The primate subarcuate fossa and its relationship to the semicircular canals part I: prenatal growth Nathan Jeffery a, * , Fred Spoor b a Department of Human Anatomy and Cell Biology, School of Biomedical Sciences, University of Liverpool, Ashton Street, Liverpool, L69 3GE, UK b Evolutionary Anatomy Unit, Department of Anatomy and Developmental Biology, University College London, London, WC1E 6BT, UK Received 8 February 2006; accepted 7 July 2006 Abstract 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 be- tween 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Petrosal lobule; Locomotion; Ontogeny; Evolution; Semicircular canals; Primate; Subarcuate fossa; High-resolution magnetic resonance imaging Introduction The semicircular canal system of the inner ear provides information on angular head movements used in reflex adjust- ments of body and head posture and eye position (i.e., stabili- zation of gaze), particularly during locomotion. Being one of the few sensory systems that leaves a direct imprint in the fos- sil record in the form of the bony labyrinth, the semicircular canals have been used to infer locomotor strategies among ex- tinct primates (e.g., Spoor, 1993; Spoor et al., 1996; Walker et al., 2003, 2004). Broadly speaking, canal arc size is consid- ered a good indicator of locomotor agility in that acrobatic, quick-moving species have relatively larger canals than slower-moving species of a similar body size (Spoor and Zonneveld, 1998; Spoor, 2003). This empirically established association is based on the notion that arc size mechanically affects the sensitivity of the canal system, and that this prop- erty needs to match the sensory requirements of specific loco- motor behaviors (Spoor, 2003; for a recent review of the biomechanics of the semicircular canal system, see Rabbitt * Corresponding author. Tel.: þ44 151 794 5514; fax: þ44 151 794 5517. E-mail address: njeffery@liv.ac.uk (N. Jeffery). 0047-2484/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2006.07.003 Journal of Human Evolution 51 (2006) 537e549