The New World monkeys (platyrrhines) are a diverse group of primates whose evolutionary history in South and Central America dates back to at least the late Oligocene. Despite a great deal of study in recent decades the evolutionary relationships between species of this group have been difficult to resolve. Studies of craniodental and postcranial anatomy, as well as molecular data, have produced conflicting results, especially with regard to marmosets and tamarins. In addition, the relationships of Saimiri (the squirrel monkeys), Cebus (capuchins), Aotus (owl monkeys), and Callicebus (titi monkeys) to the rest of the New World monkeys are equivocal in most analyses. The present study seeks to contribute to the resolution of these uncertainties by providing information from a hitherto poorly known region of anatomy, the paranasal sinuses. The paranasal sinuses are hollow mucous-lined outpocketings of the nasal cavity that occupy various portions of the facial skeleton in mammals. Sinus anatomy is well documented in Old World monkeys and apes, in which it provides diagnostic features of several genera and groups of genera. It has yet to play a role in studies of NWM relationships because our knowledge of which sinuses are present in different species is very incomplete. What little information we have indicates that these monkeys may exhibit a greater diversity of sinus patterns than any other group of primates. This project will used computed tomography scanning of dry crania as a non-invasive means of imaging the internal anatomy of the sinuses in forty-seven specimens of platyrrhines representing ten genera. Each genus is represented by an age-graded series of specimens from juvenile to adult, allowing the development of their sinuses to be documented for the first time. The taxonomic distribution of sinus anatomy documented by this analysis will be compared to several competing phylogenetic hypotheses in order to reveal which relationships it supports. By completing our knowledge of sinus patterns in higher primates, these data will also facilitate interpretation of sinus anatomy in fossil representatives of Old World monkeys and apes.
