelids have relatively longer first phalanges with larger, flattened distal articulations extending onto the volar surface, and the distal phalanx is short and flat, while ruminants have longer second phalanges with an extension of the distal facet onto the volar surface, indicating unguligrade foot posture. The sample included 18 genera of extinct camelids from the Tertiary of North America, most of which showed some morphological features typical of modern forms, and a number clustered with the extant camelids. Extinct members of the Camelinae and Protolabis show all of the morphological features of modern camelid foot posture, but the current phylogeny indicates they evolved these features in parallel. Independent evidence from trackways associated with Protolabis shows that this taxon used a pacing gait, indicating that these foot morphologies are correlated with pacing. Evolutionary change towards a modern camelid-like condition was also seen within the subfamilies Stenomylinae and Miolabinae. These parallel changes in camelid locomotor anatomy occurred in the late Oligocene or early Miocene, preceding the formation of open grassland habitats in the late Miocene.