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
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)
2) Horses in Oligocene
- Mesohippus (Intermediate Horse)
3) Horses in Miocene
- Merychippus (Ruminant Horse)
4) Horses in Pliocene
- 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 & 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.
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)
- 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.
- 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
- 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.
- 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
- 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.
- 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.
Horses in Pliocene
- Sized nearly 40 inches in height.
- Last three toed ancestors of horse.
- Teeth were straight with cusps adapted for grinding.
- 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.
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.
Evolution of Horse has resulted in development of an intelligent, long-legged, swift-running animal, suited to live & feed in open grasslands