By: George Rudzinski
To: Alan Kern
Re: Horse 1 of 9
Ä Area: Evolutionary Mechanism Theory Discussion
Author: Kathleen Hunt (firstname.lastname@example.org)
Title: Horse Evolution FAQ
This is a companion file for the transitional fossils FAQ. In this post I
will try to describe the modern view of evolution within the horse family.
I apologize in advance for the length; I didn't want to cut it down any more
than this, because horse evolution has been oversimplified too many times.
already. I wanted people to see some of the detail and complexity of the
fossil record of a fairly well known vertebrate group. (In fact, even at this
length, this post is still only a summary!) People who are in a hurry may
just want to read the intro and summary, and look at the tree.
HORSE EVOLUTION FAQ (v4.1, Jan 4 1993)
1. Historical background -- why fossil horses are famous
2. Timescale and horse family tree
3. Small equids of the Eocene
4. Medium-sized browsing equids, late Eocene and Oligocene
5. The Miohippus radiation of browsing equids (24 My)
6. Horses move onto the plains: spring-foot & high-crowned teeth (18 My)
7. The merychippine radiation of the late Miocene (15 My)
8. One-toed grazing horses of the Pliocene & Pleistocene
9. Modern equines
1. HISTORICAL BACKGROUND
In the 1870's, the paleontologist O.C. Marsh published a description of newly
discovered horse fossils from North America. At the time, very few
transitional fossils were known, apart from _Archeopteryx_. The sequence
of horse fossils that Marsh described (and that T.H. Huxley popularized) was
a striking example of evolution taking place in a single lineage.
Here, one ould see the fossil species "Eohippus" transformed into an almost
totally different-looking (and very familiar) descendent, _Equus_, through
a series of clear intermediates. Biologists and interested laypeople were
justifiably excited. Some years later, the American Museum of Natural
History assembled a famous exhibit of these fossil horses, designed to
show gradual evolution from "Eohippus" (now called _Hyracotherium_) to
modern _Equus_. Such exhibits focussed attention on the horse family not
only as evidence for evolution per se, but also specifically as a model of
*gradual*, *straight-line* evolution, with _Equus_ being the "goal" of
equine evolution. This story of the horse family was soon included in
all biology textbooks.
As new fossils were discovered, though, it became clear that the old model
of horse evolution was a serious oversimplification. The ancestors of the
modern horse *were* roughly what that series showed, and *were* clear that
evolution had occurred. But it was misleading to portray
horse evolution in that smooth straight line, for two reasons:
1. First, horse evolution *didn't* proceed in a straight line. We now know
of many other branches of horse evolution. Our familiar _Equus_ is merely
one twig on a once-flourishing bush of equine species. We only have the
illusion of straight-line evolution because Equus is the only twig that
survived. (See Gould's essay "Life's Little Joke" in _Bully for Brontosaurus_
for more on this topic.)
2. Second, horse evolution was not smooth and gradual. Different traits
evolved at different rates, didn't always evolve together, and occasionally
reversed "direction". Also, horse species did not always come into being by
gradual transformation ("anagenesis") of their ancestors; instead,
sometimes new species "split off" from ancestors ("cladogenesis") and then
co-existed with those ancestors for some time. Some species arose
gradually, others suddenly.
Overall, the horse family demonstrates the diversity of evolutionary
mechanisms, and it would be misleading -- and would be a real pity -- to
reduce it to an oversimplified straight-line diagram.
With this in mind, I'll take you through a tour of the major genera of the
horse family, _Equidae_. CAUTION: I will place emphasis on those genera
that led to the modern Equus. Do not be misled into thinking that Equus was
the target of evolution! Bear in mind that there are other *major* branches
of the horse tree that I will mention only in passing. (See the horse tree
for a lovely ASCII depiction.)
All equids (members of the family Equidae) are
members of the order of hoofed animals that bear their weight on the
central 3rd toe. (Other perissodactyls are tapirs and rhinos, and
hyraxes.) The most modern equids (descendents of _Parahippus_) are
"equines". Strictly speaking, only the very modern genus _Equus_
"horses", but I will call all equids "horses" rather
Most horse species, including all the ancestors of Equus,
2. TIMESCALE and HORSE FAMILY TREE
Recent 10,000 years ago to present
Pleistocene 2.5-0.01 My (million years ago)
Pliocene 5.3-2.5 My
Miocene 24-5.3 My
Oligocene 34-24 My
Eocene 54-34 My
And here's the tree...note that the timescale is a bit weird (e.g.
