This document discusses the evolution of horses over time from early multi-toed ancestors to modern single-toed horses. It traces the lineage from Hyracotherium, a small forest-dwelling animal, through Miohippus and Merychippus as forests shrank and grasslands expanded, to the modern horse Equus. Key evolutionary trends included a reduction in toes, lengthening of legs and body size, and adaptations like elongated muzzles and high-crowned teeth for grazing. These changes were driven by needs to escape predators on the plains and adapt to cooling climates and the dominance of grasses.

1. Horse Evolution
Geology 331Geology 331
Paleontology

2. Horses

3. Equus caballus

5. Equus burchelli: the zebra

6. Horse-Zebra hybrid
Shows that genes are discrete and not blended by
genetic recombination.

7. The extinct quagga (1883), related to today's
1870
plains zebras.

8. “Family tree”
of the horsesof the horses

10. Ungulate ToesUngulate Toes
Artiodact ls: Perissodact ls:Artiodactyls:
even-toed
Perissodactyls:
odd-toed
Bovine and swine Horse and rhinocerasBovine and swine Horse and rhinoceras

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

12. EquusMerychippusMiohippusHyracotherium
Hind feet left, front feet right.

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

14. Orohippus from the late Eocene, also a forest browser

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

16. MerychippusMerychippus
from the
MioceneMiocene,
a plains
grazer

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

18. 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

19. 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 tou g e ope . o g ed u e o
reach grass.
• Grasses are high in silica. High crownedg g
teeth are adaptive.