Contingent evolution of thick enamel by kangaroos to resist dietary abrasion

The late Cenozoic period saw widespread diversification of grazing hoofed mammals characterized by high-crowned molar teeth with low, thinly enameled crests. However, Australia's dominant grazing herbivores, the kangaroos, have much lower-crowned and higher-relief molars that are more akin to some "archaic" placental herbivores. Using x-ray microcomputed tomography scanning, we show that as kangaroos shifted to higher-abrasion grazing diets, their ancestrally vertical, interlocking mode of occlusion favored the acquisition of thick enamel as a durability adaptation. Extant grass-feeding kangaroos have relatively much thicker molar enamel than browsing taxa, especially across the cutting lophs. Data from 41 fossil assemblages reveal a progressive increase in kangaroo enamel thickness from the Late Miocene that correlates with the expansion of arid Australian habitats. The evolutionary success of kangaroos reveals unappreciated contingency in herbivore adaptation and diversification.