Oligocene is compressed almost to nothing) to keep it from being
All the names on the tree are genus names, so recall that each genus
encompasses a cluster of closely related species.
2My Old & New World Equus
\ | /
\ | /
4My Hippidion Equus
| | Neohipparion Hipparion
| | Astrohippus | |
| | Pliohippus
12My Dinohippus Calippus \ | /
| | Pseudhipparion \ | /
| | | |
15My \ | / |
\ | / Megahippus |
17My Merychippus | |
| Anchitherium Hypohippus
| | |
23My Parahippus Anchitherium
25My \ | /
\ | /
45My Paleotherium |
Propalaeotherium | Haplohippus
| | |
50My Pachynolophus | Orohippus
| | |
| | |
\ | /
\ | /
3. SMALL EOCENE HORSES
The first equid was _Hyracotherium_, a small forest animal of the
Eocene. This little animal (10-20" at the shoulder) looked
nothing at all
like a horse. It had a "doggish" look with an arched back, short
snout, short legs, and long tail. It browsed on fruit and fairly
and probably scampered from thicket to thicket like a modern
only stupider, slower, and not as agile. This famous little equid
known by the lovely name "Eohippus", meaning "dawn horse".
Some Hyracotherium traits to notice:
Legs were flexible and rotatable with all major bones present
4 toes on each front foot, 3 on hind feet. Vestiges of 1st (&
toes still present. Hyracotherium walked on *pads*; its feet
a dog's padded feet, except with small "hoofies" on each toe
Small brain with especially small frontal lobes.
Low-crowned teeth with 3 incisors, 1 canine, 4 distinct
premolars and 3
"grinding" molars in each side of each jaw (this is the
mammalian formula" of teeth). The cusps of the molars were
connected in low crests. Typical teeth of an omnivorous
At this point in the early Eocene, equids were not yet very
the other perissodactyl groups; the _Hyracotherium_ genus includes
species closely related to (or even ancestral to) rhinos and
tapirs, as well
as species that are distinctly equine.
[Note: the particular species that probably gave rise to the rest
equids, _H. vassacciense_, may be renamed, perhaps to
Though in retrospect we may consider Hyracotherium to be
was a very successful animal in its time, and seems to have found a
stable niche for itself. In fact, throughout most of the Eocene (a
20 million years), only minor evolutionary changes took place in
Hyracotherium and its near descendants. The body and feet stayed
the same, with slight changes in the toes. The major change was in
teeth; as Eocene equids started to eat more plant browse and less
they developed more grinding teeth to deal with the slightly
In the early-middle Eocene (approx 50 My), there was a smooth,
transition from Hyracotherium to a close relative, Orohippus
1976). Overall, Orohippus looked much like Hyracotherium: 10-20"
the shoulder, still "doggish" with arched back, short legs, short
snout, and fairly small brain. Orohippus still had 4 toes on front
behind, with hoofies, and was also "pad-footed". However, the
the 1st and 2nd toes vanished.
The most significant change was in the teeth. The last premolar
changed in shape to become like a molar, giving Orohippus one more
"grinding tooth". Also, the crests on the teeth were more
indicating Orohippus was eating tougher plant material.
Epihippus arose from Orohippus in the middle Eocene (approx. 47
Orohippus and Hyracotherium, Epihippus was small, doggish,
small-brained, with 4 toes in front and 3 behind. However, tooth
was continuing. Now the last *two* premolars were like molars,
Epihippus *five* grinding cheek teeth. The crests on the cheek
well-formed, and still low-crowned.
There is a late form of Epihippus sometimes called
unclear if this is a subgenus or a species of Epihippus. This
basically an Epihippus with teeth similar to, but a bit more
later Oligocene horses.
4. MEDIUM-SIZED BROWSING HORSES (Late Eocene & Oligocene)
As we move toward the Oligocene, horses start to change. The
North America was becoming drier, and grasses were just evolving.
vast forests were starting to shrink. The late Eocene horses
developing tougher teeth and becoming a bit larger and leggier (for
speed out in the open).
The species _Mesohippus celer_ appears suddenly in the late Eocene,
40 My (such sudden speciations can occur when a population
selective forces and/or becomes isolated from the parent species.
speciations are "sudden" only in geological terms, of course, where
million years is "sudden".) This animal was slightly larger than
24" at the shoulder. It didn't look as doggish, either. The back
arched, the legs a bit longer, the neck a bit longer, and the snout
distinctively longer. It had a shallow facial fossa, a depression
skull. (In later horses these fossae became complex, and handy for
identification.) Mesohippus had three toes on its hind feet *and*
*front* feet -- the 4th front toe was reduced to a vestigial
before, Mesohippus was pad-footed.
