Horse evolution

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ORDER Perissodactyla
Herbivorous or omnivorous mammals that retain a caecal digestive system. Perissodactyls have single, obliquely-ridged tibial tarsal bones (astragali), in contrast to the other major order of herbivorous mammals, the Artiodactyla, which have double, parallel-ridged astragali.
The term “Perissodactyl” comes from Greek words meaning “digits arranged around (peri-) a central toe (-dactyl)” – and this is indeed the foot-symmetry of all animals belonging to this Order. The central digit, designated by Roman numeral III, is the largest and bears most of the limb’s weight, even when the animal retains three, four, or five toes. This too is in contrast to the Artiodactyla, in which the main part of the weight is shared by closely-appressed or fused digits III and IV.
There is a strong tendency to flatten the radius and to closely appress it to the ulna, so as to inhibit or prevent supination of the manus (turning the forefoot inward or upward). In the hind limb, the tibia becomes reduced in size and loses its articulation below with the tarsus. A deep pair of grooves on the distal end of the tibia, which are obliquely oriented to match the ridges on the astragalus, forms a strong, tight, and stable articulation and likewise prevents the hind foot from turning inward.
All Perissodactyls have 18 thoracic vertebrae (and thus 18 pairs of ribs), although the number of lumbar vertebrae may be as high as 8.
Perissodactyls have bunodont (cusped) or lophodont (ridged) teeth. In the upper dentition, the teeth have the cusps or ridges are so arranged that when the tooth is somewhat worn, they make a shape that looks like the Greek letter “pi”. Worn lower teeth look like the small letter “m”.
The tooth formula in Perissodactyls is as follows: in each half of either the upper or lower jaws, there are –
3 incisors

1 canine

4 premolars

3 molars
Normally, the tooth formula is written this way: I3C1P4M3. This contrasts with the more advanced Artiodactyls such as sheep, cattle, and camels, which possess no upper incisors.

The jaw condyle is lozenge-shaped and transversely oriented. It articulates with a glenoid fossa (formed in the squamous part of the temporal bone of the skull) which is saddle-shaped, shallow, and open to the front. The chewing motion is rotatory, with the lower jaw being moved first downward, then laterally, and finally obliquely up-and-across. Food material is actually ground by this last action. Up-and-down chewing action is also possible, but the obliquely-lateral motion is charateristic of all later members of the order.
There are a number of important general trends among Perissodactyls:
(1) They tend to go for body-size increase as a way to avoid or inhibit predation. They increase not only in height but in bulk – even to extremes, such in Baluchitheres, which are the largest land mammals ever to exist.
(2) They tend to lose digits. All of them lose digit I (the thumb) early on; many also lose digit V (the pinkie). Thus, the normal condition in Perissodactyls is to have three or four toes per foot. By contrast, the normal condition in Artiodactyls is to have either four or two toes per foot.
(3) As a strategy to make the teeth last longer in eating a tough, abrasive herbaceous diet, they tend to increase the crown-height of the teeth, becoming either hypsodont (high-crowned) or sub-hypsodont.
(4) As a further strategy to increase the life of the teeth and thus the potential lifespan of the animal, they tend to increase the complexity of the teeth – which translates in lophodont teeth to increased length of the enamel bands exposed on the tooth crown.
(5) They tend to reduce the size of the first premolar tooth (P1 in both upper and lower dentitions), while increasing the size of the molars and the other premolars. There is a strong tendency to form the “cheek teeth” into a battery of uniform or near-uniform size.
(6) They tend to develop a long anterior skull with one or more toothless spaces between the cheek battery to the rear and the incisors in front.
(7) They tend to have nasal or frontal horns.
(8) They almost universally have a short trunk or proboscis, or at least a soft, strong, flexible, semi-prehensile upper lip.
All interested students of the horse will want to take every opportunity to look at the mounted skeletons of Perissodactyls which they will find in Museums of Natural History. Throughout time, there have been five major Families of Perissodactyls, to wit:
EquidsHyracotherium, Paleotherium, Parahippus, Griphippus, Astrohippus, Cormohipparion, Neohipparion, Hipparion, Onohippidion, Calippus, Anchitherium, Nannippus, Megahippus and many other fossil genera besides the still-surviving Equus.
