The primate subarcuate fossa and its relationship to the semicircular canals part I: prenatal growth.

Studies have reported a functional link between the arc size of the semicircular canals and locomotor agility across adult primates. However, canal size is spatially interlinked with the subarcuate fossa. This fossa can house the petrosal lobule of the paraflocculus, which also plays a role in coordinating head and eye movements. Consequently, it could be that it is the size of the petrosal lobule and fossa that are directly associated with locomotor agility, and not canal arc size. The apparent association of the latter would only follow from the spatial requirement of the canals to accommodate a suitably enlarged subarcuate fossa and petrosal lobule. This study aims to test the ontogenetic basis of this argument by examining high-resolution magnetic resonance images of fetal samples of Homo sapiens, Macaca nemestrina, and Alouatta caraya. Falsifiable null hypotheses examined are (1) that development of the subarcuate fossa is initiated by growth of the petrosal lobule, and (2) that growth of the semicircular canals and of the subarcuate fossa are independent. The findings confirm that the subarcuate fossa forms independently of a petrosal lobule in all three species, thereby falsifying the first hypothesis. Significant correlations were observed between size variables of the semicircular canals and the subarcuate fossa, particularly between the anterior canal and the opening of the fossa. These results falsify the hypothesis that the canals and fossa grow entirely independently. In the human sample, canal growth outpaces fossa growth, possibly because no petrosal lobule is present in humans. In the other two species, the subarcuate fossa simply seems to fill the space made available by canal growth. However, fossa enlargement cannot be excluded as an influence on size increase in the canals. Nevertheless, taken together, the results suggest that canal size is unlikely to be determined primarily by the spatial requirements of the subarcuate fossa and petrosal lobule, rather than by sensory demands reflected in the empirically established link with locomotor agility.