Other significant changes:
Cerebral hemispheres notably larger -- has distinctly equine
Last *three* premolars are like the three molars, such that
(and all later horses) had a battery of *six* similar
teeth", with one lonely little simple premolar in front.
Has same tooth crests as Epihippus, well-formed and sharp, more
for grinding tougher vegetation.
Soon after _Mesohippus celer_ and its very close relative _Mesohippus
westoni_ appeared, a similar animal called _Miohippus assiniboiensis_
arose (approx. 36 My). This transition also occurred suddenly,
but luckily a
few transitional fossils have been found that link the two genera.
Miohippus was distinctly larger than a typical Mesohippus, with a
longer skull. The facial fossa was deeper and more expanded. In
the ankle joint had changed subtly.
Miohippus also began to show a variable extra crest on its upper
teeth. In later horse species, this crest became a characteristic
the teeth. This is an excellent example of how new traits
variations in the ancestral population.
It was once thought that Mesohippus "transformed" gradually into
anagenetic evolution, so that only Miohippus continued. Recent
shows that instead, Miohippus speciated (split off) from early
via cladogenetic evolution, and then Miohippus and Mesohippus
overlapped for some 4 million years. For instance, in one place in
Wyoming there were three species of late Mesohippus coexisting with
of Miohippus. (Prothero & Shubin, 1989)
5. THE MIOHIPPUS RADIATION (Early Miocene, 24 My)
Mesohippus finally died out in the mid-Oligocene. Miohippus
a while as it was, and then, in early Miocene (24 My) began to
fairly rapidly. The horse family began to split into at least 2
main lines of
evolution and one small side branch:
1) 3-toed browsers called "anchitheres". They were very
into the Old World, and thrived for tens of millions of years.
the small, simple teeth of Miohippus. Genera include
the large _Hypohippus_ and _Megahippus_.
2) a line of small "pygmy horses", e.g. _Archeohippus_. These
horses did not
3) a line that underwent a transformation from browsing to
advantage of the new grasses. Large grasslands were just beginning
appear, thus creating a new ecological "opportunity" for grazers.
difficult to chew and wears down teeth rapidly (due to the silica
leaves) and thus a grass-eater needs tough teeth with ridges of
Open-country grass eaters, in addition, often benefit from being
runners with long legs. The evolution of this line of horses is
6.HORSES MOVE ONTO THE PLAINS: SPRING-FOOT & HIGH-CROWNED TEETH
As this third line of Miocene horses began to specialize in eating
several changes occurred. First, the teeth changed to be better
chewing harsh, abrasive grass. Small crests on the teeth enlarged
connected together in a series of *ridges* for grinding. There was a
gradual increase in the *height of the tooth crowns*, so that the
could grow out of the gum continuously as the tops were worn down
("hypsodont" teeth). And, in addition, the tooth crowns became
to the development of a *cement* layer on the teeth.
Second, these horses started to become specialized runners. There
was a simultaneous increase in *body size, leg length, and length
face*. The bones of the legs began to *fuse* together, and the leg
and musculature became specialized for efficient forward-and-back
strides, with flexible leg rotation being eliminated. Most
horses began to stand permanently on tiptoe (another adaptation for
instead of walking on doglike pads, their weight was supported by
ligaments* that ran under the fetlock to the big central toe.
All these changes occurred rapidly, and we are lucky to have a
good fossil record during this time. This was one of the most
times in horse evolution. The transitions in these characters are
_Kalobatippus_ -- this genus is not well known, but its teeth seem
intermediate between Miohippus and the later Parahippus (see below).
_Parahippus_ -- arose in early Miocene, 23 My.
A typical Parahippus was a little larger than Miohippus, with about
the same size brain and same body form. Parahippus was still
and was just beginning to develop the springy ligaments under the
Parahippus showed gradual and fluctuating changes in its teeth,
the permanent establishment of the extra crest that was so variable
Miohippus. In addition, various other cusps and crests were
join up in a series of *strong crests*, with slightly taller *tooth
Parahippus evolved rapidly and was quickly transformed into a fully
footed, hypsodont grazing horse called _Merychippus gunteri_. This
evolution took place about 18-17 My. Later fossils of Parahippus
species _Parahippus leonensis_) are so similar to early Merychippus
it's hard to decide where to draw the line between the genera.
