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

• Evolution of Horse
  • 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.

The Evolutionary History of Horses

• The evolutionary journey of horses is well-documented due to the abundance of well-preserved fossil remains found in North America.
• In contrast to other regions, 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 the completeness of the fossil record is not on par with that of North America.
• 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, with no definitive conclusion reached yet.
• 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 between North America and Europe 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 also 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: Over time, horses have undergone a significant increase in their overall body size, evolving from smaller ancestors to the larger forms we see today.
• Lengthening of Legs and Feet: Horses have experienced a gradual elongation of their legs and feet, which has contributed to their efficiency in running and moving across various terrains.
• Reduction of Lateral Toes with Emphasis on the Middle: Through the evolutionary process, horses have transitioned from having multiple toes to emphasizing the development and functionality of the middle toe for greater stability and speed.
• Straightening and Stiffening of the Back: The back of horses has evolved to become straighter and more rigid, aiding in the support of their body structure and movement.
• Widening of the Incisor Teeth: Horses have seen a widening of their incisor teeth, which has facilitated improved chewing and digestion of food over time.
• Molarisation of the Premolars: The premolars of horses have evolved to become more molar-like, enhancing their ability to grind and process food effectively.
• Increase in Height of the Crown of the Cheek Teeth: There has been a notable increase in the height of the crown of the cheek teeth in horses, aiding in the chewing and grinding of tougher vegetation.
• Development of Crests on Molars: Horses have developed crests on their molars, which assist in the breakdown of food for digestion.
• Deepening of the Front Portion of the Skull and Lower Jaw: The front portion of the skull and lower jaw of horses has deepened to accommodate the growth of high-crowned teeth for improved mastication.
• Lengthening of the Face: The face of horses has elongated in front of the eyes to make room for the high-crowned teeth, aiding in their feeding process.
• Increase in Brain Size: Throughout evolution, horses have experienced an increase in brain size, potentially linked to enhanced cognitive abilities and adaptive behaviors.

Evolution of Horses

• Horses in Eocene

  • Eohippus: Eohippus, also known as the dawn horse, was a small creature standing about 10-20 inches (25-50 cm) tall at the shoulder. It had distinct features such as an arched back, short neck, short snout, short legs, and a long tail. This ancient horse had flexible and rotatable legs with unfused major bones. Its front feet had 4 toes each, while the hind feet had 3 toes. There were remnants of other toes. Eohippus had dog-like padded feet and small hooves on each toe. It closely resembled a dog and primarily lived in forests, feeding on soft foliage. Its teeth were low-crowned, with a dental formula of 3143/3143, featuring 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of the jaw. The cusps of its molars were slightly connected by low crests. Eohippus became extinct in the Early Eocene period, approximately 49 million years ago, but before its extinction, it evolved into Orohippus.

• Orohippus

  • Orohippus: Orohippus, meaning "mountain horse," shared many similarities with Eohippus. The notable changes in Orohippus included the disappearance of toe vestiges and alterations in dental structure. It stood at a height of about 10-20 inches (25-50 cm) at the shoulder.

Evolution of Horses in Palaeontology

• Orohippus

  Orohippus, an early horse, had a dog-like appearance with padded feet. It had a small brain, arched back, and short legs. Its diet consisted of tougher plant material compared to its predecessors, resulting in molarization of the 4th premolar and more developed crests on the teeth.

• Epihippus

  Epihippus evolved from Orohippus, resembling a small, dog-like horse with pad feet. It had 4 toes in front and 3 behind. Dental evolution continued in Epihippus, with the last two premolars becoming molarized. It was slightly larger than Orohippus and became extinct by the end of the Eocene period.

• Mesohippus

  Mesohippus, an intermediate horse, descended from Epihippus. It was slightly larger than its predecessor, resembling the size of a sheep. Mesohippus had a less arched back and retained three toes on its hind feet. The third last premolar continued the trend of molarization, resulting in six grinding cheek teeth. Mesohippus survived until the Late Oligocene period.

• Miohippus

  Miohippus originated from Mesohippus in the Late Oligocene period. It was a larger horse with a longer skull and developed variable extra crests on its upper cheek teeth. Miohippus later split into two branches, one leading to three-toed browsing horses and the other to Parahippus.

• Parahippus

  Parahippus evolved from Miohippus and was a mixed feeder, both browsing and grazing. This horse appeared in the Early Miocene, slightly larger than Miohippus. As Parahippus started consuming grass, its teeth became high-crowned. It adapted to a running mode and eventually went extinct around 15 million years ago.

Interesting Fact: Hypsodont Dentition

Hypsodont dentition, characterized by high-crowned and short-rooted teeth, is seen in animals that feed on gritty and fibrous food like grasses.

