Several studies have quantified selective capability in artiodactyls using skeletal correlates such as the premaxillary shape index (PSI), incisor arcade breadth (IAB), incisor arcade curvature (DOC), and incisor width ratio (IWR). These metrics are limited in their applicability because they do not account for the potential importance of muzzle length. Herein we apply a new method for the quantification of muzzle shape (the dentary shape index [DSI]) that combines dentary length and width. We find that browsers possess the longest, narrowest muzzles, whereas the muzzles of grazers are comparatively short and wide. Discriminant function analysis demonstrates that DSI yields the highest rate of correct dietary classification of all muzzle shape proxies proposed to date. Generalized estimating equations and phylogenetically independent contrasts show that the shape of the anterior dentary (DSI) is significantly correlated with both diet and premaxillary shape (PSI) across the cetartiodactyl tree. We suggest that the match between premaxillary and anterior dentary shape is important in cropping but it is the shape of the lower incisor arcade that is under direct dietary selection. We conclude that the length and width of the anterior dentary are functional determinants of selectivity in ruminant artiodactyls and that these traits have potentially evolved in response to selection for efficient feeding as it is related to the different dietary requirements of browsers and grazers.