_Merychippus_ 17 My (as in "Merry Kippus to all!" :-)
A typical Merychippus was about 10 hands (40") tall, the tallest
The muzzle became elongated, the jaw became deeper, and the eye moved
farther back, to accommodate the large tooth roots. The brain was
larger, with a fissured neocortex and a larger cerebellum, making
Merychippus a smarter and more agile equine than the earlier horses.
Overall, Merychippus was distinctly recognizable as a horse, and
Merychippus was still 3-toed, but was fully spring-footed. This
stood permanently on tiptoe, supported and propelled by strong,
ligaments that ran under the fetlock. The side toes were still
began to be of varying sizes; some Merychippus species had
toes, while others developed small side toes that only touched the
during running. The central toe developed a large, convex,
"horsey" hoof, and
the legs became longer. The radius and ulna of the forearm fused
leg rotation was eliminated. Likewise, the fibula of the shin was
reduced. All these changes made Merychippus' legs specialized for
function: rapid running over hard ground.
Merychippus' teeth were fully high-crowned, with a thick layer of
cement, and with the same distinctive grazing tooth crests as
_Merychippus gunteri_ evolved into a slightly more advanced form,
_Merychippus primus_, in the middle/late Miocene.
7. THE MERYCHIPPINE RADIATION (Miocene, 15 My)
By the late Miocene, Merychippus was the one of the first bona-fide
plains grazers. (Simpson, 1961, called Merychippus "the horse with
look"). Merychippus underwent rapid speciation, and gave rise to
at least 19
new grazing horse species in three major groups. This explosive
horse evolution is often called the "merychippine radiation". The
major groups were:
1) Three-toed grazers known as "hipparions". These were tremendously
successful and split into 4 genera and at least 16 species,
covering a variety of niches for small and large grazers and
developed large and elaborate facial fossae. Hipparions spread
from the New
World into the Old World in several waves of migration.
2) A line of smaller horses including _Protohippus_ and _Calippus_,
collectively called "protohippines".
3) A line of "true equines" in which the side toes sometimes began to
decrease in size. In this flurry of evolution, Merychippus primus
rise to two later merychippines called M. sejunctus and M.
isonesus, who had
a mixture of "primitive" (Parahippus-like), hipparion, and equine
They, in turn, gave rise to M. intermontanus, which begat M.
M. carrizoensis. These last two looked quite "horsey" and gave
rise to a
set of larger three-toed and one-toed horses known as the "true
(see below). Crystal clear, right?
As this brief list shows, new species arose in rapid succession in
of these groups. This rapid speciation makes it hard to determine
which species arose from exactly which others.
About 10 My, the horse family reached an apex of diversity (of
of genera) and sheer numbers which it has never equalled since.
and New Worlds both seemed overrun with a wide variety of hipparions,
protohippines, and "true equines", large and small, forest browsers
plains grazers. Throughout the evolution of all these related
merychippine descendents, the facial fossae got deeper and more
With so many equine species overlapping at once, these facial
have housed species-specific glands of some sort, similar to the
marking glands of modern antelopes and deer.
8. ONE-TOED HORSES (Late Miocene, Pliocene & Pleistocene)
Let's leave the hipparions and protohippines now, and concentrate on
the merychippine line that led to the "true equines". The late
merychippine species of this line, such as M. carrizoensis, were
horses with small side toes. They gave rise to at least 2 separate
groups of horses that independently lost their side toes. This
*side ligaments* developed around the fetlock to help stabilize the
toe during running. These one-toed horses include:
_Pliohippus_ -- arose in middle Miocene (~15 My) as a three-toed
horse. Gradual loss of the side toes is seen in Pliohippus through 3
successive strata of the early Pliocene. Pliohippus was very
Equus and until recently was thought to be the direct ancestor of
except for two significant differences. First, Pliohippus's skull
facial fossae, whereas Equus has no facial fossae at all. Second,
Pliohippus's teeth are strongly curved, and Equus's teeth are very
Though Pliohippus is obviously related to Equus, it probably didn't
_Astrohippus_ (~10My) was another one-toed horse that arose
after Pliohippus. Astrohippus also had large facial fossae, and was
probably a descendent of Pliohippus.
Finally, a third one-toed horse called _Dinohippus_ (recently
discovered) arose about 12 My. The exact ancestor of Dinohippus is
known (see Evander, 1989). The earliest known species are D.
interpolatus, and D. leidyanus. They look smashingly like Equus in
morphology, teeth, and skull. The teeth were slightly *straighter*
Merychippus, and the facial fossae were significantly *decreased*. A
slightly later species was D. mexicanus, that showed even
and even smaller fossae. Dinohippus was the most common horse in
America in the late Pliocene, and almost certainly gave rise to
(Recall that Equus has very straight teeth and no fossae.)
