Vertical prism decussation, a microstructural reinforcement that toughens the cheek tooth enamel of the Rhinocerotoidea, is rare in mammals, where enamel is dominantly strengthened by horizontal decussation. Two hypotheses about the origin of this structure in rhinocerotoids are that it evolved through progressive tilting of the primitively horizontal decussation planes of early tapiroid-like ceratomorphs or that it appeared de novo in some taxon that had no decussation. However, no transitional taxa matching either of these criteria are known. New evidence from the earliest known rhinocerotoid, Hyrachyus, favors the second hypothesis, that vertical decussation arose from a primitive ceratomorph population that had lost most of the horizontal prism decussation characteristic of early ungulates. Hyrachyus has fully vertical prism decussation in its cheek teeth, confirming its relationship to rhinocerotoids, but the structure is poor in organization compared to that of later rhinocerotoids. Almost all mammals larger than insectivores generate sufficient dental stress to require microstructural reinforcement of their enamel, so the presence of poorly organized but fully vertical prism decussation and only faint traces of horizontal prism decussation in Hyrachyus indicate derivation from a taxon that had lost most of the horizontal decussation of primitive perissodactyls. In the late early Eocene ceratomorph Heptodon, whose trichotomous cladistic relationship to tapiroids and rhinocerotoids has been unresolved, there is evidence for considerable loss of enamel microstructural reinforcement, a condition that constrained these individuals to a diet of tender materials, possibly aquatic plant tissues, that required only low dental stresses during mastication. If derived from an enamel structure like that in Heptodon, the emergence of the new microstructurural organization in Hyrachyus would reflect a second dietary reversal producing higher occlusal stresses. The few other extinct lineages known to have acquired uniformly vertical prism decussation, the deperetellid tapiroids, astrapotheres and pyrotheres, may have undergone a similar sequence of dietary changes.