TapiroidsThe tapir genus Tapirus still survives, but the group was once much more common, including such forms as Homogalax, Helaletes, Hyrachyus, Chasmotherium, Lophiodon, Paleotapirus, and Protapirus. Tapirs are round-backed, moderately heavy-bodied animals with pronounced retraction of the nasal notch and development of a short proboscis. They live in forests or along the forest edge, eating a mixed diet of leaves, fallen fruit, and insects.
Rhinocerotoids – The still-surviving genera Ceratotherium, Dicerorhinus, Diceros, and Rhinoceros plus their fossil relatives, including Hyracodon, Amynodon, Menoceras, Aphelops, Baluchitherium, Teleoceras, Elasmotherium, and many more. Rhinoceroses are moderately to extremely large animals with a thick, tough, largely hairless hide. They characteristically have long, scoop-shaped heads and one or more horns (made of compressed fibers similar to hoof horn) upon their nose. Some diet mostly upon leaves and twigs, while others prefer grass.
Chalicotheres – An odd and wholly-extinct group uniquely characterized by long, heavy forelegs with claws. Chalicotheres are the only Perissodactyls that retain and even enhance the ability to supinate the manus. They are thought to have dieted primarily upon leaves and twigs, using their clawed forelimbs, slope-backed build, spout-shaped mouth and long rounded tongue to grasp branches and stip them of leaves. The best-known genus is Moropus.
Brontotheres – These animals may be likened to rhinos that have imitated Arnold Schwarzenegger: they are generally larger than rhinos and far more massive through the shoulders and chest. Instead of having one or two conical horns mounted “in line” on their nose, they possess a flat, saddle-shaped horn of bony construction mounted crosswise. Now wholly extinct, the group’s best-known members are Brontops, Brontotherium, Titanotherium, Manteoceras, and Paleosyops.
FAMILY Equidae
Perissodactyls that tend to retain lightweight bodies, and that tend to a cursorial (running) mode of locomotion. While Hyracotherium, the earliest member of this Family, retains four toes on the forefoot, and while Equus and some other late members of the Family have only one functional digit per foot, the norm is for there to be three functional digits on each limb. This contrasts with the Artiodactyls, in which the norm is for there to be two functional digits per limb.
The general tendency among Equids is to lengthen the distal elements of the limbs, i.e. those bones below the carpus and tarsus.
They also strengthen and stabilize the distal limb joints: the carpal and tarsal bones articulate closely together, and are formed as block-like rather than rounded elements. The carpal bones are arranged into proximal and distal rows. The distal row tightly articulates with, and is firmly tied by ligaments to, the top of the metacarpal bones below, so that when the carpus folds only two joints open: that between the proximal carpal row and the distal end of the radius and ulna, and that between the two carpal rows.
Equids reduce the number of lumbar vertebrae to six, while enlarging the sacrum and retaining 18 thoracic and 7 cervical vertebrae.
Dentally, Equids are unique among herbivores in not only possessing, but uniformly enlarging the upper incisors. They typically possess a strong set of incisors not only in the upper but in the lower jaws, and are thus the only mammals whose dental functioning depends upon a “three point” balance, i.e. when the jaws are closed and centered, pressure is shared by the incisors, cheek teeth, and the jaw joint.
(Please note that the taxonomist’s use of the word “Equine” – capital “E” – differs from the way your veterinarian uses the word “equine” – little “e”. In veterinary parlance, “equine” means any of the living horse-like animals, i.e. not only domestic horses but Przewalski horses, zebras, onagers, or asses. These are also Equines – but the Equinae also includes a large number of extinct forms known from the fossil record).
Equines are Equids adapted for eating grass. Thus, they have hypsodont teeth with cementum, a bone-like material, to support and strengthen each tooth. They have stout, wedge-shaped skulls in which both the maxilla and dentary are deep to accommodate the tall-crowned teeth.
