Evolutionary Trend in Equidae | Geology Optional Notes for UPSC PDF Download

Horses as Odd-Toed Ungulates

• Horses, like rhinoceroses and tapirs, are odd-toed ungulates, having 1, 3, or 5 toes on their feet.
• They belong to the Order Perissodactyla, characterized by hooves on their fingers and toes.

Even-Toed Ungulates

• Cows and camels are examples of even-toed ungulates, with 2 or 4 toes, belonging to the Order Artiodactyla.

Equidae Family

• The Equidae family includes horses, donkeys, and zebras, with horses placed in the genus Equus.

Systematic Palaeontology

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Class: Mammalia
• Order: Perissodactyla
• Family: Equidae
• Genus: Equus

Place and Time of Origin

Origin of Horses

• The evolution of horses began in the Eocene epoch of the Cenozoic era.
• North America is considered the original home of horses.

Perissodactyls and Environment

• Perissodactyls, including horses, rhinoceroses, and tapirs, are believed to have originated in a forest environment during the Early Eocene period.

Clear all your UPSC doubts with EduRev Join for Free

Join for Free

The Evolutionary History of Horses

Fossil Records

• The evolutionary journey of horses is well-documented due to the abundance of well-preserved fossil remains found in North America.
• North America boasts a nearly complete sedimentary sequence from the Eocene to the Recent, revealing a rich fossil record of horses.
• Significant horse fossils have been discovered in Europe and Asia, although their completeness does not match that of North America.

Debates on Origin

• Ongoing debates persist regarding the birthplace of horses, with some theories suggesting Europe and China as potential origins.
• Recent studies challenge the notion that Hyracotherium, an early horse ancestor, was a direct precursor to modern horses.

Biogeographic Origins and Dispersal

• The biogeographic origin of Equidae remains a topic of contention among scholars.
• During the Early Eocene, Equidae could traverse between North America and Eurasia via the DeGeer dispersal route, a land bridge over the North Atlantic Ocean.
• By the early Middle Eocene, this land bridge was no longer passable, leading to the isolation and evolution of Equidae in North America until the Oligocene.
• Subsequent land connections in the Miocene facilitated the dispersal of equids to Europe and beyond.

Continued Dispersal and Extinction Events

• Throughout the Cenozoic era, fluctuating sea levels exposed the Bering land bridge, allowing equids to migrate between North America, Europe, and the Old World during various epochs.
• Notable dispersal events occurred during the Middle Miocene, Pliocene, and Pleistocene periods, shaping the global distribution of horses.
• Horses migrated to South America during the Great Faunal Interchange around 4 to 3 million years ago via the Isthmus of Panama.
• Despite becoming extinct in North America by the end of the Pleistocene, horses persisted in other regions and were later reintroduced to the New World by early human settlers.

Interesting Fact

• The term "Old World" historically referred to Africa, Europe, and Asia, predating the discovery of the Americas, which were termed the "New World" by European explorers.

Major Evolutionary Transitions in Horses

• Increase in Body Size: From smaller ancestors to larger forms seen today.
• Lengthening of Legs and Feet: Enhanced efficiency in running and moving across various terrains.
• Reduction of Lateral Toes with Emphasis on the Middle: Transition from multiple toes to functional middle toe for stability and speed.
• Straightening and Stiffening of the Back: Aids in body structure support and movement.
• Widening of the Incisor Teeth: Facilitates improved chewing and digestion.
• Molarization of the Premolars: Premolars evolved to become more molar-like for grinding food.
• Increase in Height of the Crown of the Cheek Teeth: Supports chewing of tougher vegetation.
• Development of Crests on Molars: Assists in food breakdown.
• Deepening of the Front Portion of the Skull and Lower Jaw: Accommodates high-crowned teeth for mastication.
• Lengthening of the Face: Creates space for high-crowned teeth.
• Increase in Brain Size: Linked to enhanced cognitive abilities and adaptive behaviors.

Download the notes Evolutionary Trend in Equidae

Download as PDF

Download as PDF

Evolution of Horses

Horses in Eocene

Eohippus

• Known as the dawn horse, stood 10-20 inches (25-50 cm) tall at the shoulder.
• Features: Arched back, short neck, snout, legs, and long tail.
• Flexible and rotatable legs with unfused major bones.
• Front feet: 4 toes each; Hind feet: 3 toes; remnants of other toes.
• Dog-like padded feet, small hooves, and low-crowned teeth.
• Extinct in the Early Eocene period, approximately 49 million years ago; evolved into Orohippus.

Orohippus

• Meaning "mountain horse," shared similarities with Eohippus.
• Disappearance of toe vestiges and dental alterations.
• Height: 10-20 inches (25-50 cm) at the shoulder.

Evolution in Palaeontology

Epihippus

• Evolved from Orohippus, resembled a small, dog-like horse with pad feet.
• Dental evolution: Molarized last two premolars.
• Extinct by the end of the Eocene period.

Mesohippus

• Descended from Epihippus; size of a sheep.
• Features: Less arched back, three toes on hind feet, six grinding cheek teeth.
• Survived until the Late Oligocene period.

Miohippus

• Originated in the Late Oligocene period.
• Larger size, longer skull, variable extra crests on upper cheek teeth.
• Split into two branches: Three-toed browsing horses and Parahippus.

Parahippus

• Evolved from Miohippus; mixed feeder (browsing and grazing).
• High-crowned teeth adapted to grass consumption.
• Extinct around 15 million years ago.

Interesting Fact: Hypsodont Dentition

• High-crowned and short-rooted teeth in animals feeding on gritty and fibrous food like grasses.

Merychippus

• Evolved around 18.5 million years ago; adapted for rapid running.
• Features: Spring-footed, hypsodont dentition, fused leg bones.

Merychippine Radiation and Speciation

Rapid speciation led to three groups:

• Hipparions: Three-toed grazers.
• Protohippines: Smaller forms retaining three toes.
• True Equines: Ancestors of modern horses.

Equus Genus Emergence

• Originated approximately 5 million years ago from Dinohippus.
• Features: Pony-like size, rigid spine, long neck and legs, and deep jaw.

Take a Practice Test Test yourself on topics from UPSC exam

Practice Now

Practice Now

Evolutionary Timeline

Cenozoic Era:

• Recent and Pleistocene
• Pliocene
• Miocene
• Oligocene
• Eocene

Migration and Adaptation

• During the Late Pliocene glaciation, certain Equus species migrated to the Old World.
• Adaptations led to zebras, Kiangs, Onagers, and wild asses.

Role of Climate

• Evolution began during the Palaeocene-Eocene Thermal Maximum.
• Changes influenced by transitions from forests to grasslands.

Adaptation for Running

• Longer legs and fused bones for speed and efficiency.

Impact of Pleistocene Climate Change

• Climate shifts and overhunting caused extinctions, including the modern horse, Equus, in North America.

Discontinuous Evolution

• Evolutionary changes in equids were not always gradual.
• Environmental factors drove abrupt adaptations like toe reduction and body size increase.