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 3rd digit.
  • Perfection of hoof on 3rd 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)
Horse Evolution through different periods

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 & 5th) 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.
Forelimb – Eohippus
Hindlimb – Eohippus

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.
  • 3rd & 4th 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 – 2nd & 4th 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.
Forelimb – Merychippus
Hindlimb – Merychippus

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-2nd & 4th 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.
Forelimb – Pliohippus
Hindlimb – Pliohippus

Horses in Pleistocene

1) Equus (The modern horse)

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

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


Horse Evolution Part 1
Horse Evolution Part 2

Harjeet Kaur

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