The Isthmus of Panama arose at this point. Some very early
species gave rise to the "hippidions", stocky, short-legged, one-toed
horses with odd boxy skulls (~4 My). They travelled into the South
and thrived there briefly.
Throughout the end of the Pliocene, Dinohippus showed a gradual
decrease in the facial fossae, straightening of the teeth, and
changes, as Dinohippus smoothly graded into Equus. (Hulbert, 1989)
_Equus_ -- arose in late Pliocene about 4 My.
Finally we arrive at Equus, the genus of all modern equines.
Equus were 13.2 hands tall (pony size), with a classic "horsey"
body -- rigid
spine, long neck, long legs, fused leg bones with no rotation, long
flexible muzzle, deep jaw. The brain was a bit larger than in early
Dinohippus. Like Dinohippus, Equus was (and is) one-toed, with
that prevent twisting of the hoof, and has high-crowned, straight
teeth with strong crests lined with cement.
Members of Equus still retain the genes for making side toes.
these express themselves only as the vestigial "splint bones" of
toes 2 and
4, around the large central 3rd toe. Very rarely, a modern Equus
with small but fully-formed side toes. (see Gould, "Hen's Teeth
The earliest known Equus species were a set of three "simple Equus"
species collectively known as the _Equus simplicidens_ group. They
some primitive traits from Dinohippus, including a slight facial
had zebra-like bodies (relatively stocky with a straight shoulder
and thick neck), and short, narrow, donkey-like skulls. They
stiff, upright manes, ropy tails, medium-sized ears, striped legs,
at least some striping on the back (all traits shared by modern
They quickly diversified into at least 12 new species in 4
in a burst of evolution reminiscent of the great merychippine
All these Equus species coexisted with other one-toed horses (such as
_Astrohippus_) and with various successful hipparions and
which had been merrily evolving on their own paths.
During the first major glaciations of the late Pliocene (2.6
_Equus_ species crossed to the Old World. Some entered Africa and
diversified into the modern zebras. Others spread across Asia, the
N. Africa as desert-adapted onagers and asses. Still others spread
Asia, the Mideast, and Europe as the true horse, _E. caballus_.
_Equus_ species spread into South America. The Equus genus was
the most successful perissodactyl genus that ever lived -- even
domestication by humans.
Compare Equus to Hyracotherium and see how much it has changed. In
no way can Equus and Hyracotherium be considered the same "kind".
change from Hyracotherium to Equus is truly long-term, large-scale
9. MODERN EQUINES (Recent
The three-toed horses gradually died out, perhaps outcompeted by the
phenomenally successful artiodactyls (or not). Most of the
in North America also died out, as the Ice Ages started. (The
these extinctions are unknown.) However, one-toed Equus was very
Until about 1 million years ago, there were Equus species all over
Asia, Europe, North America, and South America, in enormous
that must easily have equalled the great North American bison
herds, or th
huge wildebeest migrations in Africa.
In the late Pleistocene there was a set of devastating
killed off most of the large mammals in North and South America.
horses of North and South America died out (along with the mammoths
saber-tooth tigers). These extinctions seem to have been caused by
combination of climatic changes and overhunting by humans, who had
reached the New World. For the first time in tens of millions of
were no equids in the Americas.
The only members of Equus -- and of the entire family Equidae --
survived to historic times were
_Equus burchelli_ -- the Plains zebra of Africa, including
zebra", "Burchell's zebra", "Chapman's zebra", the
Quagga, and other subspecies. The Plains zebra is what peopl
usually think of as the "typical zebra", with rather wide
stripes, and thick horizontal stripes on the rump.
_Equus zebra_ -- the Mountain zebra of South Africa. This is
zebra with the dewlap and the gridiron pattern on its rump
_Equus grevyi_ -- Grevy's zebra, the most horse-like zebra.
the big zebra with the very narrow vertical stripes and
_Equus caballus_, the true horse, which once had several
_Equus hemionus_ -- the desert-adapted onagers of Asia & the
including the kiang (formerly E. kiang)
_Equus asinus_ -- the true asses & donkeys of northern Africa.
African wild asses are sometimes called E. africanus.
[I have a separate file about the relationships & current status of
surviving wild equines, including information about captive
E-mail for details.
