Ecological and morphological correlates of infraorbital foramen size and its paleoecological implications
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The infraorbital nerve (ION) transmits sensory information from mechanoreceptors of the upper lip and vibrissae (whiskers) to the brain via the infraorbital foramen (IOF). Vibrissae are special sensory hairs used by mammals to explore their surroundings. Researchers have used the size of the IOF to infer vibrissa count, which in turn has been incorporated into phylogenetic and ecological interpretations of fossil taxa. However, these interpretations are based on untested assumptions linking IOF size, ION size, vibrissae, and ecology. The purpose of this dissertation is to determine the functional significance of IOF size and to apply the results to the fossil record. It is hypothesized that ecological differences among mammals affects maxillary mechanoreceptivity (touch sensitivity of the rostrum), and that IOF area may be used as a measure of maxillary mechanoreceptivity. Three questions are posed to appraise this hypothesis: (1) Does IOF area correlate with ION area and vibrissa count? (2) How do the IOFs of primates differ from those of other mammals? (3) How do diet, substrate preference, and activity pattern affect IOF size? IOF area, ION area, and vibrissa count were collected from cadaver of extant mammals as well as museum osteological specimens. Results indicate that: (1) IOF and ION areas show a strong positive correlation. Based on this finding, it is hypothesized that IOF area may be a good measure of maxillary mechanoreception. (2) Vibrissae count significantly correlates with IOF area. (3) Euarchontans have relatively smaller IOFs than most other mammals. (4) The IOFs of primates co-vary with diet, where frugivores have relatively larger IOFs than both insectivores and folivores. Infraorbital foramen areas of 14 adapoid, six omomyoid, and 15 plesiadapiform species were measured. Two questions were addressed: (1) Do the sampled fossils share a similar reduction in IOF area to extant primates? (2) Do extinct frugivores have larger IOFs than insectivores and folivores? Results show that, adapoids and omomyoids have relatively small IOFs similar to euarchontans, but plesiadapiforms retain larger IOFs, comparable to most non-euarchontan mammals. Dietary analyses indicate that both frugivorous adapoids and omomyoids have larger IOFs than both insectivorous and folivorous species.