Grit not grass: Concordant patterns of early origin of hypsodonty in Great Plains ungulates and Glires

A major step in mammalian evolution was the shift amongst many herbivorous clades from a browsing diet of leaves to a grazing diet of grasses. This was associated with (1) major cooling and increasing continentality and the enormous spread of grasslands in most continents, replacing closed and open forests, and (2) hypsodonty, the possession of high-crowned teeth. Hypsodonty is traditionally linked with eating grass because of the contained phytoliths, silica-rich granules, which are presumed to wear away mammalian dental tissues. However, we present evidence from the Great Plains of North America that the origins of hypsodonty in different clades of ungulates (hoofed mammals) and Glires (rodents and lagomorphs) were substantially out of synchrony with the great spread of grasslands, 26–22 Myr ago (latest Oligocene/earliest Miocene). Moderate hypsodonty was acquired by some Oligocene artiodactyls and several rodent families (mainly burrowers) at least 7 Myr earlier. Highly hypsodont ungulates and hypselodont (= ever-growing cheek teeth) rodents post-date the spread of grasslands by 4 to 9 Myr. Lagomorphs follow a different trend, with hypselodont forms present from near the Eocene–Oligocene boundary. These results indicate that hypsodonty was not a simple adaptation for eating grasses, and may have originated in some clades to counteract the ingestion of grit and soil.