Evolution of the occlusal morphology of hominin postcanines as modeled through the inhibitory cascade.

The inhibitory cascade is an experimentally-derived model that describes changes in the relative occlusal area of mammalian postcanines. The inhibitory cascade has been tested in samples of rodents, ungulates, carnivores, and platyrrhines and may be useful for interpreting dental trends observed in fossil hominins. However, the inhibitory cascade has not yet been validated in a comprehensive sample of catarrhines. This study examines relative postcanine sizes in the mandibular dentitions of 11 extant, non-hominin catarrhine genera (N>250), modern humans (N=20), and three fossil hominin genera (N=43). Mesiodistal and buccolingual diameters were measured and used to compute occlusal area, and relative occlusal areas were analyzed in the context of the inhibitory cascade. The results validate the application of the inhibitory cascade to catarrhine postcanines, specifically showing that cercopithecoids are best described by relative occlusal area proportions of M1<M2 <M3, and hominoids are best described by relative occlusal area proportions of M1<M2≈M3. However, Paranthropus boisei does not follow the predictions of the inhibitory cascade. Instead, it has unusually large P4s and M2s for catarrhines and differs from its congener Paranthropus robustus in this regard. The sort of differences seen between the inhibitory cascades of P. boisei and P. robustus are not found in extant catarrhine genera for which there is sound evidence of monophyly. Evidence from the inhibitory cascade suggests that we should revisit the hypothesis that the megadontia and hyper-megadontia seen in the taxa presently included in the hominin genus Paranthropus are homologous traits. [ABSTRACT FROM AUTHOR]