For many people, the horse family remains the classic example of
As more and more horse fossils have been found, some ideas about
evolution have changed, but the horse family remains a good example
evolution. In fact, we now have enough fossils of enough species
genera to examine subtle details of evolutionary change, such as
In addition to showing that evolution has occurred, the fossil
show the following characteristics of evolution
1. Evolution does not occur in a straight line toward a goal, like
rather, evolution is like a branching bush, with no predetermined
Horse species were constantly branching off the "evolutionary
and evolving along various unrelated routes. There's no discernable
"straight line" of horse evolution. Many horse species were
at the same time, with various numbers of toes, adapted to various
different diets. In other words, horse evolution had no inherent
We only have the impression of straight-line evolution because only
genus happens to still be alive, which deceives some people into
that one genus was somehow the "target" of all the evolution.
one genus is merely the last surviving branch of a once mighty and
The view of equine evolution as a complex bush with many
contemporary species has been around for several decades, and is
recounted in modern biology and evolution textbooks.
2. There are no truly consistent "trends".
Tracing a line of descent from Hyracotherium to Equus reveals
apparant trends: reduction of toe number, increase in size of
lengthening of the face, increase in body size. But these trends
are not seen
in all of the horse lines. On the whole, horses got larger, but
(Archeohippus, Calippus) then got smaller again. Many recent horses
evolved complex facial pits, and then some of their descendants
again. Most of the recent (5-10 My) horses were three-toed, not
and we see a "trend" to one toe only because all the three-toed
recently become extinct.
Additionally, these traits do not necessarily evolve together,
or at a
steady rate. The various morphological characters each evolved in
starts, and did *not* evolve as a suite of characters. For example,
throughout the Eocene, the feet changed little, and only the teeth
Throughout the Miocene, both feet and teeth evolved rapidly. Rates
evolution depend on the ecological pressures facing the species
The "direction" of evolution depends on the ecological challenges
facing the individuals of a species and on the variation in that
on an inherent "evolutionary trend".
3. New species can arise through several different evolutionary
Sometimes, new species split off suddenly from their ancestors
(e.g., Miohippus from Mesohippus) and then co-existed with those
Other species came into being through anagenetic transformation of
ancestor, until the ancestor had changed appearance enough to be
new name (e.g. Equus from Dinohippus). Sometimes only one or a few
species arose; sometimes there were long periods of stasis (e.g.
Hyracotherium throughout the early Eocene); and sometimes there were
enormous bursts of evolution, when new ecological opportunities
merychippine radiation). Again, evolution proceeds according to the
ecological pressures facing the individuals of a species and on the
present within that species. Evolution takes place in the real
diverse rates and modes, and cannot be reduced to a single, simple
A Question for Creationists:
Creationists who wish to deny the evidence of horse evolution should
careful consider this: *how else can you explain the sequence of
fossils?* Even if creationists insist on ignoring the transitional
(many of which *have* been found), again, how can the unmistakable
SEQUENCE of these fossils be explained? Did God create
then kill off Hyracotherium and create some Hyracotherium-Orohippus
intermediates, then kill off the intermediates and create
kill off Orohippus and create Epihippus, then allow Epihippus to
"microevolve" into Duchesnehippus, then kill off Duchesnehippus and
Mesohippus, then create some Mesohippus-Miohippus intermediates, then
create Miohippus, then kill off Mesohippus, etc.....each species
coincidentally similar to the species that came just before and
Creationism utterly fails to explain the sequence of known horse
fossils from the last 50 million years. That is, without invoking
Created Everything To *Look* Just Like Evolution Happened" Theory.
[And I'm not even mentioning all the *other* evidence for
is totally independent of the fossil record -- developmental biology,
comparative DNA & protein studies, morphological analyses,
The fossil record, horses included, is only a small part of the
Truly persistent and/or desperate creationists are thus forced
illogical, unjustified attacks of fossil dating methods, or
and usually flat-out wrong proclamations about a supposed "lack" of
"transitional forms". It's sad. To me, the horse fossils tell a
magnificent and fascinating story, of millions of animals living out
their lives, in their natural world, through millions of years. I
dedicated horse rider and am very happy that the one-toed grazing
survived to the present. Evolution in no way impedes my ability to
the beauty and nobility of these animals. Instead, it enriches my
appreciation and understanding of modern horses and their rich
"All the morphological changes in the history of the Equidae can
be accounted for by the neo-Darwinian theory of microevolution:
genetic variation, natural selection, genetic drift, and speciation."
(Futuyma 1986, p.409)
"Because its complications are usually ignored by biology textbooks,
creationists have claimed the horse story is no longer valid.
main features of the story have in fact stood the test of time...."
