HORSE EVOLUTION
The impetus for producing this major, gorgeously-illustrated feature was a telephone
conversation with a man whom I otherwise enjoy. He’s a cowboy, and grew up on a
ranch. He’s a kind fellow who has done a good job with not only a lot of colts, but also
his kids, who are likewise very fine people. He’s a Christian – a commitment that I
admire – and his children were not only raised in their church, but have gone on to attend
Bible College. All of this certainly deserves praise. However, this fellow knows that I’m
a paleontologist interested in fossil horses. As we were discussing some of this, at one
point in the conversation he raised his voice and said, “I can’t understand it! Belief in
evolution is simply stupid!”
Well, it’s with any such point of view that I will never be able to agree. In the first place,
such an attitude invalidates my experience and beliefs in a way that I would never impose
in return – not to mention that it goes against nearly every piece of advice St. Paul ever
gave. What could possibly cause an otherwise reasonable man to make such a statement
so vehemently to someone he knows it is going to offend? Why -- because the fellow
cares about me, of course; he wants my soul to be saved, even at the cost of his own.
Why does he think my soul is in danger of damnation? Because he believes that the
words he reads in the Bible are literally true – no room for metaphorical interpretations or
alternative readings. To this guy, there is one translation of the Bible (I don’t know which
one) that is the true and accurate recording of God’s thoughts, desires, and commands for
us. Fundamentalism prevents him from being able to reconcile the time-span that geology
and paleontology propose for the world, because what mention of that subject there is in
Scripture indicates that the world was created less than 5,000 years ago. Fundamentalism
has crippled his ability to accept any evidence that would indicate otherwise. But to deny
the existence of the (as you are about to see, thousands) of fossilized skeletons of horses
and other animals known to science is simply foolish. Worse, to claim as some folks have
that they have been “carved out of the rock” in an effort to deceive the public, is an
irresponsible lie. And yet, this guy, so schooled in looking for the truth within Scripture,
is inclined to believe such lies and distortions. He even gave me a link to a website by a
creationist with a Ph.D. degree (in Economics) that spouts even more of them!.
I think the news media have also helped to destroy some peoples’ faith in the reliability
of science. This even applies to straightforward science reporting. Mis-teaching in the
public schools hasn’t helped, either – but the fault for all of this lies squarely at the
doorstep of my own profession, which has failed to make an adequate public educational
effort. Thus, evolutionist vs. creationist debates are still staged; in Kansas the legislature
once again wants to ban the teaching of evolution in public schools; and the people of the
United States have elected a string of conservative legislators and Presidents who,
responding to strong lobbying by conservative and fundamentalist Christians, have cut
Museum funding to the bone.
The concept that the public has of what evolution is and how it works simply does not
match that which paleontologists hold. In fact, it hasn’t matched for more than a century.
When your High School biology teacher taught you that evolution means one animal
changing into another animal over a long span of time – and the picture he conveyed was
of morphing -- he taught you badly; for there is no known mechanism for “crossfading”
among living things. Instead, change through time is iterative by nature – change usually
occurs one “click” at a time, like working a lock with a roller dial. When you go to your
local Museum and you see an exhibit that shows a single series of horses gradually
getting bigger through time – what I call an “inflate-a-horse” exhibit – this is a story that
gives the idea that one horse species has morphed into the next. It so far fails to convey
the real story that it is almost a crime. This review seeks to convey a more correct idea.
Evolution means “change through time”. Charles Darwin said that it occurred as a result
of predation, accident, and disease acting upon variability within a population of animals.
This was sloganized by Huxley into the “survival of the fittest.” But it needs to be
remembered that that what Darwin was proposing was one possible mechanism by which
animals in later generations might accumulate enough changes to be recognizable as a
species different from the ancestor. Other mechanisms for change through time have also
been proposed – the two most famous alternatives to Darwinism coming from Ernst Mayr
and Stephen Jay Gould (see the extensive bibliography at the end of this feature for
citations to their writings). My personal belief is that there is no incompatibility at all
between believing that animals have changed or evolved through time, and that God
created the universe. For paleontology says nothing at all about how the universe began;
its only interest is in what has unfolded afterward. It is not paleontologists but
fundamentalist creationists who arrogantly insist that they know exactly how God has
nourished, or how He continues to nourish, this unfolding.
