Horse Evolution

Geology 331Geology 331Paleontology

Horses

Equus caballus

Equus burchelli: the zebra

Horse-Zebra hybridShows that genes are discrete and not blended by

genetic recombination.

The extinct quagga (1883), related to today's 1870

plains zebras.

“Family tree” of the horsesof the horses

Ungulate ToesUngulate Toes

Artiodact ls: Perissodact ls:Artiodactyls: even-toed

Perissodactyls: odd-toed

Bovine and swine Horse and rhinocerasBovine and swine Horse and rhinoceras

Horses reduced their toes from five to one, while also lengthening the , g gbones, during their evolution.

EquusMerychippusMiohippusHyracotherium

Hind feet left, front feet right.

Hyracotherium or Eohippus from the Eocene, a forest browsera forest browser

Orohippus from the late Eocene, also a forest browser

Miohippus from the Oligocene. Forests were shrinking, grasslands were expanding.g, g p g

MerychippusMerychippusfrom the MioceneMiocene, a plains grazer

The modern horseThe modern horse Equus from the Pl i t dPleistocene and

Recent

Evolutionary trends in horses andEvolutionary trends in horses and many other ungulates

• Reduction in toes• Elongation of legs by standing on the toes• Increased body size• Elongation of the muzzle• High crowned specialized teeth• High crowned, specialized teeth

Environmental Causes of Evolutionary trends in horses and

many other ungulatesmany other ungulates• Need for greater speed and body size to

escape predatorsescape predators.• Cooling climate during the Cenozoic

replaced forests with grasslands Simplereplaced forests with grasslands. Simple hooves and elongated legs are adaptive for running in the open. Elongated muzzle to u g e ope . o g ed u e oreach grass.

• Grasses are high in silica. High crowned g gteeth are adaptive.