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 Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

is the primary driver of

hypsodont Hypsodont is a pattern of dentition with high-crowned teeth and enamel extending past the gum line, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritt ...

tooth development, not the

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

-rich composition of

grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...

, as was previously thought.

Traditional co-evolution 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 A grassland is an area where the vegetation is dominated by grasses ( Poaceae). However, sedge (Cyperaceae) and rush ( Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur nat ...

. During the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Period (145-66 million years ago), the Great Plains were covered by a shallow inland sea called the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...

which began to recede during the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

to the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pala ...

(65-55 million years ago), leaving behind thick marine deposits and relatively flat terrain. During the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58niche Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development * Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ecology, the ...

for mammals, including many

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...

s 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.

Modern evolutionary hypothesis

Early evidence

In 2006 Strömberg examined the independent acquisition of high-crowned cheek teeth (hypsodonty) in several ungulate lineages (e.g.,

camelid Camelids are members of the biological family Camelidae, the only currently living family in the suborder Tylopoda. The seven extant members of this group are: dromedary camels, Bactrian camels, wild Bactrian camels, llamas, alpacas, vicuñas, ...

equid Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus' ...

s, 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 The Central Great Plains are a semiarid prairie ecoregion of the central United States, part of North American Great Plains. The region runs from west-central Texas through west-central Oklahoma, central Kansas, and south-central Nebrask ...

by early late Arikareean (≥21.9 Million years ago), at least 4 million years prior to the emergence of hypsodonty in

Equidae Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...

. 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

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

s and

perissodactyl Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) o ...

s. The results suggested that high-crowned teeth represent are adapted for a particular feeding environment, not diet preference.

Morphology

More recent 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. Hence the relationship between high-crowned mammals and the source of tooth wear in the fossil record may not be straightforward and the spread of grasslands in South America, traditionally linked with the development of notoungulate hypsodonty, was called into question.

Temporal discontinuity

Most importantly, evidence has shown, that the development of hypsodonty in Cenozoic mammals is out of sync with the flourishing of grasslands both in North America and South America, where grasslands spread 10 million years earlier. Observations of this temporal discontinuity between the spread grasslands and the development of hypsodonty in mammals is also supported by earlier evidence of hypsodonty in dinosaurs. For example,

hadrosaurs Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which incl ...

, a group of herbivorous

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is the ...

, 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.

Modern Examples of Hypsodonty

Hypsodonty is observed both in the fossil record and the modern world. It is a characteristic of large

clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English te ...

(equids) as well as subspecies level specialization. For example, the

Sumatran rhinoceros The Sumatran rhinoceros (''Dicerorhinus sumatrensis''), also known as the Sumatran rhino, hairy rhinoceros or Asian two-horned rhinoceros, is a rare member of the family Rhinocerotidae and one of five extant species of rhinoceros. It is the o ...

and the

Javan rhinoceros The Javan rhinoceros (''Rhinoceros sondaicus''), also known as the Javan rhino, Sunda rhinoceros or lesser one-horned rhinoceros, is a very rare member of the family Rhinocerotidae and one of five extant rhinoceroses. It belongs to the same ge ...

both have brachydont, lophodont cheek teeth whereas the

Indian rhinoceros } The Indian rhinoceros (''Rhinoceros unicornis''), also called the Indian rhino, greater one-horned rhinoceros or great Indian rhinoceros, is a rhinoceros species native to the Indian subcontinent. It is listed as Vulnerable on the IUCN Red Li ...

, has hypsodont dentition. A mammal may have exclusively hypsodont molars or have a mix of dentitions. Hypsodont dentition is characterized by: * High-crowned

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, ...

* A rough, flattish occlusal surface adapted for crushing and grinding *

Cementum Cementum is a specialized calcified substance covering the root of a tooth. The cementum is the part of the periodontium that attaches the teeth to the alveolar bone by anchoring the periodontal ligament.Illustrated Dental Embryology, Histology, ...

both above and below the

gingiva The gums or gingiva (plural: ''gingivae'') consist of the mucosal tissue that lies over the mandible and maxilla inside the mouth. Gum health and disease can have an effect on general health. Structure The gums are part of the soft tissue l ...

l line * Enamel which covers the entire length of the body and likewise extends past the gum line * The cementum and the enamel

invaginate Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegeta ...

into the thick layer of dentine

References

{{reflist, 2 Evolutionary biology Evolution of mammals Biological hypotheses