The superior teeth of Equines have two fossettes. Each fossette is filled, or largely filled, with cementum.
Except for the first premolar, which is greatly reduced in size, the premolar and molar teeth of equines are large, square in cross-sectional shape, and tightly appressed to each other to form “cheek batteries” for the efficient grinding of grass blades.
There is a strong tendency among equines to retract the nasal notch, i.e. to “undercut” the nasal bone, even to an extreme degree (vis., Hippidium and Onohippidion). In such forms, deep pits or “facial fossae” tend to be present on the face, the complex as a whole indicating the presence of a small trunk or a strongly prehensile upper lip longer than that in the living horse. Where there is little or no retraction of the nasal notch (Neohipparion, Nannippus), the face will be smooth, lacking facial fossae. Forms that have smooth faces tend to have the most complex and most highly hypsodont teeth.
Equines are universally unguligrade, having hoofs rather than claws and never locomoting with the hock, carpus, or “ankles” (the joint between the distal end of the cannon bone and the pasterns) touching the ground. Generally speaking, they are are tall and have proportionally long distal limb elements. They are adapted for straight-line flight over firm substrates in open terrain.
Equines characteristically develop joints between the “wings” of the sacrum and the transverse processes of the last lumbar vertebra. They also have articulations between the transverse processes of the last several lumbar vertebrae. Moreover, the joints between the accessory articular processes in the lumbar vertebrae of equines are vertically-oriented, and they are shaped to articulate like dovetail joints. These “inter-transverse” and “dovetail” articulations almost totally inhibit rotation and lateral flexion among the lumbar and lumbo-sacral joints, while promoting up-and-down coiling of the lumbo-sacral joint and loin-span. This is in sharp contrast to Artiodactyls, which retain long, relatively loosely-articulated lumbars which permit twisting and a greater degree of lateral flexion.
INFRAFAMILY Protohippini
Protohippine equines are distinguished by possessing high-crowned teeth that are nevertheless comparatively simple in structure (the genus Equus has the most complex teeth within the Protohippine lineage).
An important distinguishing characteristic of the Protohippine dentition which is easy to see is that the protocone loop of enamel is joined to the hypoloph, not set off (as it is in Hipparionines) as a separate circlet.
Two kinds of skull and skeletal structure may be found within the Protohippini: one is the “normal” or mesomorphic build we associate with the genus Equus. These animals have flat or slightly-rounded backs, nasal notches only moderately retracted, moderately prehensile upper lips, and smooth faces. The other type re-echoes the “chalicomorph” or “okapi-like” body design in having forelimbs longer than hind limbs, a sloping back, deeply-retracted nasal notch, and deep facial fossae for the attachment of the muscles to move a long, strongly prehensile upper lip or short trunk. Pliohippus is an outstanding example of this, and study of Pliohippus should remind students never to fall back into the old 19th-century “evolutionary progression”, proposed by O.C. Marsh, that supposedly led from Protohippus through Pliohippus to Equus. Equus is the descendant of Protohippus and Dinohippus, both flat-backed and smooth-faced genera, not Pliohippus.
Not surprisingly, to go along with the two different body-styles found within the Protohippini, there are two sorts of dentition. Equus and earlier members of its direct lineage have relatively straight teeth, with the angles of the “tables” or occlusal surfaces of the cheek batteries set at from 7 to 10 degrees of slope. Pliohippus and other “okapi-like” forms have upper teeth with a lot of curve to them, and thus table angles ranging from 10 to 25 degrees of slope. These teeth have exceptionally heavy outer styles and buttresses, and very simple lower teeth; such a design is less for dealing with grass than with “chop”, i.e. twigs, leaves, and bark rather than grass. The supposition is that Pliohippus and similar forms were eating a diet similar to a deer’s.