In the horse family, change through time is very well documented, based on literally
thousands of fossil skeletons that have been unearthed and studied over the past two and
a half centuries. Students who wish to understand the subject will want to pay attention to
two areas in particular: logical systems for classifying living things, and age
determination for the remains of animals that have been found within rock strata. In this
feature, I present the information on stratigraphy and age-determination primarily in
pictorial form. The rules and procedures of taxonomy (the science of classifying living
things) are explained below. The example used throughout is, of course, the horse – so
here you will also get a complete classification of the horse along with the actual,
technical reasons for that classification. I have also supplied many wonderful photos
showing fossil specimens exhibited in museums around the world, so that this issue of
“The Inner Horseman” functions as a virtual World Museum Tour. I certainly hope that
you find studying this material fun as well as enlightening!
Classification
Living things are classified on the basis of similarity in structure. This means that you
must first notice the structure of whatever living thing you mean to study, noting
particularly any features (taxonomists call them “characters”) unique to that creature.
Similarities shared by a range of organisms, or features unique to only one or a few of
them, can then be tabulated.
Characters common to a large number of organisms are said to be primitive. “Primitive”
in this sense does not imply crudeness, for all organisms living at all times must have a
body plan that works very well in order to stay alive at all. Nor does the term “primitive”
necessarily imply that the feature occurs early in time, for in fact most of the characters
of most organisms now alive have been inherited unchanged through untold generations,
over the span of millennia.
Because of this, characters which are unique to a given organism – characters which of
necessity must be modifications, great or small, of earlier designs – have special
significance in classification, for only such “derived” features can define the boundaries
of taxonomic units -- groups of related species. The logical system of classification,
called cladistics, makes use of derived features shared by all members of a taxonomic
unit to define that unit. Because derived features can only arise from pre-existing designs
(those which are comparatively more widespread or more primitive), a cladistic logicdiagram also shows us the most likely order in which the descendants of any given
lineage acquired derived features.
The science of classification formally began in the mid-18th century with the work of
Swedish scientist Carl Linné. Linné (whose name is usually Latinized to Carolus
Linnaeus) proposed the system of binomial nomenclature which is still used today. In this
naming system, all species receive two names – the first signifying the genus to which the
species belongs. When the second term, called the trivial, is paired with the genus name,
the two terms taken together uniquely name and identify the species. The example most
relevant to our interests would be:
Equus – the genus for horses, asses, onagers, quaggas, and zebras. The Latin word
“equus” means “weighting all four feet equally”.
caballus – the trivial term is a Latin word meaning “nag”. Note that by itself, the trivial
term does not identify the horse -- or any other animal. There is, for example, at least one
beetle and also one plant whose genus names are followed by the trivial “caballus”. Thus,
in order to be sure that we’re discussing the living horse, we need to write:
Equus caballus – this binomial term is the scientific name, used in taxonomic
classification, that uniquely identifies the horse.
Please note the importance of capitalization and italicization. In scientific writing, it is
mandatory to capitalize the first letter of the genus name, and to not capitalize the first
letter of the trivial. The genus name, which is OK to use alone, or the full binomial when
it is used, must be italicized or in some other manner set off from the surrounding text
(i.e., boldface or underline are OK too). While these rules are mandatory in any technical
journal, good usage demands that they be followed in all publications (so for example,
the better-edited newspapers and monthlies such as Equus Magazine scrupulously follow
these rules).
Naming Species: Importance of the Type Specimen
There are also important rules concerning the naming of species. The person who is the
first to describe and publish, in an encyclopedia or peer-reviewed journal, any new
species gets permanent credit for doing so. The most important rule of taxonomy is that
the trivial term is permanently associated with a “type specimen.” Ideally this would be a
complete skeleton; practically in horse paleontology it is often a skull, a partial dentition,
or even a single tooth. The type specimen must be housed in a Museum of Natural
History or other permanent and professionally curated collection, it must bear a unique
specimen number, and it must remain accessible in perpetuity for any scientist who
wishes to examine it.
