The Basicranial Axis in the Rock Hyrax, Procavia capensis (Pallas, 1766) (Mammalia, Afrotheria, Hyracoidea): Novel Structure of the Frontal Bone

Abstract

The composition of the basicranial axis has been reported in the comparative literature in numerous extant and extinct mammals. The primary components are unpaired endochondral bones: the ethmoid, presphenoid, basisphenoid, and basioccipital. Ventral to these are several dermal elements: included are the unpaired vomer, which is widely present, and three bones with more limited distributions, the paired palatines, the paired pterygoids, and the unpaired parasphenoid. Here, we describe novel dermal structures on the dorsal aspect of the basicranial axis in the rock hyrax, Procavia capensis. This taxon has a medial process of the paired frontal bones that meets on the midline and overlies the presphenoid at its juncture with the ethmoid; we term this the presphenoid process of the frontal. Ventral to the presphenoid process is a second smaller process that is off the midline and presents a facet contacting a boss on the presphenoid; we term this the presphenoid facet of the frontal. Occupying the notch between these two structures is the ethmoidal nerve. We sampled the taxonomically diverse collections of the Section of Mammals, Carnegie Museum of Natural History, and the literature. We found additional instances of interfrontal contact dorsal to the ethmoid and presphenoid in the artiodactyl Neotragus and in various platyrrhine and catarrhine primates resembling the presphenoid process of the frontal of P. capensis. However, we did not find a presphenoid facet of the frontal in Neotragus and the primates, which, therefore, thus far is unique to P. capensis. Additionally, we report the presence of a midline suture on the ventral surface of the presphenoid of neonatal P. capensis, which, to our knowledge, is also unique to this mammal.