Ontogenetic,intraspecific, and interspecific variation of the prehallux in primates: Implications for its utility in the assessment of phylogeny

The prehallux is a sesamoid bone occurring in the region of the hallucial tarso-metatarsal joint in a number of metatherian and eutherian orders and in some nonmammalian tetrapods. Within the order Primates, it occurs invariably in ceboids and Hylobates, with extreme infrequency in pongids and Homo, and is absent in other primates groups. It has been suggested that, first, the prehallux is homologous both within and across the infraclasses Metatheria and Eutheria; second, it has functional significance in that it contributes to joint stability and is an adaptation to arboreality; third, its presence results in diagnostic features on the entocuneiform and hallucial metatarsal, so that original presence or absence can be unambiguously assessed in instances when the bone itself is not preserved; and fourth, because of presumed homology, it may be employed in the reconstruction of phylogenetic relationships. The present study concludes that the homologous nature of the bone is open to reasonable doubt, the assumption of homology does not yield significantly more parsimonious phylogeny reconstructions than does the assumption of analogy, there are no invariant diagnostic features associated with its presence, and functional explanations currently offered are of questionable validity. Thus, the prehallux is at present of little utility in either establishing or precluding phylogenetic relationships among primates.