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Horse Evolution

Introduction - The evolution of the horse was not smooth and gradual, it didn't proceed in a straight line toward a goal, it was rather complicated. Many horse species were usually present at the same time, with various sizes and numbers of toes, adapting to various different diets. Sometimes, new species split off suddenly from their ancestors and then co-existed with those ancestors. Other species came into being through transformation of the ancestor, until the ancestor had changed appearance enough to be given a new name. Sudden specie changes can occur when a population encounters new selective forces and/or becomes isolated from the parent species. I have concentrated on showing the changes from the first equid Hyracotherium to the current Equus there were many others along the way, links to more detailed studies at the end of the page. The major genera of the horse family are referred to as Equidae. All equids (members of the family Equidae) are perissodactyls (members of hoofed animals that bear their weight on the central 3rd toe) The most modern equids (descendents of Parahippus) are called "equines" or 'horses'. Tracing a line of descent from Hyracotherium to Equus reveals several apparent trends: reduction of toe number, increase in size of cheek teeth, lengthening of the face, increase in body size. But these trends are not seen in all of the horse lines. On the whole, horses got larger, but some horses (Archeohippus, Calippus) then got smaller again. Many recent horses evolved complex facial pits, and then some of their descendants lost them again. Most of the recent horses were three-toed, not one-toed, and we see a "trend" to one toe only because all the three-toed lines became extinct.

 Hyracotherium (54 million years ago “Eohippus” meaning dawn horse) The first equid, a small forest animal, looking more like a dog than a horse and ranging from 10-20 inches at the shoulder. It had an arched back, short neck, snout and legs, and long tail.  It walked on pads with small ‘hoofs’ on the top of each toe instead of a claw. It had legs that were flexible and rotatable with all major bones present and unfused. There were four toes on each front foot and three toes on the hind. The vestigial toes - two on the front feet and one on the hind - were still present. It had a small brain with especially small frontal lobes. In its mouth it Low-crowned teeth, 3 incisors, 1 canine, 4 premolars and 3 molars in each side of each jaw, cusps of the molars were slightly connected in low crests. It browsed on fruit and fairly soft foliage.It had a choppy, up-down gait and was not very fast. It was a very successful animal in its time and it existed for a period of about 20 million years.

Orohippus (approx 50 Million years ago) Similar to its ancestor Hyracotherium, The only major differences in the two were the absence of the vestigial toes that were present in Hyracotherium and that the last premolar changed shape into a molar, giving them another grinding tooth. Also the crests on the teeth were more pronounced, indicating that they probably fed on tougher plants

 

Epihippus (approx. 47 Million years ago) Like its ancestors Orohippus, but with the teeth making the biggest change, with the last two premolars changing shape to be like molars, giving Epihippus five grinding cheek teeth. The crests on the cheek teeth were well-formed, and still low-crowned. There is a late form sometimes called Duchesnehippus.

 

Mesohippus  (40 Million years ago) This animal was slightly larger than Epihippus, standing at 24" at the shoulder.  It began to look more horse like, as its back was less arched, legs and neck being a bit longer, and the snout and face being distinctively longer. It had a shallow facial fossa, a depression on the skull. The 4th front toe on the front feet was reduced to a vestigial nubbin. Cerebral hemispheres in the brain notably larger. The Last three premolars are like the three molars, so it had six similar grinding "cheek teeth", with one little premolar in front.

 

Miohippus (36 Million Years ago).

Larger than the previous Mesohippus (they co-existed fo some 4 million years) with a slightly longer skull. The facial fossa was deeper and more expanded. In addition, the ankle joint had changed subtly. Miohippus also began to show a variable extra crest on its upper cheek teeth. Mesohippus died out, while Miohippus grew rapidly.

 

The Miohippus Radiation (24 Million Years ago) Miohippus continued for a while as it was, then began to change fairly rapidly. The horse family began to split into 2 main lines of evolution and one small one.

anchitheres - 3-toed browsers. They were very successful, and thrived for tens of millions of years. They retained the small, simple teeth of Miohippus. Generally include Anchitherium and the large Hypohippus and Megahippus.

true equines - Transformed from browsing (fruit eating) to grazing (grass eating) Large grasslands were just beginning to appear, but grass is difficult to chew and wears down teeth rapidly (due to the silica in the leaves) and thus a grass-eater needs tough teeth with ridges of some sort. Open-country grass eaters, benefit from being swift runners with long legs.

