100 - Evolutionary Polarity of Primate Neurocranial Shape

Poster Board Number: 100
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Abstract Body : Among extant anthropoid primates, New World (“platyrrhine”) monkeys from the cebid family, including cebines and callitrichines, exhibit distinct “dolichocephaly,” a cranial morphology in which anteroposterior length of the neurocranium far exceeds its mediolateral width. Previous research indicates that body size alone cannot explain observed variation in dolichocephaly across anthropoid taxa; cebids such as Saimiri are significantly more dolichocephalic than similarly-sized Old World (“catarrhine”) monkeys such as Miopithecus. Extreme dolichocephaly appears to be unique to cebid monkeys rather than an artifact of allometric scaling. However, it remains unclear whether dolichocephaly is an evolutionarily derived cebid trait, or an ancestral phenotype that was convergently lost in catarrhines and platyrrhines outside the cebid family.

To address this question, caliper measurements of neurocranial length (glabella-lambda) and biparietal breadth were collected for 800 primate specimens spanning extant New World monkeys, Old World monkeys, and apes. Neurocranial length and breadth were also measured from CT scans of fossil primates Parapithecus, Aegyptopithecus, and Homunculus, representing stem anthropoids, stem catarrhines, and basal platyrrhines respectively. Neurocranial length was divided by breadth to calculate species-average dolichocephaly. Using maximum parsimony methods in Mesquite 3.70, dolichocephaly was reconstructed at ancestral nodes across an anthropoid phylogenetic tree. Given ongoing debate regarding platyrrhine phylogeny, several alternative trees were evaluated. Our analysis indicates that stem catarrhine Aegyptopithecus groups with the non-dolichocephalic extant catarrhines, while the stem anthropoid Parapithecus and basal platyrrhine Homunculus both exhibit degrees of dolichocephaly similar to modern cebids, supporting the hypothesis that a reduction in dolichocephaly occurred independently in catarrhines and non-cebid platyrrhines.

These results suggest that dolichocephalic neurocranial shape is a plesiomorphic anthropoid trait retained in the cebid lineage; furthermore, they imply that platyrrhine and catarrhine ancestors potentially diverged in neurocranial morphology as early as the Oligocene epoch. In reconstructing the evolutionary history of anthropoid neurocranial shape, we also lay a foundation for prospective work on the significance of neurocranial shape variation observed across nonhuman primates. Future directions include investigating adaptive hypotheses for cebid dolichocephaly, such as the potential role of obstetric cephalopelvic disproportion in constraining neurocranial shape; and comparisons of the ontogeny of neurocranial shape in dolichocephalic cebids and like-sized, non-dolichocephalic Old World monkeys.