The evolutionary origin of primates has attracted considerable attention over the past century. Several competing hypotheses have been proposed to account for the evolutionary divergence of primates from other mammals. These hypotheses emphasize the importance of variables such as diet, substrate preference, and activity pattern when reconstructing the behavioral and ecological profiles of extinct primates and their predecessors. Morphological analyses of cranial features are frequently employed to test these hypotheses. The infraorbital foramen (IOF) is one morphological feature that has been used in this capacity. The IOF is located below the inferior orbital margin of the maxilla (face). The infraorbital nerve (ION) passes through the IOF, and transmits sensory information from specialized touch cells (mechanoreceptors) of the upper lip, rhinarium, and vibrissae to the brain. For decades, researchers have used the size of the IOF to assign evolutionary relationships and ecological profiles to fossil primates. However, these interpretations are based on untested assumptions linking IOF size to ION size, vibrissal development, and ecology.
The purposes of this doctoral dissertation research project are to establish what IOF size indicates functionally and ecologically and to apply the results to the fossil record in order to evaluate competing hypotheses proposed for the origin of primates. The doctoral student hypothesizes that behavioral and ecological differences among mammals directly affect facial sensory acuity (maxillary mechanoreceptivity) and that the size of the IOF is a measure of relative sensory acuity of the face among mammals. This study poses three questions that will assist in appraising this hypothesis. 1) Does IOF size correlate with IOF area, ION area, and vibrissa number? 2) How does ecology (e.g., diet) affect IOF size? 3) How does IOF size in primates compare to that of other mammals? Data on IOF/ION area and vibrissa counts will be collected on a phylogenetically and ecologically diverse sample of mammalian cadavers, osteological museum specimens and skin pelts, representing 18 mammalian orders. Once the relationships among IOF area, maxillary mechanoreception, and ecology are determined among extant mammals, the results will be applied to questions regarding the evolution and diversification of primates. Testing these hypotheses is difficult at times, given the fragmentary nature of the fossil record. However, the IOF is one anatomical feature that is frequently preserved in fossilized remains. Therefore, the results of this study will allow for an expansion of previous paleoecological analyses and will enhance our current understanding of the evolution of early primates.
In terms of broader impacts, this project will help train a graduate student. Because the questions addressed in this study will be explored using a diverse mammalian sample, the determined correlates of IOF size will be applicable to evolutionary questions in both paleoanthropological and general mammalian paleontological research.
