Evolutionary stages of the equine species - equids 
(Horse Origins)

    The most organized group of animals – mammals -started to appear on the earth at the beginning of the Mesozoic Era (the upper Triassic Period).  It is plausible that to this period belongs the evolution of primitive mammals out of a group of some instinct reptiles (more likely Theromorpha).  Some typical similarities with the mammals were noticeable through the formation of teeth and the skeletal structure. During the following Mesozoic Eras – Jurassic and Cretaceous period – the reptiles were dominating the land and reaching their evolutionary peak. However, at the end of the Cretaceous period – due to vast changes in the earth “crust”, accompanied with drastic climatic changes – the greater part of the Mesozoic reptiles were nearing extinction. During that time period, the land was becoming dominated by mammals that had an easier time adjusting to the ever changing living conditions.  In the beginning of the Cenozoic Era - Eocene Epoch – an “explosive” evolution of mammals was taking place and they adjusted well to their new environment and quickly multiplied into numerous evolutionary branches according to the various living conditions.  Some of these groups/branches can be followed directly to mammals of this age.

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tapir

rhino

    The evolutionary stages of the Equidae family serve as a classical illustration of the zoological evolution, because it is possible to observe a step by step change in the shapes of the body, the build of the limbs, the structure of the teeth etc.  In accordance with the changes in the environment, development continued from a five-toed mammal the size of a fox, to the present size of a horse.

    At the “door-step” of this evolutionary line, it becomes very difficult to recognize the ancestors of horses from the ancestors of tapir and the rhinoceros. Both of them have obviously similar origins, and similarities in the structure of teeth, odd-toed limbs, obvious mobility of the upper lip and other similarities according to which they join the evolutionary line of odd-toed hoofed mammals, the Perissodactyls. The tapirs and rhinoceroses remained “faithful” to their original style of life and also kept their original forms suitable for life in the tropical forests, however the evolutionary line of horses led to life on dryer land in much harsher climatic conditions of the steppes.

     The first predecessors of horses needed to walk on several spread-out toes to accommodate for living in the primeval forests, walking mostly on soft and moist ground. As the main land was drying out, the steppes began to appear and with it large numbers of dry land predators, therefore, the horse’s predecessors required more speed in order to survive. This was accomplished by the lengthening of the limbs, lifting of the toes from the ground in such a way that gradually, but eventually, the weight of the body was resting on one of the longest toes, the third. On solid ground, pushing-off the single toe, equipped at the last link with a hoof, the horse was able to reach fast speeds. The present horse is a single-toed hoof-mammal, while his predecessors were multi-odd-toed animals.

     This fact shows us (in theory only) very strong evidence of how living creatures change under the influence of altered climate and readily adapt/adjust to a new living environment.

     This evolutionary development of the odd-toed predecessors to the present single-toed Equidae family can be observed in North America, wherein the Tertiary Period, layers of many well-preserved skeletons (fossils) of “older” and “younger” predecessors of horses were discovered.  No other line of mammals has such a “complete” evolutionary line compiled by the Paleontologists. 

Eohippus
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    The evolutionary line of horses began in the lower Eocene Epoch in a form called Eohippus approximately the size of a fox and had various characteristics reminding us of his older predecessors – the predators: relatively short head (see pic), 44 teeth* with uneven, dull and bumpy molars, short neck, “springy”, arched back, the “wrist” and hock joints (carpus and tarsus) still low to the ground, but the limbs are relatively long, obviously showing the beginnings of adaptation to gaining more speed. The forelimbs had developed five toes out of which only four were equipped with a small hoof; the fifth “large toe – thumb” was off the ground. The hind limbs had three out of the five toes equipped with small hooves, while the first and fifth toes were not touching the ground (see pic).

    During the Eocene Epoch, which lasted many millions of years, the Eohippus branched out into various types resembling the fox, mainly in size (from 25 cm to 45 cm in height). Thousands of complete, fossilized skeletons of these animals were found in the Eocene layers in North America, mainly in the Wind River basin in Wyoming, USA. Similar limbs of four-toed ancestors of horses were also discovered in the Eocene layers in Europe.  This mammal was called Hyracotherium.

The Evolution Table

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    The successor of Eohippus, in the middle of Eocene period, was Orohippus, also called Protorohippus, who still very much resembled the Eohippus; same size, slimmer body, elongated head, slimmer forelimbs and longer hind legs, all of which are characteristics of a good jumper. The outer toes of Eohippus are no longer present by the Protorohippus; hence on each forelimb there were four fingers (toes) and on each hind leg three toes, and the first of his premolar teeth were dwarfed.

