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.

Traditional Coevolution Hypothesis

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, 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, leaving behind thick marine deposits and a relatively flat terrain. During the Miocene and Pliocene epochs, 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, which wear away dental tissue more quickly. So the spread of grasslands was linked to the development of high-crowned teeth in grazers.

Modern Evolutionary Hypothesis

Early Evidence

In 2006 Strömberg examined the independent acquisition of high-crowned cheek teeth in several ungulate lineages 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 were established in the Central Great Plains by early late Arikareean, 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.

Morphology

Further, examination of mammalian teeth suggests that it is the open, gritty habitat and not the grass itself which is linked to diet changes. Analysis of dental microwear patterns of hypsodont notoungulates from the Late Oligocene Salla Beds of Bolivia showed shearing movements are associated with a diet rich in tough plants, not necessarily grasses. That the relationship between high-crowned mammals and the source of tooth wear in the fossil record may not be straightforward, hence the spread of grasslands in South America, traditionally linked with the development of notoungulate hypsodonty, is in question.
Observations of hypsodonty development in Cenozoic mammals is also supported by the evolution of hypsodonty in other groups. For example, hadrosaurs, a group of herbivorous dinosaurs, likely grazed on low-lying vegetation and microwear patterns show that their diet contained an abrasive material, such as grit or silica. Grasses had evolved by the Late Cretaceous, but were not particularly common, so this study concluded that grass probably did not play a major component in the hadrosaur's diet.

Temporal Discontinuity

Most importantly, evidence has shown, that the development of hypsodonty is out of sync with the flourishing of grasslands both in North America and South America, where grasslands spread 10 million years earlier.

Hypsodonty

Hypsodonty is observed both in the fossil record and the modern world. It is a characteristic of large clades as well as subspecies level specialization. For example, the Sumatran rhinoceros and the Javan rhinoceros both have brachydont, lophodont cheek teeth whereas the Indian rhinoceros, has hypsodont dentition. A mammal may have exclusively hypsodont molars or have a mix of dentitions. Hypsodont dentition is characterized by: