During mammalian evolution the simple radial enamel, where prisms follow a straight course from the dentin-enamel junction to the tooth surface, gave rise to the typical arrangement of layers of prisms forming Hunter-Schreger bands (HSB). The formation of HSB is believed to have improved the physical properties of enamel. The pattern of HSB has been used to infer relationships between feeding habits and enamel structure. The evolution of enamel structure in the South American faunas is poorly known. Most studies have relied mainly on fossil teeth of Europe and North America mammals, and the majority of the taxa studied are from the Eocene or younger ages. In the present work the patterns of HSB in the enamel of herbivorous mammals from the Paleocene of the Sao Jose de Itaborai basin, Brazil were investigated. Vertical HSB were present in the teeth of large animals (first lower molar width > 8 mm), while transverse HSB were present in taxa with first lower molar width ranging from 5.6 to 3 mm. The only exception was Lamegoia conodonta (first molar width = 10.3 mm). However, maybe to compensate for its larger size, in L. conodonta the transverse HSB were highly undulated. Our results indicate that the development of vertical HSB in large lophodont and bunolophodont herbivors may have occurred as a functional adaptation. Vertical HSB wear at a slower rate than transverse HSB, and this adaptation would enhance functional durability of enamel, extending the overall longevity of the teeth and the animal's life span.254924928