    In the early stages of the Oligocene Epoch the North American environment was changing. During the still warm and dry weather conditions, the forests were yielding to flat lands growing grass and various brush in places or were otherwise covered in sand; hence the type of environment resembling our present prairies.

    The older Pliocene Epoch revealed the Mesohippus, one of the widely spread type of mammals in North America.  The Mesohippus was walking in front, as well as in hind, on three toes (fingers) from which the third was stronger than the outer ones and thus more weighted. Albeit, his first and fifth toe only remained dwarfed metapodia (front – metacarpal, hind (hocks) metatarsal). Judging by his slim limbs, the Mesohippus, who was about 50 cm tall, was a fast animal. The narrow molar teeth evidently did not have cement, nor did his predecessors’.

Evolution Table of the limbs

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    At the end of Oligocene Epoch and at the beginning of the Miocene Epoch, the Mesohippus changed into a form known as Miohippus. The Miohippus had some parts bigger than the former. This Miohippus has branched out through his further development into two branches from which one adjusted to the life in the primeval forests, while the other remained “loyal” to life on the prairies. The forest form led (Miohippus intermedius) to the birth of Kalobatippus, whose second and fourth finger again elongated in order to travel on softer grounds of the primeval forest. The Kalobatippus managed to relocate to Asia through a wide strip of land (today the Bering Strait) and from there onto Europe, where his relics were described under the name Anchitherium. During the millennia, this Kalobatippus transferred into a form known as Hyohippus, which at the beginning of Pliocene Epoch completely became extinct.

    From the steppe form of Miohippus evolved the North American breed of Parahippus. This little “horse” was the size of a small pony, with a prolonged skull and the facial structure resembling horses of today; the third toe became stronger, larger, and hence it carried the main weight of the body. His four premolars resembled the molar teeth and the first almost disappeared. **) The incisive teeth of the Parahippus, like his predecessors, had a crown as humans do,  however, the top incisors had a trace of shallow crease marking the beginning of the core/cup, hence the higher evolutionary stage of the equine species.

     In the middle of the Miocene Epoch after the Parahippus followed the form of Meryhippus, whose “crunching” on hard steppe grassy plants caused him to grow relatively longer and wider molars.  The hind legs which were relatively short still had the side toes equipped with small hooves, but with calm movement were not touching the ground. From the numerous varieties of Meryhippus evolved three new kinds of Equids, the Hipparion, Protohippus and Pliohippus.

Meryhippus & Neohipparion
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     According to shapes, the most diverted of the three was the Hipparion ,mainly in the structure of tooth enamel. In comparison with other horses, the inside or tongue side had completely isolated parapet. On his slim legs, the Hipparion had three toes equipped with small hooves, but the side toes were not touching the ground. The American Hipparion, also known as Neohipparion, proliferated in several kinds of Equids several of which managed to migrate to Asia and Europe during the Pliocene Epoch. (The European Hipperia differ from the American Hipparion in the smaller body size – the most known discovery of these fossils were near Athens.)
    The complete and well-preserved skeleton of the North American Hipparion-Whitney shows an animal the size of a small pony. They were very slim similar to antelopes, and equally timid.  They easily adjusted to life on dry prairies. According to newer research, they are being excluded from the direct ancestry of horses and more likely led to the evolution of zebras and asses.  Outside the diversities in the head, today’s horses, asses, and zebras differ only slightly in the teeth and bones. The branch of  Protohippus died off in the Pliocene Epoch and took part in further evolution of the horse. The form of Meryhippus leading to present horses, was the Pliocene Pliohippus, who still had long metapodia on both sides, but externally barely visible callused stubs. The long and slim limbs of Pliohippus show a quick-footed steppe animal. The Pliohippus then transforms into the form of Plesippus (discovered by Matthew) that further completes the evolution into the form of genuine Equus during the upper Pliocene Epoch of the Tertiary period.

    It still remains uncertain how these horses came from their “home land” of North America to Europe.

    At the end of the Pliocene Epoch, the climate in North America began to cool down significantly. The animals were forced to move southerly. One part of the Plesippus species escaped to South America and the other across the land “bridge” around the Bering Strait to Asia and Europe.  A portion also remained in the southern section of North America. The Ice Age spread five times over Europe and North America and five times again receded (the interglacial periods). This of course lasted many millennia and it is estimated that approximately one million years elapsed from the Ice Age (the Quaternary Period) to our era.

