The Internal Otic Region of Oromerycids (Artiodactyla, Oromerycidae), Early Camelids (Artiodactyla, Camelidae), and the Vicuña (Artiodactyla, Camelidae), Including Notes on Intraspecific and Subadult Ontogenetic Variation.

Abstract

The taxonomic composition of the suborder Tylopoda is an ongoing debate. Recently, the internal otic region (petrosal and bony labyrinth) has been intensively studied as a source of additional morphological data, but the morphology of this region in extinct tylopods is not well documented. To remedy this, we used µCT scanning to image and describe the petrosal and bony labyrinth of two oromerycids (Protylopus, Eotylopus), four early camelids (Poebrotherium wilsoni, Poebrotherium eximium, Paratylopus primaevus, Stevenscamelus franki), and the living vicuña (Vicugna vicugna). Several early camelid specimens also preserved the ear ossicles (malleus, incus, stapes), enabling us to describe their morphology for the first time. Our sample allows us to not only compare among taxa, but to also examine variation within taxa and during ontogeny. We found that the morphology of the petrosal is far more variable than that of the bony labyrinth, both within and across taxa. There is no notable ontogenetic variation between the juveniles and adults in our sample. Protylopus has an unusual petrosal morphology, and its bony labyrinth is somewhat reminiscent of early dichobunoid artiodactyls. Conversely, Eotylopus has a transitional morphology that seemingly links it to camelids. Poebrotherium wilsoni and Po. eximium do not noticeably differ in their morphology, but there are identifiable differences in Pa. primaevus and S. franki, suggesting that the petrosal of camelids is diagnostic at a genus level. The early camelids were distinct from the vicuña in petrosal, bony labyrinth, and ossicular chain morphology, highlighting the importance of examining basally branching members to resolve evolutionary relationships.