Comparative Basicranial Anatomy of Extant Terrestrial and Semiaquatic Artiodactyla

ABSTRACT Comparative data from the ear region has played an important role in recent combined-data phylogenetic analyses of the relationships of living and extinct Artiodactyla and the position of that clade among Euungulata. These studies have also been important for establishing the phylogenetic position of Cetacea and for understanding the relationships of a diversity of euungulate species to their fossil relatives. Detailed and standardized descriptive reference works of the basicranium for a range of living artiodactylans are not, however, readily available. Here I describe exemplar species from the four major extant terrestrial and semiaquatic artiodactylan clades (Hippopotamidae, Ruminantia, Suina, and Camelidae) and illustrate the anatomy of the ear region with the auditory bulla both in place and removed. Terrestrial artiodactyls exhibit varying degrees of expansion of the bony external acoustic meatus laterally relative to the mediolateral dimensions of the rounded, medial aspect of the auditory bulla, a characteristic that is least developed in Tragulidae. A relatively elongate external acoustic meatus has previously been described as entirely absent in living and fossil cetaceans and in some fossil species such as Diacodexis pakistanensis. Variation also exists in the proximity of the petrosal-bullar complex to midline basicranial bones. Isolation of these bones from other basicranial structures has been previously interpreted as functionally important for underwater hearing in Cetacea. Many artiodactylans have contact between the auditory bulla and the basioccipital but no contact between the deeper pars cochlearis of the petrosal bone and the basioccipital/basisphenoid. Exceptions are species of Hippopotamidae in which both the bulla and the petrosal are separated from midline bones. The functional interpretation of this separation has previously been linked to aquatic hearing, but this association may be more complex than originally thought. Other features observed in the basicrania of terrestrial artiodactylans described here are a general coalescence of basicranial foramina (i.e., the basicapsular fissure, carotid foramen, piriform fenestra, and sometimes the foramen ovale), the development of large and ornate styliform processes in species of Ruminantia, and widespread contact between the auditory bulla and the paracondylar process of the exoccipital.