Once the type specimen has been designated and named, the real work concerning it
begins. This consists of observation, discussion, and even controversy among all the
paleontologists who have examined it. Because the trivial can never appear, and has no
scientific meaning, unless it is assigned to and paired with a genus name, the original
namer must assign it to some genus. But that by no means guarantees that’s where it’s
going to stay! After examining the type, another scientist may feel justified in reassigning the specimen to a different genus. These re-assignments may go on for years –
often because other discoveries, whether in the area of the geology and stratigraphy or as
a result of further study of the skeletal material itself, help to put the type specimen into
better perspective. There so many examples of this in the paleontological literature that I
can only say, “let the student be forewarned”. Here’s a table containing a few examples;
boldface type indicates instances where the current expert opinion is that the original
namer assigned the species to the correct genus:
Original Date & Namer
Original Assignment
Erwin Hinckley Barbour, 1914 Hypohippus matthewi
Present Assignment
Megahippus matthewi
Edward Drinker Cope, 1873
Anchitherium cuneatum
Mesohippus cuneatus
E.D. Cope, 1873
Protohippus sejunctus
Protohippus sejunctus
E.D. Cope, 1879
Anchitherium praestans
Kalobatippus praestans
E.D. Cope, 1889
Hippotherium isonesum
Calippus isonesus
E.D. Cope, 1889
Hippotherium sphenodus
Griphippus sphenodus
E.D. Cope, 1889
Hippotherium retrusum
Calippus retrusus
E.D. Cope, 1892
Equus simplicidens
Equus simplicidens
E.D. Cope, 1893
Hippidium interpolatum
Pliohippus interpolatus
E.D. Cope, 1893
Protohippus lenticularis
Nannippus lenticulare
E.D. Cope, 1893
Equus eurystylus
Neohipparion eurystyle
J.W. Gidley, 1903
J.W. Gidley, 1907
Joseph Leidy, 1850
J. Leidy, 1856
Neohipparion whitneyi
Merychippus campestris
Palaeotherium baridii
Hipparion occidentale
Neohipparion whitneyi
Pliohippus campestris
Mesohippus bairdii
Cormohipparion occidentale
J. Leidy, 1858
J. Leidy, 1869
J. Leidy, 1869
J. Leidy, 1869
Eohippus perditus
Protohippus placidus
Protohippus supremus
Hipparion gratum
Othniel C. Marsh, 1874
Pliohippus pernix
Pseudhipparion perditus
Calippus placidus
Pliohippus supremus
Griphippus gratus
Pliohippus pernix
Henry Fairfield Osborn, 1918
Miohippus gidleyi
Miohippus gidleyi
H.F. Osborn, 1918
Merychippus republicanus
Pseudhipparion republicanum
H.F. Osborn, 1918
Pliohippus leidyanus
Dinohippus leidyanus
O.A. Peterson, 1907
Parahippus nebrascensis
E.H. Sellards, 1916
Hipparion minor
William Berryman Scott, 1893 Anchitherium equinum
Parahippus nebrascensis
Nannippus minor
Hypohippus equinus
In all cases, however, even where later research has proven the generic assignment
incorrect, the original namer continues in perpetuity to receive credit for naming and
describing the type – his name is forever associated with it. So, for example, even though
it was Morris Skinner and Bruce MacFadden who did the work and had the insight to reassign the type specimen of Hipparion occidentale to Cormohipparion, the full correct
citation for “Cormohipparion occidentale” is Cormohipparion occidentale (Leidy,
1856).
The Classification Hierarchy
Linnaeus’ original system of classification was hierarchical. A hierarchical classification
functions like bowls that stack one inside the other: into the biggest bowl go many
organisms, which are themselves bundled into groups. Each of these groups, in turn,
functions like a bowl which holds still smaller bowls, and so on until we reach the level
of the genus, species, and subspecies.
Figs. XXX – XXX graphically illustrate the whole hierarchical classification of the living
horse, Equus caballus. Students of the horse need to be familiar with the meaning of such
terms as Perissodactyl (vs. Artiodactyl), and Equid vs. Equine (capital “E”) vs. equine
(little “e”). All taxonomic terms are defined in terms of structure. In other words, a
member of the Order Perissodactyla is identified as such because it possesses certain
unique, derived features that are visible on the skeleton. This, at root, is what makes a
horse a horse – and not a deer, a goat, a dog, or a primate. Each of the accompanying
“learning units” is designed to familiarize you with the particular characteristics that
define and characterize each taxonomic unit, whether Kingdom, Phylum, Class, Order,
Family, Genus, or species.
