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. Editor's summary: Convergent evolution has been seen across many systems, but perhaps it is nowhere as clear as in Australia, where isolation led marsupials to fill many niches from large carnivores to tiny, mouse-like insectivores. The Australian herbivore niche was largely filled by kangaroos, a group that included many more species in the past than extant taxa would suggest. Couzens et al. looked at tooth structure in this group and found that they developed thick enamel in vertically crushing teeth in response to the increase in grasses. Conversely, placental herbivores elsewhere around the world developed teeth with thin enamel and high crowns that are often used transversely. The isolation of Australia shows how different evolutionary solutions can emerge for similar challenges. —Sacha Vignieri [ABSTRACT FROM AUTHOR]