The other infrafamiliar taxon is the Hipparionini. Students of the horse should take every opportunity, by visiting Museums of Natural History, to familiarize themselves with these animals as well. Hipparionines are typically small, narrow-bodied, lightweight, agile and dainty. While Protohippines tend to be large and heavy, Hipparionines are relatively small, some even becoming dwarfs no larger than the African dik-dik. All Hipparionines have extremely hypsodont teeth with highly complex structure, good for processing dry forage. These animals were, evidently, filling the ecological niche now exclusively occupied by antelopes.
Members of the genus Equus have but one functional toe per foot.
It should be noted that Equus is not the only horse genus to become monodactyl; for example, there is a species of Neohipparion from the Ashfall Quarry in northern Nebraska that achieved this condition long before either Equus (or its direct ancestor Dinohippus) came into existence. Nevertheless, monodactyly is a derived character shared by all members of Equus, and is thus a diagnostic feature of the genus.
As previously stated, Equus has highly hypsodont teeth which have complex, more wrinkled enamel structure than other members of its lineage.
Once again, however, Equus does not have the most high-crowned teeth known among Equids: Hipparionines have teeth which are equally or more high-crowned than Equus, and there is even one species of Nannippus known from Florida which actually develops hypselodont teeth in which the roots never close and the tooth is “ever-growing”. Nor does Equus have the most complex enamel structure; that honor belongs to Cormohipparion, Neohipparion, and some of the European species of Hipparion.
Students of the horse should, by visiting a Museum of Natural History, study the remains of all the fossil horses. There are many more genera than the old “hippuses” ladder-of-descent litany tends to include, and, although I repeatedly admonish students to visit their local Museum, I forewarn you that many of the exhibits you see there are likely to be sadly out of date (this is because since the Carter Administration, and much more seriously since the Reagan Administration, federal funding for Museums and for all forms of so-called “esoteric” research has been cut to the bone. This is not the only reason, but it is an important one, why the author herself does not work in a Museum). So, when you visit the Museum you may see a “horse evolution exhibit” that still features the hackneyed and incorrect series that typically starts with “Eohippus” (Hyracotherium), and then proceeds through Orohippus, Mesohippus, Miohippus, Merychippus, and Pliohippus, finally to terminate with Equus. But you may know that paleontologists are currently aware also of the following genera (in alphabetical order):


















SPECIES Equus caballus
Equus caballus is the heaviest and one of the tallest species in its genus, and it also happens to be one of the largest and heaviest of all equids. It has the broadest and deepest skull and the stoutest limbs. The upper cheek teeth are large, usually with long, bipartate protocones that look somewhat like a Dutch shoe (Fig. XXX). In general, the enamel plications visible upon the working surfaces of the upper teeth are quite complex; a pli caballin is typically present. The inferior cheek teeth show a “U”-shaped enamel re-entrant (the ectoflexid). The incisor teeth are large and all of them, even the lateral incisors, possess both infundibulae (“cups”) and dental marks (“stars”), which can both, at a certain stage of wear, simultaneously be present in a given tooth. Although the incisor row is broad, it is not as broad as in some fossil genera, e.g. for example Calippus, nicknamed by paleontologists the “lawn mower horse” (Fig. XXX).
There is little inflation of the frontal sinuses in most animals, so that they have straight faces and flat foreheads. One very useful characteristic (if one has a complete or nearly-complete skull to identify) is that the temporal bone of the skull forms a broad triangle behind the ear region in this species, broadly separating the postglenoid and paramastoid processes. This is a reflection of the fact that the occiput slopes backwards rather than being tucked under. The practical result is that you can tell the skull of a horse from that of an ass, mule, zebra, or onager by standing it up on end. The horse skull will balance; skulls of the mule and of the other species will fall forward.
Living Species of Equus
Students of the horse should help themselves to put the horse into context by comparing it with the other eight living (or recently extinct) species in the genus Equus. You should realize at this point how closely related to the horse these animals are; the tragedy is that all of them are now on the brink of extinction (and the horse itself has been, since 1947, extinct in the wild). Since most of these very interesting animals can be seen in zoos, you should familiarize yourself with what they look like:
Equus zebra – the “true zebra”. There are two subspecies, the Hartmann’s zebra (E. zebra hartmanni), and the Mountain zebra (E. zebra zebra). This species is distinguished by its donkey-like body size and form, by a “gridiron” pattern of stripes over the top of the croup, and by the fact that there is a flap of skin on the underside of the throat.