How Species are Defined
The smallest unit recognized by the science of taxonomy is the subspecies. However,
where horses are concerned, it is obvious that we often benefit from looking at still
smaller units – for example, herds within a subspecies, and even the individual within the
herd. The reason that taxonomic classification stops at the level of the subspecies is that
this is the level where panmixia occurs.
“Panmixia” is a term relating to the genetics of populations. Remember that, in
classifying animals, we do not look at their genes; instead, we look at the results of gene
action which are manifested as the structural peculiarities of the visible body. Panmixia is
the condition which occurs within a population of animals in which every breeding
individual has an equal chance of mating with every other individual. When this is the
case, any given gene has its best shot at showing up in the maximum number of
individuals. “Gene flow” through a panmictic population is free of restrictions or
bottlenecks. Subspecies are, by definition, panmictic populations.
A species, which may contain one or more than one subspecies, is a group of populations
in which any individual is capable of breeding with any other individual to produce
viable offspring that can themselves breed to produce viable offspring. In other words, a
species is defined by reproduction and gene flow (which in turn creates a certain limited
range of skeletal structures within that population). When a species is spread out over a
lot of territory – and the horse at one time represented one of the best examples of such –
inevitably there will be barriers to reproduction. For example, a mountain range may
intervene between the easternmost and westernmost populations of a species. The
difficulty in crossing the mountains will make it less likely that individuals from the
eastern vs. western populations will meet and interbreed.
When there are barriers to panmixia, and when environmental conditions differ from one
range to another, populations or herds inhabiting different regions will tend to become
adapted to the particular conditions found in that region. This is, of course, not only
because they tend to breed only with each other, but also because individuals less
physically suited to a given regime of terrain, forage, and climate tend to reproduce less
often and produce fewer offspring. They may also be more susceptible to predation,
accident, and disease. Over time, the physical type they represent becomes rare and may
become altogether extinct. This is how the gene pool of a population becomes
characteristic of that population. We detect the range of different gene combinations
within a given gene pool through its expression in the physical bodies of individuals.
When dealing with the fossil record, we are, of course, lucky to find preserved even one
member of any population. It has been estimated that somewhere between 1% and 15%
of the skeletons of all the mammals that ever lived were fossilized. Of this small number,
only a tiny fraction are actually found, collected, studied, and reported. These statistics
are often used as a kind of apology for any mistakes that paleontologists might make in
interpreting the fossil record – we have to work from very partial data. However, the
same statistic should also be used in reverse -- as a measure of the number and diversity
of animals that have lived on Earth. For example, the student may consider the fossil
record of horses: in the five largest collections of fossil mammals in North America (the
American Museum of Natural History in New York City, the National Museum of
Natural History/Smithsonian Institution in Washington D.C., the University of Nebraska
at Lincoln, the University of California at Berkeley, and the University of Florida at
Gainesville) there are tens of thousands of horse bones and teeth sufficiently complete to
permit identification at least to the level of genus. There are also numerous other, smaller
collections: The University of Kansas at Lawrence, the University of Michigan at Ann
Arbor, the Page LaBrea Tar Pit Museum in Los Angeles, the Los Angeles County
Museum of Natural History, and the University of Colorado Museum at Denver, not to
mention the National Museum of Mexico in Mexico City, the National Museum of
Canada in Ottawa, and the Drumheller Museum in Alberta. If this is the case, it is simply
awesome to contmplate the number of animals belonging to the horse lineage that have
lived on Earth.
A Classification of Horses
KINGDOM Animals
Animals are multi-cellular animals characterized by the power to move. Although as
adults some become rooted by a stalk or pedicle (example: clams, crinoids, anemones),
for at least part of their lives they are motile. The cells of animals are thin-walled, lacking
stiff cellulose reinforcement.
PHYLUM Chordates
Animals that have a stiff rod of cartilage extending along the dorsal part of the body. This
rod, called a notochord, functions to protect the central nerve cord, which is also located
dorsally. In most, the notochord is present only at an early embryonic stage, but in a few
(the tunicates) it is present in early life, or (in Amphioxus) it persists throughout life.
Chordates possess the full compliment of body systems, i.e. they have a complete
digestive tube with a front opening (mouth) and rear opening (anus); central and
peripheral nervous systems; segmented muscles; a heart that recirculates blood through a
closed circulatory system. There is a brain and sense organs concentrated at the front end
of the body, and the body itself is bilaterally symmetrical. The creature breathes by
means of a long series of gills, and although there is a mouth opening, there are no jaws.
