Comparative Morphology of Premolar Foramen in Lagomorphs (Mammalia: Glires) and Its Functional and Phylogenetic Implications

Lagomorphs (a group that consists of pikas, hares, rabbits and allies) are notable for their conservative morphology retained for most of their over 50 million years evolutionary history. On the other hand, their remarkable morphological uniformity partly stems from a considerable number of homoplasies in cranial and dental structures that hamper phylogenetic analyses. The premolar foramen, an opening in the palate of lagomorphs, has been characterized as an important synapomorphy of one clade, Ochotonidae (pikas). Within Lagomorpha, however, its phylogenetic distribution is much wider, the foramen being present not only in all ochotonids but also in leporids and stem taxa; its morphology and incidence also varies considerably across the order, even intraspecifically. In this study, we provide a broad survey of the taxonomic distribution of the premolar foramen in extant and fossil Lagomorpha and describe in detail the morphological variation of this character within the group. Micro-computed tomography was used to examine the hard palate and infraorbital groove morphology in Poelagus (Leporidae) and Ochotona. Scans revealed the course and contacts of the canal behind the premolar foramen and structural differences between the two crown clades. We propose that the premolar foramen has evolved independently in several lineages of Lagomorpha, and we discuss development and function of this foramen in the lagomorph skull. This study shows the importance of comprehensive studies on phylogenetically informative non-dental characters in Lagomorpha.