Grit, not grass hypothesis

The Grit, not grass hypothesis is an evolutionary hypothesis that explains the evolution of high-crowned teeth, particularly in New World mammals. The hypothesis is that the ingestion of gritty soil is the primary driver of hypsodont tooth development, not the silica-rich composition of grass, as was previously thought.

Since the morphology of the hypsodont tooth is suited to a more abrasive diet, hypsodonty was thought to have evolved concurrently with the spread of grasslands. During the Cretaceous Period (145-66 million years ago), the Great Plains were covered by a shallow inland sea called the Western Interior Seaway which began to recede during the Late Cretaceous to the Paleocene (65-55 million years ago), leaving behind thick marine deposits and a relatively flat terrain. During the Miocene and Pliocene epochs (25 million years), the continental climate became favorable to the evolution of grasslands. Existing forest biomes declined and grasslands became much more widespread. The grasslands provided a new niche for mammals, including many ungulates that switched from browsing diets to grazing diets. Grass contains silica-rich phytoliths (abrasive granules), which wear away dental tissue more quickly. So the spread of grasslands was linked to the development of high-crowned (hypsodont) teeth in grazers.

In 2006 Strömberg examined the independent acquisition of high-crowned cheek teeth (hypsodonty) in several ungulate lineages (e.g., camelids, equids, rhinoceroses) from the early to middle Miocene of North America, which had been classically linked to the spread of grasslands She showed habitats dominated by C3 grasses (cool-season grasses) were established in the Central Great Plains by early late Arikareean (≥21.9 Million years ago), at least 4 million years prior to the emergence of hypsodonty in Equidae. In 2008 Mendoza and Palmqvist determined the relative importance of grass consumption and open habitat foraging in the development of hypsodont teeth using a dataset of 134 species of artiodactyls and perissodactyls. The results suggested that high-crowned teeth represent are adapted for a particular feeding environment, not diet preference.

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