The sense of “hearing” is diffuse, the whole body being sensitive to vibrations in the
water; there are no ears.
SUBPHYLUM Vertebrates
Chordates in which the notochord becomes replaced during embryonic life by a chain of
vertebrae which enclose and protect the dorsal nerve cord. Generally, there are additional
parts as well – limbs -- to compose an internal skeleton (endo-skeleton). In most
vertebrates, the skeleton in the adult creature is bony, but in some specialized forms, such
as sharks, the skeleton is cartilagenous. A skull, or at least the lower part of the skull (the
palate area and basicranium) is present, and part of the gill series is modified to become
jaws that articulate with the skull. A specialized, “concentrated” organ of hearing appears
on the side of the head in the area of the jaw joint. The mouth possesses rows of hard
teeth.
CLASS Mammals
Vertebrates which have fur, secrete milk for their young, and whose jaw joint is
specifically formed by the articulation of the dentary element of the jaw with the
squamosal part of the temporal bone of the skull. This latter is, of course, the criterion by
which paleontologists distinguish mammals from near-mammals and non-mammals.
Generally speaking, mammals possess teeth of several different shapes – i.e., incisors,
canines, premolars and molars; but some mammals lose all teeth or reduce them to
vestiges (armadillos, pangolins, aardvarks, sloths), others modify them to fantastic new
structures (baleen whales), and still others return to having teeth of uniform morphology
(dolphins, killer whales). No matter how modified, however, mammals have only one
row of teeth along the margins of the jaws, and they replace only the anterior part of the
dental arcade only one time. The teeth develop in, and erupt through the gums out of,
sockets called alveoli.
Primitively, the limbs of mammals terminate in five digits of about equal length arranged
in a fan shape. From this original design many modifications have been tried, including
loss of some digits, lengthening or shortening of one or more digits, fusion of digits to
form a single stout digit, change from claws to either fingernails or hoofs, or change of
the whole limb from a paw into a flipper or a wing.
With few exceptions, mammals have exactly 7 cervical vertebrae (whales and dolphins,
and also giant ground sloths, have less than 7 vertebrae).
Each half of the mandible (the jawbone) in mammals is made up of but one, single
element, the dentary bone (in reptiles, amphibians, fish, and birds the mandible is formed
either of more than one bony element, or is formed from a different bone than the
dentary).
The pelvis of mammals is likewise relatively simple, each half being composed of only
three elements, the ischium, ilium, and pubis (the pelvis in other vertebrates tends to have
additional elements). Importantly, mammals possess a sacrum composed of five or more
vertebrae which fuse to make a rod above the pelvis. Birds and frogs also show fusions in
this area, but they are far more extensive and serve to prevent flexion of the pelvis on the
lumbar vertebrae. In mammals, up-and-down flexion of the spine is a crucial and
characteristic element of locomotion, in contrast to the characteristically side-to-side,
sinusoidal motions of the vertebral chain in reptiles, amphibians, and fishes (you can tell
a fish from a whale or dolphin by the orientation of its tail fins: the fish’s tail fin is
vertical, because in order to swim, he “wags his tail” from side to side. Marine mammals,
by contrast, have their flukes oriented horizontally, because the main swimming motion
is “humping” or up-and-down action of the tail).
Every student of the horse should be able to name and know the characteristics of all five
of the classes of vertebrates: mammals, birds, reptiles, amphibians, and fishes. By
contrasting mammals with the other four, we derive a richer picture of the unique nature
of all mammals, and of the horse in particular.
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 saddleshaped, 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 (highcrowned) 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:
Equids – Hyracotherium, Paleotherium, Parahippus, Griphippus, Astrohippus,
Cormohipparion, Neohipparion, Hipparion, Onohippidion, Calippus, Anchitherium,
Nannippus, Megahippus and many other fossil genera besides the still-surviving Equus.
Tapiroids – The 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 heavybodied 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.
SUBFAMILY Equinae
(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 verticallyoriented, 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 lumbosacral 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 “okapilike” 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.
