Equus: a genus containing horses, asses and zebras: a total of SEVEN LIVING SPECIES

Equus quagga (plains zebra) Equus africanus (African wild ass)

Equus : (Prezewalkski’s Horse) * only remaining “wild horse” population * It went extinct in the wild in about 1966, but some had been captured. Using only NINE, they were bred and reintroduced. Now there are +/- 300 living in Mongolia.

Equus ferus caballus (domestic horse)

http://evopalaeo.blogspot.com/2012/04/tree-thinking-2.html Look here for an explanation of monophyletic, paraphyletic and polyphyletic

Three (front) and four (hind) toes, with pads (like dogs have). Hyracotherium: (55 million years ago) 2 ½ feet long; 20 lbs Three (front) and four (hind) toes, with pads (like dogs have). Browser of Forest Habitat (soft leaves, fruits)

Evolution has definitely produced some larger horses over time… ? Evolution has definitely produced some larger horses over time… … but is the story really this straightforward and simple?

The grinding teeth (molars) of browsers can have crowns of average height. Hyracotherium

Browsing Horse Miohippus – A Larger, Three-Toed More Adapted to Grasslands

Miohippus was almost twice the size of Hyracotherium, (so its skull and teeth were bigger too), but its molars still had crowns of average height. Miohippus

Where did our grasslands come from? - Cretaceous/Western Interior Seaway was gone by the time Hyracotherium came on the scene, but North American interior climates were becoming drier - During the Miocene, many plants (including grasses) evolved a type of photosynthesis known as C4 photosynthesis, which is better in dry environments.

Merychippus – A grazing, Three-Toed Horse of North America

Merychippus was even larger than Miohippus, and its teeth started to show higher crowns.

Why might grazing require teeth with higher crowns? Grasses are high in silicates; hard, gritty minerals based on the element Silicon (this is the same element that makes up sand, quartz crystal, and glass!) Grass is abrasive and wears teeth down fast. Higher molar crowns means more tooth to wear down! Kangaroos are also grazers; but they wear their molars all the way down until they move forward and fall out, and are replaced with new molars that grow from the backs of their mouths.

Merychippus had a worldwide Distribution

Pliohippus – A One-Toed Grazing Horse

Pliohippus: getting bigger, and molar crowns getting taller!

Zebras, Asses, Prezewalski’s Horse, Domestic Horse Equus Zebras, Asses, Prezewalski’s Horse, Domestic Horse

Equus – Historic World-wide Distribution

Is there a relationship between grazing and the evolution of large body size? – Perhaps. Grasses are pretty low in nutrition. Animals that eat mainly grass (such as cows and horses) may need to be larger than browsers (such as deer and goats, which eat more diverse plant foods). Why? Perhaps it’s so they can ingest and store more food in their stomachs to get the nutrition they need.

So the evolutionary pathway leading up to the modern horse (Equus) involved a reduction in toe number, an increase in overall size, and changes in foot shape and molar crown height that may have been better adapted for a grazing lifestyle.

HOWEVER… Was the evolutionary pathway leading to our modern horse (Equus) the only evolutionary pathway that occurred in horses?

NO! The evolutionary tree of horses shows many lineages – some of which remained small, some of which did not change in size, some of which remained browsers. Thus the *complete* story of horse evolution is not a single, linear progression from small many-toed browsers to large single-toed grazers.