Evolution of Horse

Classification of Modern Horses

Kingdom: Animalia (Multicellular, eukaryotic organisms)

Phylum: Chordata (Possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail)

Class: Mammalia (Endothermic amniote possessing a neocortex, hairs, three middle ear bones and mammary glands)

Order: Perissodactyla (Odd – toed ungulates)

Family: Equidae (Thick skulled with stocky bodies)

Genus: Equus – Only recognized extant genus in the family Equidae

Where & When Horses Originated?

The history of Horse’s Evolution dates back to beginning of Eocene Period of Coenozoic Era. The first known ancestors of horse were dog-sized, living on moist ground & soft leafy vegetation.

First known fossil of Horse from Europe: Hyracotherium

&

Its contemporary fossil of Horse from America: Eohippus

Evolutionary Trends

1) Changes in Limbs

• Foot posture changes; from Plantigrade to Unguligrade.
• Reduction in the number of digits from 4/5 to 1 in each foot.
• Gradual enlargement & elongation of 3^rd digit.
• Perfection of hoof on 3^rd digit.
• Development of spring mechanism.
• Length of limbs increased.

2) Change in Neck

• Elongation of neck, enabling horse for ground grass grazing.

3) Changes in Teeth

• High crowns developed on molars & premolars. (Brachydont Dentition was replaced by Hypsodont Dentition).
• Loss of Canines.
• Appearance & Increase of Diastema (space between incisors & grinding teeth).

4) Changes in Brain & Skull

• Increase in size & complexity of cerebral hemispheres & cerebellum.
• Olfactory lobes hidden under cerebrum.
• Preorbital Region of skull elongated.

Horses through different Periods

1) Horses in Eocene

• Hyracotherium (Old World Horse)& Eohippus (Dawn Horse)
• Orohippus (Mountain Horse)
• Epihippus

2) Horses in Oligocene

• Mesohippus (Intermediate Horse)
• Miohippus

3) Horses in Miocene

• Parahippus
• Merychippus (Ruminant Horse)

4) Horses in Pliocene

• Hipparion
• Pliohippus (Pliocene Horse)

5) Horses in Plistocene

• Equus (Modern Horse)

Horses in Eocene

1) Hyracotherium & Eohippus

• Descended from Five Toed Hoofed mammals.
• Hyracotherium (Old World Horse) fossils obtained from London Clay.
• Eohippus (Dawn Horse) fossils obtained from Big Horn Basin in North America.
• Hyracotherium & Eohippus were very similar to each other.

Features of Eohippus…

• Small, dog sized browsing animal.
  • Skull and Neck were short.
  • Hindlimb was slightly longer than forelimb.
  • Forelimb had four digits (2, 3, 4 & 5^th) whereas hindlimb had three digits.
  • All the toes touched the ground & had pads underneath.
  • Ulna & fibula were stout and were well built free from radius & tibia respectively.
  • Low crowned teeth i.e. Brachydont.
  • Teeth were 44 in number.
  • Cerebral Hemispheres were small & smooth and they neither covered the olfactory bulb nor the midbrain.

Hyracotherium also had similar features some of them are mentioned below:

Features of Hyracotherium…

• Forest dweller & used to browse on soft vegetation.
  • Small, dog sized, nearly 60cm in length.
  • Forelimbs had 4 toes & hindlimb had 3.
  • Had a pad on underside of toes.
  • Low crowned teeth i.e. brachydont. Teeth were 44 in number.
  • It had a short diastema (space between front teeth and cheek teeth)

2) Orohippus

• Fossils were recovered from Bridger Beds, New Mexico
• Little higher than Eohippus.
• Browsing animal.
• Forelimbs had 4 toes & hindlimbs had 3.
• Middle digit in both toes was prominent.
• 3^rd & 4^th premolars exhibited a trend towards molarisation.

3) Epihippus

• Slightly larger than Orohippus.
• Forelimbs had four digits & hindlimbs had 3 digits.
• Forest residing browser.
• Last two premolars were molar-like in appearance.

These forms became extinct by end of Eocene & got replaced by Mesohippus.

Horses in Oligocene

1) Mesohippus

• Sheep sized, measuring 18-24 inches in height.
• Neck was short.
• Long & slender legs.
• No.of toes in both limbs i.e. forelimbs & hindlimbs reduced to three.
• Trunk was long & slender with more arched back.
• All the three digits touched the ground but middle one was larger.
• Feet had pad under the toes showing beginning of hoof.
• Dentition was brachydont i.e. molars were still low crowned.
• Diastema appeared.
• Cerebral Hemispheres were enlarged.

2) Miohippus

• Larger than Mesohippus.
• Replaced Mesohippus in Late Oligocene.
• Forest dweller & a browser.
• Forelimbs as well as hindlimbs were three toed.
• Still were still low crowned i.e. brachydont.

Horses in Miocene

1) Parahippus

• Although these had three toes in both fore & hind limbs, the third digit in both became more predominant.
• Limbs elongated.
• Dentition became nearly hypsodont i.e. high crowned.
• Snout considerably elongated.

2) Merychippus

• First three-toed grazers feeding on grass.
• The side toes – 2^nd & 4^th were reduced and no longer touched the ground.
• Third i.e. central one had large convex hoof.
• Ligaments of the muscles were very well developed.
• Spring mechanism present.
• Foot pads absent.
• Diastema was very well developed.
• Hysodont teeth i.e. high crowned.
• Cerebral Hemispheres were large, convulated & showing fundamental pattern of fissures.

Horses in Pliocene

1) Hipparion

• Sized nearly 40 inches in height.
• Last three toed ancestors of horse.
• Teeth were straight with cusps adapted for grinding.

2) Pliohippus

• First one-toed horse.
• Sized 40 inches nearly in height.
• Side toes-2^nd & 4^th reduced. In some species they changed to splints as in modern horse.
• Crowns of upper molars were similar to those of modern horse.
• Increased in size.
• Increased preorbital length of skull.
• Increased size & complexity of molar teeth.

Horses in Pleistocene

1) Equus (The modern horse)

• Sized increased as compared to Pliohippus.
• About 60 inches in height.
• 2^nd & 4^th digits were represented by splints whereas 1^st & 5^th were completely lost.
• Whole body weight is balanced on 3^rd digit. It also bears well developed hoof.
• Molar crowns were much elongated.
• Enlarged brain; cerebral hemispheres have grooved surface.

Evolution of Horse has resulted in development of an intelligent, long-legged, swift-running animal, suited to live & feed in open grasslands

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