(Futuyma 1982, p. 85)
"When asked to provide evidence of long-term evolution, most
turn to the fossil record. Within this context, fossil horses are
most frequently cited examples of evolution. The prominent Finnish
paleontologist Bjorn Kurten wrote: 'One's mind inevitably turns to
inexhaustible textbook example, the horse sequence. This has been
incorrectly more often than not -- as evidence for practically every
evolutionary principle that has ever been coined.' This cautionary
notwithstanding, fossil horses do indeed provide compelling
support of evolutionary theory."
(MacFadden 1988, p. 131)
"The fossil record [of horses] provides a lucid story of descent
for nearly 50 million years, and we know much about the ancestors of
(Evander 1989, p. 125)
"It is evolution that gives rhyme and reason to the story of the
as it exists today and as it existed in the past. Our own
existence has the
same rhyme and reason, and so has the existence of every other living
organism. One of the main points of interest in the horse family
is that it
so clearly demonstrates this tremendously important fact."
(Simpson, 1961, p. xxxiii)
I've tried to incorporate all the recent research I could find into
For more information, non-scientists may want to start with Simpson's
1961 book, _Horses_. This book is a classic, readable account of
evolution, and though it's now somewhat outdated, I think it's
still the most
accessible introduction to the topic. However, I *strongly*
Simpson's book be supplemented with newer information from
summary (1988) and/or Prothero & Schoch's _The Evolution of
(1989). These and other selected references are listed below.
Thanks to Larry Moran for the prototype of the ASCII horse tree and
other various notes.
Bennett, D.K. 1986? (year not on my xerox! argh.) The origins of
Equus 110:33, 11:37, 112:37.
This is a three-part series in a good-quality trade magazine,
horse owners who have some interest in science and evolution.
references are in the articles.) The author is a vertebrate
who specializes in the evolution, form, and function of modern
Her analysis shows that E. caballus had at least 5 subspecies
Colbert, E.H. 1980. _Evolution of the Vertebrates_, 3rd edition.
& Sons, New York.
Carroll, R.L. 1988. _Vertebrate Paleontology and Evolution_. WH
& Co., New York.
(These are two standard texts on vertebrate fossils & evolution.
has a 4th edition out now.)
Futuyma, D.J. 1982. _Science on Trial: The Case for Evolution.
Books, New York.
(A well-written book on the evidence for evolution, written for the
Futuyma, D.J. 1986. _Evolutionary Biology_. Sinauer Associates,
(A standard text covering theories of *how* evolution occurs --
stress evidence for evolution per se.)
Gould, S.J. (year?) _Hen's Teeth And Horse's Toes_.
Gould, S.J. (year?) _Bully for Brontosaurus_.
(Collections of essays written for _Natural History_ magazine.
Teeth..." has essays on horse side toes and zebra stripes;
essays on "fox-terrier size" Hyracotherium and on the fallacy of
a direction of evolution in the horse family. Other essays are
too. Sorry I don't have more precise references handy....my copy
Teeth is in Boston, and Bully for B is at home.)
Hildebrand, M. 1987. The mechanics of horse legs. Amer. Sci.
(not about evolution, but interesting & useful nonetheless.)
Janis, C. 1976. The evolutionary strategy of the Equidae and the
rumen and cecal digestion. Evolution 30:757-774.
(An interesting analysis of the significance of hindgut
equids, and on why the Equidae tend not to have high species
Lowenstein, J.M., and O.A. Ryder. 1985. Immunological systematics
extinct quagga (Equidae). Experientia 41:1192-1193.
(The authors studied molecules from skins of the extinct quagga, and
conclude it was a subspecies of the plains zebra.)
MacFadden, B.J. 1976. Cladistic analysis of primitive equids with
other perissodactyls. Syst. Zool. 25(1):1-14.
(An analysis of the interrelationships of Hyracotherium, Orohippus,
Epihippus, the paleotheres, and other early perissodactyls.)
MacFadden, B.J. 1984. Systematics and phylogeny of the _Hipparion_,
_Neohipparion_, _Nannippus_, and _Cormohipparion_ (Mammalia,
from the Miocene and Pliocene of the New World. Bull. Am. Mus.
(*Extremely* detailed analysis of evolution and interrelationships
hipparions. [Okay, okay, I didn't read the whole thing.] Relies
large extent on the distinctive morphology of the facial fossae in
MacFadden, B.J. 1984. _Astrohippus_ and _Dinohippus_ from the
Yepomera local fauna (Hemphillian, Mexico) and implications for
the phylogeny of one-toed horses. J. Vert. Paleon. 4(2):273-283.
(Description & discussion of Pliohippus, Astrohippus, and Dinohippus.