Equus burchelli – the Bontequagga, Plains zebra, or common zebra. This animal has a pony-like body form and an undulating facial profile. It completely lacks infundibulae in the lateral incisors. Its stripes are broad, although some forms may have thinner stripes lying in the white zones between broader black stripes. The belly is white or cream-colored and unstriped; sometimes the light tone extends far up on the body, while there are also melanistic individuals whose white color is confined to rows of white dots. Many zoos distinguish different forms based on the exact pattern of striping, e.g., , but in actuality they are all members of a single species. This is the commonest zebra to be seen in zoos, may often also be seen in the circus, and is the most populous in the wild (although still an endangered species).
Equus grevyi – the narrow-striped zebra, Abyssinian zebra, or Grevy’s zebra. This animal is the oldest of all African equids, its fossil record there extending back at least five million years. It is tall, sometimes reaching over 15 hands in height, and has fairly prominent withers. The skull is long and narrow, and the facial profile may be either straight or slightly undulating. It has the simplest enamel pattern on its teeth of any Equus; in the lower teeth, the “U”-shaped ectoflexid re-entrant reaches all the way across the teeth.
Equus onager – the Indian and west-Asian Onager, Onaigre, Khur, or Kulan. One of three so-called “hemione” (meaning “half like an ass”) species, the Onager is an orange-colored animal with a buffy to white underbelly. It has ears longer than those of a horse but shorter than those of an ass; they are pointed like those of the horse and ass, rather than being rounded like those of zebras. Onagers and other half-asses are distinguished by having proportionally long, narrow cannon bones, short wedge-shaped heads, and lower cheek teeth in which the ectoflexid re-entrant does not penetrate all the way across the tooth.
Equus hemionus – the Dzhungarian (Chinese and Mongolian) hemione, Chiggetai or Dziggetai. This is the tallest and most gracile of the half-asses, and also the rarest to be seen in Western zoos. Its coat is a dusty-gray color and the back and belly contrast less than in the onager and kiang.
Equus kiang – the Tibetan Kiang. This is hemione has the shortest, stoutest limbs and overall the stoutest build. Its pelage is a rich blackish-brown, standing in sharp contrast to the nearly-white fur of the underbelly and the inner flanks.
Equus asinus – the ass. Besides the domestic donkey, there were once wild donkeys in the Arabian Peninsula, and until the recent series of civil wars in Somalia, there was a population surviving there also.
Equus quagga – the South African Quagga. This animal became extinct in 1904, having once roamed the plains of South Africa in countless numbers. It was exterminated by the Afrikaaners as the buffalo were nearly exterminated by Anglo-Americans. The animal had a body form similar to that of a Burchell’s zebra, but was typically striped only upon the legs and over the top of the croup. My own analysis of quagga skulls – there are only five known to science – indicates that these animals deserve to be recognized as a separate species more similar to a horse than to a zebra. However, amino-acid assay tests performed on preserved quagga hides indicate that quaggas are zebras. Until I see the results from a DNA test performed with modern techniques, I am remain inclined to believe what the morphological analysis tells me, for unlike any zebra, the lateral incisors of quaggas possess full infundibulae – just like a horse.
Along with the horse, the onager and the ass were both brought into domestication at least 4,000 years ago. In addition, all three species of zebra have in historical times been captured, tamed, ridden, driven, and trained for performance in the circus. Mules as normally bred in Europe and the Americas are the product of a human-planned cross between a jack ass and a mare (i.e. E. caballus X E. asinus), but there are other ways to get mules: African farmers frequently make use of zebra semen to get mules from mares, because they have much greater immunity to African parasites and stand the climate better (horses never occurred in sub-Sarharan Africa until people began bringing them there in the 16th century). Zebra-horse mules are called by various names such as “zebrules” or “zorses”.

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