GENUS Equus
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-ofdescent 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):
Anchitherium
Archaeohippus
Astrohippus
Calippus
Cormohipparion
Dinohippus
Epihippus
Griphippus
Hipparion
Hippidium
Hypohippus
Kalobatippus
Megahippus
Nannippus
Neohipparion
Onohippidion
Palaeotherium
Parahippus
Pseudhipparion
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 reentrant (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 nearlycomplete 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 creamcolored 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 orangecolored 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”.
Fossil Species of Equus
As if the abundance and diversity of living members of the genus Equus were not
enough, we also have an extensive record of this animal in the Northern Hemisphere
beginning with its origin at the end of the Tertiary Period. The genus immediately
ancestral to Equus is Dinohippus. This animal is similar to Equus in every respect – so
much so that I cannot find any characteristics to distinguish them. I continue to use the
term Dinohippus, therefore, merely as a way of distinguishing certain species that lived in
North America during the Hemphillian and Blancan Land Mammal Ages, between 7 and
5.5 million years ago. (This is a good example of the sometimes uneasy marriage
between the logic trees generated by cladistic analysis and the older method of drawing
“phylogenetic trees” showing the descent of a species through time. Properly, some day,
it will probably be necessary to re-classify Dinohippus by making it a sub-genus, and
then we will write, for the best-known species in the genus, “Equus (Dinohippus)
interpolatus”).
Dinohippus gave rise to a species, Equus francescana, that migrated from California
across the Bering Land Bridge during the earliest phase of the Pleistocene when sea-level
was lowered. Fossils of this species found in the Old World are called Equus stenonis,
and in this form it spread throughout that continent and into Europe and Africa. In
Europe, it gave rise to such species as Equus sussenbornensis, the ancestor of Equus
mosbachensis, and Equus bressanus – the latter being the probable ancestor of asses. In
western Asia and north Africa, Equus stenonis appears to have given rise to Equus
hydruntinus, another ass-like form, and Equus teilhardi, which is probably an onager. In
Africa, it ultimately gave rise to Equus grevyi.
In North America, Equus francescana gave rise to Equus simplicidens. Equus francisi,
an onager-like species, also appears very early; it may be derived from E. francescana or
from the back-migration of a Eurasian form. Likewise, E. simplicidens is a form which in
many respects parallels Equus grevyi, having a very long head, big body, stout legs, and a
relatively simple dental pattern. The “stilt-legged” Equus francisi, on the other hand, is
not only longer in the shanks but smaller and lighter-bodied, with a more complex
enamel pattern in the teeth.
From Equus francescana, and also possibly from forms back-migrating from the Old
World, there came an enormous proliferation of North American species of the genus
Equus. There really are quite a few of these, but those of you wishing to read the techical
literature should be forewarned that paleontologists eager to make their mark in this
fertile field of investigation have, over the past two centuries, almost hopelessly muddled
matters by naming hundreds of species, most of which must be regarded, for a variety of
reasons, as invalid “duplicates”. I therefore discuss here only those species that I think are
really unique and valid.
Bloodlines of the genus Equus continue through three Land Mammal Ages: the Blancan,
which is the tail-end of the Tertiary; the Irvingtonian, which is the earlier part of the
Pleistocene; and the Rancholabrean, which is the later part. In each of these time periods,
Equus species can be classified into either “stout-legged” (horse or zebra-like) or “stiltlegged” (onager-like) forms. (Ass-like forms are difficult to distinguish, and apparently
rare, in the fossil record from the New World):
Blancan
Stout-legged forms: long head, relatively simple teeth, large size
Equus francescana
Equus simplicidens (old names for this form that you might still see in a museum exhibit
are Plesippus, Equus shoshonensis). This form is commonly exhibited because a large
number of skeletons have been recovered from the famous Hagerman Quarry near
Hagerman, Idaho. There is a complete skeleton of this species on exhibit in a small
Museum run by the National Park Service in Hagerman, and at some seasons you can
also take a bus tour to view the actual quarry itself. Equus simplicidens skeletons are also
on exhibit at the U.S. National Museum of Natural History/Smithsonian Institution in
Washington, D.C.).
Stilt-legged form: shorter head with broader snout, relatively complex tooth structure,
smaller size
Equus francisi (first appearance)
Typically Irvingtonian (though some range from late Blancan up into the
Rancholabrean):
Stout-legged forms:
Equus niobrarensis
Equus hatcheri
Equus scotti (See a complete skeleton of this form on exhibit at the American Museum of
Natural History in New York City).