Evolution of Horses

Merychippus: The Transition to Grazing

• Merychippus evolved around 18.5 million years ago from Parahippus into a spring-footed, hypsodont horse.
• Standing at about 40 inches (1 meter) high at the shoulder, Merychippus was a fully spring-footed horse, constantly on tiptoe.
• Adapted for rapid running over hard ground, Merychippus had fused leg bones and reduced leg size to eliminate rotation.
• It underwent a significant transformation by increasing the height of its teeth due to a diet dominantly focused on grass.
• The evolution of Merychippus marked the transition of horses from browsing to grazing, earning it the title of a ruminant horse.

Merychippine Radiation and Horse Speciation

• Merychippus underwent rapid speciation known as "merychippine radiation," leading to the emergence of three major groups of new horse species.

Hipparions: The Three-Toed Grazers

• Hipparions, ranging from small to large pony-like horses, appeared around 16 million years ago.
• Examples include Comohipparion, Pseudhipparion, Neohipparion, Hipparion, and Nannippus.
• These horses migrated from North America to the Indian subcontinent approximately 14-13 million years ago.

Protohippines and True Equines

• Protohippines, including Protohippus and Calippus, evolved around 16 million years ago as smaller forms retaining three toes.
• In the Middle Miocene, Pliohippus emerged from the protohippines line, eventually evolving into a single-toed species.
• Pliohippus gave rise to the "hippidions," stocky, short-legged, single-toed horses with distinctive boxy skulls.
• True equines encompass present-day horses and their ancestors, featuring both three-toed and single-toed varieties such as Dinohippus and Equus.

Horses in the Pliocene

• The descendants of hipparions and protohippines continued into the Pliocene, spreading to the Old World and South America.
• Dinohippus, a true equine, dominated North America in the late Pliocene and gave rise to the modern-day horse, Equus.

Evolution of Modern Horses

Equus Genus Emergence

• Equus is the genus encompassing all present-day equines, originating approximately 5 million years ago from Dinohippus.
• The initial Equus stood at 52 inches (1.3 m) tall, akin to a pony, featuring characteristics like a rigid spine, long neck and legs, fused leg bones without rotation, long nose, flexible muzzle, and a deep jaw.
• Compared to Dinohippus, Equus had slightly larger brains and straighter, higher-crowned teeth.
• Equus evolved into various species that coexisted with other horse species.

Evolutionary Timeline

• Cenozoic Era
• 
  
  • Recent and Pleistocene
  • Pliocene
  • Miocene
  • Oligocene
  • Eocene
• Body Form Evolution
• 
  
  • Equus
  • KM
  • Pliohippus
  • Merychippus
  • Parahippus
  • Mesohippus
  • Orohippus

Morphological Changes

Various changes occurred in horses during their evolutionary journey. Refer to the diagram for visualization.

Migration and Adaptation

• During the Late Pliocene glaciation, around 2.5 million years ago, certain Equus species migrated to the Old World.
• Some of these species settled in Africa and evolved into modern zebras.
• Others dispersed across Asia, the Middle East, and Africa, adapting to desert environments as Kiangs, Onagers, and wild asses.

Evolution of Horses

The Origins of Horses

• Horses have a fascinating evolutionary history, with some species evolving in North America and others spreading to Asia, Africa, South America, and Europe.
• Equus, the true horse, originated in North America but went extinct there around 11,000 years ago, likely due to climate change and overhunting. It was later reintroduced by humans in the 16th century.
• In the Siwalik Hills of the Indian Subcontinent, fossils of hipparionine equids like Hipparion have been found, indicating their migration from North America.

Role of Climate in Evolution

• The evolution of horses began close to the Palaeocene-Eocene boundary during a period of global warming known as the Palaeocene-Eocene Thermal Maximum.
• Changes in horses during evolution included a reduction in the number of toes and the development of deeper cheek teeth, influenced by environmental shifts like the transition from forests to grasslands.
• As the Earth cooled in the Late Eocene, vegetation patterns changed, leading horses to shift from browsing to grazing, adapting their teeth to cope with the tough silica content of grass.

Block 3: Introduction to Palaeontology

• Adaptation of Horses for Rapid Running

  The grazing horses evolved to excel in rapid running on open grasslands. They developed longer legs for increased speed, with leg bones fusing together to support efficient forward-and-back strides. This adaptation also led them to stand permanently on tiptoe, transitioning from three-toed to single-toed creatures, enhancing their speed.

• Impact of Pleistocene Climate Change

  During the Pleistocene era, climate shifts and human overhunting contributed to the extinction of various mammals in North America, including the modern horse, Equus. This period also witnessed global sea level fluctuations, creating land bridges that facilitated the migration of horses from North America to other regions.

• Discontinuous Evolutionary Changes in Equids

  The fossil record of equids indicates that the evolutionary progression among different species was not always gradual. While certain adaptations, such as the molarization of premolars, occurred gradually, others like the reduction in toes and increase in body size were abrupt. These changes were driven by the necessity for horses to survive changing climates, showcasing the influence of environmental factors on evolution.