Evolutionary Changes in Horses

Chris SmithMo BermudezKevin Hatton

Questions??

• What are the evolutionary changes of horses?

• How has the climate affected their evolutionary process?

• How has recent human interaction affected their evolution?

Eocene

• 34-54 million years ago• Most of the land mass was warm and tropical

leading to a great environment for mammals to live

• This era had little fluctuation for 20 million years

What the earth looked like during Eocene

Eocene equids

• Hyracotherium• Orohippus• Haplohippus *• Epihippus

Hyracotherium

Hyracotherium

• Legs were flexible and rotatable with all major bones present and unfused

• 4 toes on each front foot and 3 on the hind feet

• Walked on pads like a dog but instead of nails had small hoofs on each toe

Courtesy of web.fccj.org

Hyracotherium

• Small brain• Low crowned teeth with 3

incisors, 1 canine, 4 premolars, and 3 molars

Courtesy of flickr.com

Orohippus

• Diverged from Hyracotherium 50 million years ago

• Height still approx 20 inches at shoulder• Still had arched back• Short legs, neck, snout• Small brain• Lost first and second toe vestiges

Orohippus

• Major changes occurred in teeth• Last premolar became molar• Crests on the teeth more pronounced most

likely a result of eating tough plant material

Epihippus

• Descendant of Orohippus• Again teeth continued to change with the last

two premolars becoming molars giving it now a total of five teeth suitable for grinding tough vegetation

• Crest of cheek teeth well formed and low crowned

• Body structure remains relatively unchanged

Oligocene(24-34 million years ago)

• Started when the earth began to cool down. The earth as a whole saw a decline of approx 10 degrees centigrade

• As a result of changing temperature the forests began to recede and grassy plain areas developed

• Animals developed longer legs for running over planes

• Dietary changes were made to either meat eating or plant eating

What the earth looked like during Oligocene

Compare world change after 20 million years.

Mesohippus• Transitional horse• Climate becoming drier and grasses evolving• Appeared 40 Million years ago• Slightly larger 24 inches at shoulder• Back loosing arched appearance• Legs, neck, snout getting longer• Shallow facial fossae• Three toes on each foot • Fourth toe on front greatly reduced and

nonfunctional

Notice the change in the vertebral arch

Mesohippus

• Cerebral hemispheres larger• Six molars now for grinding

Miohippus

• Arose approx 34 million years ago• Larger then Mesohippus• Slightly longer skull• Deeper facial fossae• Overlapped with Mesohippus• Extra crest on upper molar

Differences in skull length.Courtesy of Florida museum of Natural Hist.

Miocene Period(24-5.3 Million Years Ago)

Miocene Period

• The horse family began to split into 2 main lines of evolution

• The first line included family Anchiterinae, which included Anchitherium, Hypophippus, and Megahippus.

• The second line included Parahippus and Merychippus

Anchitherium

• Evolved in North America in the early middle Miocene period

• Members of the subfamily Anchitherinae• Had low-crowned, lophodont cheek teeth

lacking cement and three-toed feet supported by pads

• Both in the New and the Old World the fossil consisted of isolated teeth and often very small

Hypohippus

• Size increase occurred in the North American Hypohippus, but was not clearly evident in the North American Anchiterium

• Lived from 17-11 million years ago and was found in Nebraska, Colorado, and Montana

• All of their species retained three toes that bore a lot of the animals weight and were well adapted for the soft ground of forests

Megahippus• Megahippus were very

large horses and specialized leaf eaters

• Found on the Great Plains and lived 15-11 million years ago

• They were the last of the browsing horses in North America

• Their front teeth have a curve in a narrow U-shape, an adaptation for browsing

Late Miocene Period • Climate cooling increased • Seasonal aridity turned the forests into more open

woodland and woodland savannas with rich herb layers

• Large grassland were just beginning to appear increasing the edge effect and changed both in quality and quantity of food.

• Began to specialize in eating primarily grasses• Their increased fiber-content in the form of grasses

has initiated the change in size increase

Late Miocene Period• Small crests on the teeth

enlarged and connected together in a series of ridges for grinding

• There was increase in the height of tooth crown (hypsodont) and became harder due to the development of the cement layer on the teeth

• Teeth could also grow out of the gum continuously as the tops were worn down (hypselodont)

Late Miocene Period• Started to become specialized

runners• Bones of the legs began to

fuse together and the leg bones became more muscular eliminating flexible leg rotation, which specialized for efficient forward-and-back strides

• Began to stand permanently on their tiptoe, which instead of walking on doglike pads, their weight was supported by spring ligaments that ran under the fetlock to the big central toe

Parahippus

• Arose 23 million years ago • Found in Great Plains and Florida• Between 18-17 million years ago Parahippus

developed springy ligaments under the foot• Showed fluctuation changes in its teeth,

including the permanent establishment or a stronger crest with slightly taller tooth crowns

Merychippus

• Arose 17 million years ago• Tallest equine at that time 40”• The muzzle became elongated, the jaw

became deeper, and the eye moved farther back, to accommodate the large tooth roots

• Developed full high-crowned teeth with a thick layer of cement

• Brain became bigger with a fissured neocortex and larger cerebellum

Merychippus cont..