Stilt-legged forms:
Equus francisi (later occurrences)
Equus arellanoi
Equus calobatus
Equus zoytalis
Typically Rancholabrean (though some originate in the Irvingtonian):
Stout-legged forms:
Equus caballus
Equus occidentalis (See a complete skeleton and many skulls of this form on exhibit at
the Page La Brea Tar Pit Museum in Los Angeles, California, and at the U.S. National
Museum of Natural History/Smithsonian Institution in Washington, D.C.)
Equus amerhippus
Equus excelsus (See a complete skeleton of this form on exhibit at the Nebraska State
Museum of Natural History in Morrill Hall on the campus of the University of Nebraska
at Lincoln).
Stilt-legged forms:
Equus altidens
Equus quinni
Equus conversidens (See a complete skeleton of this form on exhibit at the American
Museum of Natural History in New York City).
Subspecies of Equus caballus
In a recent publication, my co-author Robert Hoffmann and I recognize seven subspecies
within the species Equus caballus. (This paper is posted in its entirety at the Equine
Studies Institute website, www.equinestudies.org, under “Knowledge Base”).
The first item to take care of under this heading are all the synonyms for Equus caballus
– “duplicate names” proposed, as previously mentioned, by paleontologists eager to name
horse types. Here is a list of published synonyms of Equus caballus with their namer and
the publication date; you can see that this tendency for the proliferation of names goes
right back almost to the beginning of the science:
Equus ferus (Boddaert, 1785)
Equus sylvestris (Brincken, 1828)
Equus przewalskii (Polyakov, 1881)
Equus mosbachensis (Reichenau, 1903)
Equus hagenbecki (Matschie, 1903)
Equus gmelini (Antonius, 1912)
Equus laurentius (Hay, 1913)
Equus niobrarensis alaskae (Hay, 1913)
Equus abeli (Antonius, 1914)
Equus mexicanus (Hibbard, 1957)
Equus midlandensis (Quinn, 1957)
Equus algericus (Bagtache, Hadjonis and Eisenmann, 1984)
Dr. Hoffmann and I recognize seven subspecies within the species. These are forms that
we distinguish on the basis of their morphology as well as their mapped geographic
occurrence. The seven valid subspecies are:
Equus caballus alaskae – the Lamut or Alaskan wild horse. Became extinct about 10,000
years ago, near the end of the Pleistocene. Once ranged in northeastern Asia and across
the Bering Land Bridge into Alaska.
Equus caballus mexicanus – the American peri-glacial horse. Became extinct at the end
of the Pleistocene. Once ranged from the glacial margin west of the Mississippi, south
into northern Mexico.
Neither of these forms were ever domesticated, as they were already extinct by the time
horse domestication began.
Equus caballus przewalskii – the Przewalski horse or Mongolian wild horse. Its last
known range was a narrow strip in the Gobi Desert, but it once ranged broadly across
Russia and Siberia from the Ural Mountains to northeastern Asia. You may frequently
see this animal in zoos. It became extinct in the wild in 1947, but survives in zoos and
preserves today, all living animals being the descendants of only 13 wild-caught
ancestors. The Przewalski horse has contributed in a minor way to a restricted number of
Asian breeds in Mongolia, northern China, and eastern Tibet. It is, as I have explained in
both the “Mammalian Species” and “Origin of the Mustang” papers posted in our
Knowledge Base, most definitely NOT the ancestor of the domestic horse.
Equus caballus ferus – the Tarpan. This animal became extinct in 1913, a herd being kept
up until that time in the Bialovesh Forest Preserve in Poland. Once ranged from eastern
Poland east to the Ural Mountains. This form was the first wild horse to be domesticated.
Its characteristics are preserved within several Russian, eastern European, and west Asian
breeds including the Konik, Hucul, and Akhal-Teke and related Turkmenian breeds.
Through these in turn, Tarpan characteristics have been incorporated into the
Thoroughbred.
Equus caballus pumpelli – the Afro-Turkic horse. This is the immediate ancestor of our
“Oriental” breeds, including the Arabian, Old Hittite, Persian, Bashkir, Lokai, Marwari,
and Barb.