Concludes that Dinohippus was probably the ancestor of Equus, and
was probably the ancestor of Astrohippus.)
MacFadden, B.J. 1985. Patterns of phylogeny and rates of evolution
in fossil horses: hipparions from the Miocene and Pliocene of
North America. Paleobiology 1(3):245-257.
Analyzes the evolution of hipparion species. Of the 16 known
appear too suddenly for the mode of speciation to be determined.
Of the 10
that appeared gradually enough for speciation mode to be
determined, 5 have
originated by anagenesis (transformation of an ancestor species
descendent species, such that the ancestor "disappears") and 5 by
cladogenesis (splitting off of a new species from an ongoing ancestor
species, such that the 2 species continue to exist together.)
MacFadden, B.J. 1986. Late Hemphillian monodactyl horses (Mammalia,
Equidae) from the Bone Valley formation of central Florida. J.
(Description of two recent discovered advanced one-toed horse
_Astrohippus stocki_ and _Dinohippus mexicanus_.)
MacFadden, B.J. 1988. Horses, the fossil record, and evolution:
perspective. Evol. Biol. 22:131-158.
(A useful and readable update on current evidence & theories of horse
MacFadden, B.J. 1993. [a new book about horse evolution. I have
it yet but am trying to get a copy. Over $70! sheesh]
MacFadden, B.J., J.D. Bryant, and P.A. Mueller. 1991. Sr-isotopic,
paleomagnetic, and biostratigraphic evidence of horse evolution:
evidence from the Miocene of Florida. Geology 19:242-245.
This is an interesting example of the variety of dating methods
paleontologists use to date their finds. MacFadden et al. dated the
Parahippus --> Merychippus transition at a Florida site with
data and Sr/Sr dates, and also by cross-correlation to other sites
with Sr/Sr, K/Ar, Ar/Ar, zircon fission-track, and paleomagnetic
methods. Surprise, surprise, all the dates were consistent at
roughly 16 My.
MacFadden, B.J., & R.C. Hubbert. 1988. Explosive speciation at
the base of
the adaptive radiation of Miocene grazing horses. Nature
(An interesting summary of the merychippine radiation. Has a nice
tree, too. MacFadden's horse tree is used by almost everyone these
MacFadden, B.J., & M.F. Skinner. 1981. Earliest holarctic
_Cormohipparion goorisi_ n.sp. (Mammalia, Equidae) from the
(medial Miocene) Texas gulf coastal plain. J. Paleontology
(Description of a hipparion that was found to have crossed into the
from the New World sooner than previously realized.)
Prothero, D.R., & R.M. Schoch, eds. 1989. _The Evolution of
Clarendon Press, New York.
A compilation of current research and theories of perissodactyl
The following chapters were particularly useful:
Evander, R.L. Phylogeny of the family Equidae. pp. 109-126
MacFadden, B.J. Dental character variation in paleopopulations
morphospecies of fossil horses and extant analogs. pp.
Hulbert, R.C. Phylogenetic interrelationsihps and evolution of
American late Neogene Equinae. pp. 176-196.
Prothero, D.R., & R.M. Schoch. Origin and evolution of the
summary and synthesis. pp. 504-529.
Prothero, D.R., & N. Shubin. The evolution of Oligocene horses.
Winans, M.C. A quantitative study of North American fossil
the genus _Equus_. pp. 262-297.
Radinsky, L. 1983. Allometry and reorganization in horse skull
Science 221 (16 Sept):1189-1191
(Analysis of horse skull changes around the time that horses
high-crowned teeth, between 15 and 25 million years ago.)
Renders, E. 1984. The gait of _Hipparion_ sp. from fossil
Laetoli, Tanzania. Nature 308:179-181.
(Interesting paper describing fossil hoofprints of an adult female
hipparion and her foal. They were using a gait called a "running
Simpson, G.G. 1961. _Horses_. Doubleday & Co., New York.
(An interesting and readable, though outdated, account of horse
Written for the intelligent non-scientist by a prominent
Thomason, J.J. 1986. The functional morphology of the manus in the
tridactyl equids _Merychippus_ and _Mesohippus_: paleontological
from neontological models. J. Vert. Pal. 6(2):143-161.
(Analysis of the pad-foot to spring-foot transition.)
"[Fossils] are animals, just as full of life as you are, even
occur at different points in the endless stream of time. Within
segments of this stream, they breathe, eat, drink, breed, fight,
their own lives..."
(Simpson, 1961, p. xxxiv)
don't know yet if there's anything substantially new in there.