Archeohippus - A line of small "pygmy horses", These horses did not survive long.

 

Kalobatippus (24 - 19 million years ago) Part of the group called anchitheres this is Greek for stilt-walking horse its descriptive name came from its elongated metapodials (the bones between the ankle/wrist and the toes). These bones gave the animal longer legs. They had low-crowned teeth, useful for browsing on leaves, sprouts, and possibly fruits. They could not have eaten much grass with these teeth, because the abrasive grasses would have quickly worn the teeth down. The three-toed, short-toothed browsing anchitheres became extinct but the grazing horses diversified.

 

Parahippus (23 million years ago) Typically a little larger than its ancestors, with about the same size brain and same body form. It was still three-toed, and was just beginning to develop the springy ligaments under the foot. It showed gradual and fluctuating changes in its teeth, including the permanent establishment of the extra crest as well as various other cusps and crests beginning to join up in a series of strong crests, with slightly taller tooth crowns. 

 

Merychippus (17 Million years ago) A typical Merychippus was about 10 hands (40") tall, the tallest equine yet. The muzzle became elongated, the jaw became deeper, and the eye moved farther back, to accommodate the large tooth roots. The brain was notably larger, with a larger cerebellum, making them a smarter and more agile equine. Merychippus was recognizable as a horse, was still 3-toed, but was fully spring-footed. This animal stood permanently on tiptoe, supported and propelled by strong, springy ligaments that ran under the fetlock. The side toes were still complete, but began to be of varying sizes; some Merychippus species had full-size side toes, while others developed small side toes that only touched the ground during running. The central toe developed a large convex hoof, and the legs became longer. The radius and ulna of the forearm fused so that leg rotation was eliminated. Likewise, the fibula of the shin was greatly reduced. All these changes made their legs specialized for rapid running over hard ground. Its teeth were fully high-crowned, and with the same distinctive grazing tooth crests as Parahippus. They gave rise to two later merychippines species [merychippines sejunctus and merychippines isonesus], who had a mixture of Parahippus, hipparion, and equine features. They gave rise to merychippines intermontanus, merychippines stylodontus and merychippines carrizoensis, all of which began to look alot more like th modern day horse.

 

The Merychippine Radiation (15 Million Years ago) Merychippus was the one of the first bona-fide speedy plains grazers, and gave rise to at least 19 new grazing horse species in three major groups.  

hipparions - Three-toed grazers, these were successful and split into 4 genera and at least 16 species, eventually covering a variety of niches for small and large grazers and browsers. They developed large elaborate facial fossae.

protohippines - A line of smaller horses including Protohippus and Calippus.

true equines - in which the side toes sometimes began to decrease in size. Some of them looked quite similar to modern day horses and gave rise to a set of larger three-toed and one-toed horses known as the "true equines"

 

Pliohippus (15 Million years ago) Arose as a three-toed horse. Gradual loss of the side toes. Pliohippus was very similar to Equus and except for two significant differences. First, Pliohippus's skull has deep facial fossae, whereas Equus has no facial fossae at all. Second, Pliohippus's teeth are strongly curved, and Equus's teeth are very straight.

Dinohippus (12 Million years ago) a one-toed horse. The earliest known species are Dinohippus spectans, Dinohippus interpolatus, and Dinohippus leidyanus. They look smashingly like Equus in foot, teeth, and skull. The teeth were slightly straighter than Merychippus, and the facial fossae were significantly decreased. A slightly later species was Dinohippus mexicanus, that showed even straighter teeth and even smaller fossae. Dinohippus showed a gradual decrease in the facial fossae, straightening of the teeth, and other gradual changes.

 

Astrohippus (10Million years ago) another one-toed horse that had large facial fossae.