Equus caballus mosbachensis – the Central European horse. The most ancient and
primitive form of Equus caballus, the Mosbach horse is the immediate ancestor of the
Warmblood breeds, including the Latvian, Grönigen, Friesian, Cleveland Bay, and all the
German Warmbloods and their derivatives now bred in Sweden, Denmark, and the Low
Countries.
Equus caballus caballus – the West European horse. This is the cold-and-wet adapted
subspecies, the form that has the stoutest, most rounded body, the shortest ears, the
broadest and deepest head, the shortest legs, the broadest feet, and the thickest fur, mane,
and tail. It is native to those parts of Europe that lie west of the Rhine and Rhone rivers,
from Norway and Sweden in the north through the Low Countries, the British Isles,
western France, and Spain. E. c. caballus is the immediate ancestor of all the draft breeds,
such as the Belgian, Clydesdale, Brabant, and Suffolk Punch and their derivatives now
bred in other countries, such as Russia and the Americas. It is also the immediate
ancestor of all the pony breeds, large and small, including the Fjord, Shetland, Exmoor,
Dartmoor, Mehrens, Galician and Asturian. Ponies and draft horses all derive from a
single ancestry, the overall size of a given form being remarkably concordant with the
area of land upon which the form developed, i.e. Continental forms are the largest, those
occurring on the main British island next biggest, and those occurring on offshore islands
like Man, the Shetlands, and the Orkneys being the smallest.
INTRODUCTION TO THE FAMILY-LEVEL REVIEW
The following text is reprinted almost exactly as it originally appeared in the Elsevier
World Encyclopedia of Animal Science, Volume C-7, “Horse Breeding and
Management”. It is a review, written in semi-technical language, of the horse family. It
includes a cladogram with characters. The interested student will want to carefully study
this logic-diagram, noting that only ONE “shared-derived” character is sufficient to
define any given clade (though more such characters are always desirable).
This paper contains many citations – that is the custom in peer-reviewed scientific
reporting. All of the citations are listed in the bibliography which follows. In addition to
works cited in “The Evolution of the Horse Family”, the bibliography also lists a large
number of other papers with annotations as to which fossil species may have been
discussed or illustrated in those papers. Many of these works are very old – you will note
the long series by the prolific 19th-century American paleontologists Joseph Leidy and
Edward Drinker Cope. These men lived at a time when many fossil horse remains first
came to light. They named and were the first to describe most of the species. That most
of these species have now been placed in genera other than the one originally assigned
does not, by the rules of taxonomy already explained, invalidate the type or the species
name. It certainly does not invalidate the work of these brilliant and dedicated men,
either.
There continues to be large interest in research on the horse family. The genus Equus
actually has its own “fan club” (a technical interest-group) within the Friends of the
Pleistocene of the Society of Vertebrate Paleontology. I have been welcomed as a
member of this group and for that I am grateful. Thanks to the continuing, diligent efforts
of paleontologists -- professionals and amateur fossil-hunters alike – new discoveries
continue to be made which add to the thousands of skeletons already known. From these
comes our only hope of correctly recording and interpreting diversity, environmental
adaptation, and change through time in the horse family.
THE TECHNICAL READING LIST
The following is a list of publications from which information used to compile this article
comes. If you want to pursue the technical literature, you should print this bibliography
out and take it to the library with you, where you will find that it saves you a great deal of
time. The ordinary public library probably will not have or be able to locate most of these
listings for you; you will have to access either a University library or a library within a
large Museum of Natural History, such as the American Museum in New York City, the
British Museum in London, the National Museum of Canada in Toronto, the National
Museum of Mexico in Mexico City, or the U.S. National Museum/Smithsonian
Institution in Washington, D.C. Online Internet listings of the U.S. National Library may
also be of assistance; go to http:/catalog.loc.gov or do a Google search for “Library of
Congress catalog”.
Abusch-Siewert, S. 1983. Gebissmorphologische Untersuchungen an eurasiatischen
Anchitherien (Equidae, Mammalia) unter besonder Berücksichtigung der Fundstelle
Sandelzhausen. Courir Forschungsinstitut Senkenberg 62:1-361.
Agassiz, L. and A. Gould. 1851. Principles of Zoology. Gould and Lincoln Publishers,
London.
Akersten, W.A., H.A. Lowenstam, and A. Walker. 1984. “Pigmentation” of soricine
teeth: composition, ultrastructure, and function, in Am. Soc. Mammal. 64th Ann.
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