 

The phylogenic development
 of the hind leg
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     The oldest form of horse, Equus Stenonis,  was discovered in Italy, and had evolved from the Plesippus at the end of the Tertiary and beginning of the Quaternary period (diluvia). The Equus Stenonis suddenly proliferated into two branches; one lighter in massiveness of the body and one heavier. The North American genuine form of the diluvium horse was named Equus Scotti and did not differ in any way from the European form; however, some types exceeded the modern horse (Equus Scotti var, giganteus) in size.

     In South America the Plesippus was evolving into a form named Hippidium.  The Hippidium was relatively short-legged with an especially long nose part-low skull, and continued to live on the South American pampas for a long time, but was no longer evolving and eventually died off.

     In North America all the horses became extinct as well, but more likely due to some mass contagium. At the end of the 15th century, when the first Europeans came to America, there were no horses; the cultural tribes of Indians (in today’s Mexico and Peru) did not have a name for the animal. The Spanish imported predecessors of all the horses in America for the first time. The runaway horses (also cattle) went wild on the pampas and soon proliferated into large herds, only to be caught later and again domesticated.

     Like the modern horses, the genuine horses were stepping mainly on the end of the third toe, and from both side toes (2 & 4) remained hardly any exterior trace, but their skeletal remnants alone are obvious on the back of both sides of metacarpal (metacarpus III) and metatarsal (metatarsus III) bones commonly called the “splint bones”. They are the remnants of the second and the fourth metapodium (toe). It is often believed that the splint bones on the modern horse are just useless attachment, but they indeed play an important role in supporting the carpal joints (front knee) and even the tarsal joints (hock).

dog - tapir - horse
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a phylogenetic atavism caused 
by arrested development in a
 certain embryonic stage

 

    It is not unheard that foals were born with three toes equipped with hooves. This is called a phylogenetic atavism caused by arrested development in a certain embryonic stage.

     From the phylogenetic perspective of the single-toed animals, the Equids’chestnuts still remain unresolved. Equids grow chestnuts on the inside front limbs, above the carpal joint. Genuine horses grow them on the inside of the hind limb above the tarsal joint. Zebras and asses do not have chestnuts on the hind legs; K. Hinz saw remnants of calluses in chestnuts that developed by flat walking mammals as they pushed off hard ground. According to Hintz, the toe callus by the single-hoof mammals relates to the “frog.” The metacarpal callus, resp. metatarsal, (the so-called “spur”), or the horny, wart like growth on the skin located on the back part of the ankle (fetlock), the metacarpal and metatarsal calluses relate to the “chestnuts”.

    According to other authors, the chestnuts came from keratinized sebaceous glands that excreted foul smelling secreta.

    Throughout the phylogenetic development, the teeth also underwent significant changes. The type of the original omnivorous teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, have gradually changed into the teeth of herbivorous (plant-eating) mammal. They were long (as much as 10cm) “cubical” molars equipped with a flat grinding surface. In conjunction with the teeth, during the evolutionary stages, the elongation of the facial part of the skull becomes apparent, which can be also observed in the backward set eyeholes. Also, the relatively short neck of the equine ancestors was becoming longer with equal elongation of the legs, as they were forced to find food on the steppes.

   Finally, the size of the body was growing as well, not only due to plentiful food, but also due to the increase in natural choices. 

Edited by R.A. 02/21/04

Related Articles: Zoological Division of Today’s Equidae Family (Equids). 
                              Equine Groups of Origins  

Translated by Ludvik K Stanek a.k.a. Lee Stanek from the 1953 Special Zoo-Technique - Breeding of Horses
Published in 1953 by the Czechoslovakian Academy of Agricultural Science and certified by the Ministry of Agriculture.
Written by: MVDr Ludvik Ambroz, Frabtisek Bilek, MVDr Karel Blazek, Ing. Jaromir Dusek, Ing. Karel Hartman, Hanus Keil, pro. MVDr Emanuel Kral, Karel Kloubek, Ing. Dr. Frantisek Lerche, Ing. Dr Vaclav Michal, Ing. Dr Zdenek Munki, Ing. Vladimir Mueller, MVDr Julius Penicka, pro. MVDr Emil Pribyl, MVDr Lev Richter, prof. Ing. Dr Josef Rechta, MVDr Karel Sejkora and Ing. Dr Jindrich Steinitz.