Taxeopody in the carpus and tarsus of Oligocene Pliohyracidae (Mammalia: Hyracoidea) and the phyletic position of hyraxes

Abstract

Recent hyracoids and elephants share a taxeopode arrangement of tarsal and carpal bones, a condition in which bones are aligned with minimal interlocking between adjacent elements. Taxeopody has often been interpreted as a synapomorphy reflecting a close phyletic link between Hyracoidea and Proboscidea, but recently it has been suggested [Fischer, M. S. (1986) Cour. Forschungsinst. Senckenberg 84, 1-132] that hyracoid taxeopody is an independent acquisition resulting from selection favoring increased midcarpal and midtarsal rotation and that Hyracoidea is actually allied with Perissodactyla. As a test of this hypothesis, isolated carpal and tarsal bones of primitive Oligocene hyracoids from the Fayum, Egypt, have been examined to determine whether these indicate a taxeopode or diplarthral carpus and tarsus. Four complete astragali from the Fayum, representing at least three taxa, show a single, slightly convex articular surface on the head for articulation with the navicular and lack a facet for the cuboid. Two complete magna representing two species have a single proximal facet for articulation with the lunar, and they lack a facet for the scaphoid. Thus, both the carpus and tarsus of Fayum hyracoids are taxeopode. Taxeopody in hyracoids cannot be attributed to selection for carpal and tarsal rotation in climbers because the Oligocene, Miocene, and Recent species show great diversity in body size and probably locomotor specializations, despite relative uniformity of structure in the carpus and tarsus. The shared taxeopody of hyracoids and proboscideans, along with other osteological characters and similarities in hemoglobin, eye lens proteins, and other molecules, all suggest that Hyracoidea belongs within Paenungulata.