THE PRIMATE ECTOTYMPANIC TUBE: CORRELATES OF STRUCTURE, FUNCTION, AND DEVELOPMENT

The primate ectotympanic bone form is frequently used in anthropology as a discrete characteristic with strong phylogenetic signaling among primates; however, the proximate mechanism by which this variation operates has not been shown. Some primates (catarrhines and tarsiers) have been described as having an elongate bony ectotympanic tube; others (strepsirrhines and platyrrhines) maintain just a ring of bone. While some functional theories have been suggested, the present dissertation project suggests that the variation in the ectotympanic bones is more likely a morphological response to overall cranial shape. Structural correlates were identified using geometric morphometric methods in two sets of analyses, one using large intraspecific sample sizes of two catarrhine species and the other looking across all primates. The relative length of the ectotympanic bone in both of these analyses is tested for correlations with overall cranial structure. Two fossil catarrhines are included that have been noted for their unusual ectotympanic bones: Aegyptopithecus zeuxis and Pliopithecus vindobonensis. This dissertation project also illustrates the growth of the ectotympanic bone in primates via an in-depth discussion of the human, catarrhine, tarsier, and lorisiform growth patterning. This subproject is presented using non-metric analyses and qualitative description. Across analyses, these three subprojects show that the relative length of the ectotympanic tube is most correlated and potentially driven by the width of the cranium relative to the length of the cranium. Relatively wide crania and wide brains tend to present with long ectotympanic bones. Thus, while ectotympanic bone morphology is still useful in descriptions of primate phylogeny, it is more likely that it is a consequence of the shape of the cranium rather than an independent character. The ectotympanic bones of the two fossil species are shown to be abnormally short for catarrhine taxa, more similar to the platyrrhine condition. There is much more diversity in ectotympanic morphology than previously appreciated, particularly among lorisiforms. Some of that diversity is accounted for in the way in which the ectotympanic develops across taxa; lorisiforms and tarsiiformes are born with fully developed ectotympanic rings that lengthen to variable degrees likely due to relative cranial width