• Radius and ulna of the forearm fused eliminating leg rotation

• Fibula of the shin was reduced• Side toes became smaller and only touched

the ground while running• Stood permanently on its tiptoe• This line gave rise to the “true equines”

Dinohippus

• Arose about 12 mya• Very similar to Equus,

modern horse, in foot morphology, teeth, and skull structure

• Teeth became straighter than the Merychippus

• Facial fossae were decreased

• Later species, D. mexicanus, evolved straighter teeth and smaller facial fossae

• Dinohippus-most common in N. America in late pliocene epoch

Late Pliocene(2.6 million years ago)

• During the late Pliocene era, the Isthmus of Panama arose

• Some Dinohippus species travelled into S. America and gave rise to hippidions

• Equus arose and is the current genus of the modern horses

• Zebra-like bodies with short, narrow, donkey-like skulls

• Probably the most successful of all the genus’ that ever lived

Pleistocene(15,000 to 2 million years ago)

• This era in the horses evolutionary time frame happened to be during and after the last Ice Age

• Many animals did not survive this drastic shift in climate

Why did horses survive this drastic shift in climate while other large animals did not?

THE CECUM!!!!!!!

Canine Equine Rhinocerous

Pleistocene(15,000 to 2 million years ago)

• Horses adapted to surviving almost solely off of grasses

• They evolved a cecum to help digest the cellulose found in stems and leaves of plants

• The cecum allows horses to survive in very poor-quality areas

• This adaptation allows them to withstand a high-fiber diet and to avoid competition among other species

• This may have also lead to the increase in their size

Courtesy of The Nature of Horses by Stephen Budiansky

Pleistocene(15,000 to 2 million years ago)

• Evolved a larger brain that may have been related to more sensitive lips for effective foraging

• Horses evolved the diastema, a gap between their incisors and molars, which helped them watch for predators while continuing to eat

• 3-toed to 1-toed helped make them more efficient at fleeing from predators

Courtesy of The Nature of Horses by Stephen Budiansky

Pleistocene(15,000 to 2 million years ago)

• Other traits that evolved were some juvenile characteristics (Neoteny)– Higher curiosity– Flexibility or adaptability to environmental changes– A more playful nature– Submissiveness– Dependency

• These characteristics or traits lead to an easier domestication of the horse by man

Pleistocene(15,000 to 2 million years ago)

• After the Ice Age, approximately 15,000 years ago, the horse became extinct in North America

• This extinction was due to overhunting by the people occupying North America at the time

• Horses were close to extinction in Europe and Asia but their domestication saved them as a species

Recent(15,000 years ago to Present)

• Domesticated horses were first used as a weapon of war by the Hittites in about 2000 B.C.

• These horses very closely resembled present-day Arabians due to the low set eye, dish-faced profile and a high set tail

• Later on other tribes such as the Huns, and Greeks and Romans started using horses for war

• Horses weren’t introduced to Egypt until between 1715-1689 B.C.

• Even later, Cortez and De Soto introduced Spanish horses to North America

Bibliography• Anderson, J.K. 1961. Ancient Greek Horsemanship. University of California

Press. Berkeley and Los Angeles, California. pp. 1-40.• Budiansky, Stephen. 1997. The Nature of Horses: Exploring Equine Evolution,

Intelligence, and Behavior. The Free Press. New York, New York. pp. 9-59.• Crowell, Pers. 1951. Cavalcade of American Horses. McGraw-Hill Book

Company, Inc. New York, New York. pp. 1-39.• Forsten, Ann. 1991. Size Trends in Holarctic Anchitherines. The Paleontological

Society. pp. 147-159. Helsinki, Finland• Hunt, Kathleen. 1995. Horse Evolution. (Accessed March 5, 2009.)

http://www.talkorigins.org/faqs/horses/horse_evol.html• MacFadden, Bruce J. 1992. Fossil Horses Systematics, Paleobiology, and

Evolution of the Family Equidae. Florida Museum of Natural History, University of Florida. Press Syndicate of the University of Cambridge. New York, New York.

• Self, Margaret Cabell. 1946. The Horseman’s Encyclopedia. A.S. Barnes and Company, Inc. United States of America. pp. 178-188.