 

 Equus (4 Million years ago) The first Equus were 13.2 hands tall (pony size), with a classic "horsey" body, a rigid spine, long neck, long legs, fused leg bones with no rotation, long nose, flexible muzzle and deep jaw. They had zebra-like bodies, stocky with a straight shoulder and thick neck, short, narrow, donkey-like skulls. They probably had stiff, upright manes, ropy tails, medium-sized ears, striped legs, and at least some striping on the back. Equus is one-toed, with side ligaments that prevent twisting of the hoof, and has high-crowned, straight grazing teeth with strong crests. The brain was a bit larger than in early Dinohippus. The earliest known Equus species were a set of three "simple Equus" species collectively known as the Equus simplicidens group. They still had some primitive traits from Dinohippus, including a slight facial fossa. They quickly diversified into at least 12 new species in 4 different groups. Some certain Equus species entered Africa and diversified into the modern zebras. Others spread across Asia, the Mideast, & N. Africa as desert-adapted onagers and asses. Still others spread across Asia, the Mideast, and Europe as the true horse, E. caballus. Other Equus species spread into South America.

 

Members of Equus still retain the genes for making side toes. Usually these express themselves only as the vestigial "splint bones" of toes 2 and 4, around the large central 3rd toe. Very rarely, a modern Equus is born with small but fully-formed side toes.

 

Modern Equines (Recent) The three-toed horses gradually died out. Most of the one-toed horses in North America also died out, as the Ice Ages started. However, one-toed Equus was very successful. Until about 1 million years ago, there were Equus species all over Africa, Asia, Europe, North America, and South America, in enormous migrating herds. All the horses of North and South America died out.  For the first time in tens of millions of years, there were no equids in the Americas. The only members of Equus, and of the entire family Equidae that survived to historic times were:-

   Order Perissodactyla    -  means "odd-toed." This group of ungulates includes horses, tapirs, and rhinos. The name of their order derives from the fact that their middle toe is larger than the others. Most species have three digits on the hindfoot and three or four on the forefoot, but in some only a single digit, the third, remains.

   Family Equidae    - This family, made up of the horses, asses and zebras, contains one genus with nine species. Domestic equids range worldwide; in the wild equids occur mainly in East Africa and the Near East to Mongolia. They inhabit a variety of habitats from lush grasslands and savanna to sandy and stony deserts.

   Genus Equus    - The single Recent Equus.

contains six subgenera (Asinus, Hemionus, Equus, Dolichohippus, Hippotigris, Quagga)

and eight species (Asinus, Hemionus, kiang, caballus, Grevyi, zebra, burchelli, quagga)

 

Subgenera - Asinus ; Species - Equus asinus: (Ass)

African Ass, donkey or burro.(The African wild asses are sometimes called E. africanus.)

once found from Morocco to Somalia and from Mesopotamia to Oman,

now occurs only in domestic situations and some wild populations.

 

Subgenera - Dolicohippus ; Species - Equus Grevyi: (Grevys Zebra)

Grevy's zebra, This is the big zebra with the very narrow vertical stripes and huge ears.

found in southern and eastern Ethiopia, Somalia, northern Kenya.

 

Subgenera - Equus; Species - Equus Caballus: (Horse)

the true horse, which once had several subspecies.

once found in the wild throughout Poland and Hungary to Mongolia,

now occurs in domestication throughout the world and wild populations.

 

Subgenera - Hemionus; Species - Equus Hemionus: (Kulan and Onager)

originally found in the desert from Ukraine and Palestine to Manchuria and west India.

the desert-adapted onagers from Asia & the Mideast.

 

Subgenera - Hemionus; Species - Equus Kiang: (Kiang)

originally from Tibet and adjecent highland regions.

 

Subgenera - Hippotigris; Species - Equus Zebra: (mountain zebra)

This is the little mountain zebra with the dewlap and the gridiron pattern on its rump.

Originally from southwest Angola, Namibia and western and southern south Africa.

 

Subgenera - Quagga; Species - Equus Burchelli: (common zebra)

lives in open country, the Plains zebra of Africa.

this is what people usually think of as the "typical zebra",

with rather wide vertical stripes, and thick horizontal stripes on the rump.

includes "Grant's zebra", "Burchell's zebra", "Chapman's zebra",

originally from southern Ethiopia to central Angola and eastern South Africa;

 

Subgenera - Quagga; Species - Equus Quagga: (half-striped zebra)

formerly found in South Africa.

similar to the common zebra but only half striped.

 

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For further information -

http://members.ozemail.com.au/~xenophon/horses/faq01.html

 

To see skeletons of all equids -

http://chem.tufts.edu/science/evolution/HorseEvolution.htm