The relationship between the infraorbital foramen, infraorbital nerve, and maxillary mechanoreception: Implications for interpreting the paleoecology of fossil mammals based on infraorbital foramen size

Osteological cranial features, such as foramina, assist in phylogenetic and ecological interpretations of fossil mammals. However, the validity of using foramina in these interpretations when their contents are not well documented is questionable. For decades, the infraorbital foramen (IOF) has been used to interpret aspects of the fossil record, yet there are conflicting accounts about what passes through the foramen and little known about how neural and vascular structures contribute to its contents. This study tracks and documents the neural and/or vascular anatomy of the IOF and examines the correlation of infraorbital nerve (ION) and IOF cross-sectional area. To address this question, 161 mammalian cadavers, including 80 primates, were injected with latex dye to track the vascular anatomy associated with the IOF. All ION fibers were then removed from the infraorbital canal, and ION cross-sectional area was calculated from histological slides. Latex injections and histological slides revealed that only the ION and a small infraorbital artery pass through the IOF. Variation in ION size explains 85% of variation in IOF area, and the artery represents a negligible portion of the foramen. The strong positive correlation between the ION and IOF size suggests that, in the absence of nerve tissue, the IOF can serve as a proxy for ION area. IOF area may be used to evaluate differences in maxillary mechanoreception in both extinct and extant taxa.