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| Prehistoric Horses |
Writing
this week in the journal Science, paleontologist Bruce J.
MacFadden said the evolution of horses involved many more twists and
turns than previously imagined. Modern steeds did not follow a
relatively smooth transition from the diminutive, foxlike forest
browsers that were their earliest ancestors to those impressive,
open-plains athletes we know today. Rather, horses fluctuated
considerably in form and size over time. MacFadden,
who is the vertebrate-paleontology curator at the Florida Museum of
Natural History in Gainesville, said horses have proved especially
popular with evolutionary scientists.
Kathleen Hunt, a biologist at the University of
Washington in Seattle, said the modern-day horse 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." MacFadden, the study author, agrees
that horse evolution was, in fact, a pretty messy affair—a jumble of
evolutionary processes such as random genetic variation and natural
selection. "Any changes in morphology [physical form and
function], such as in tooth or limb evolution, can be explained
within this framework," he said. "Equine mammals are adaptable
critters whose size, diet, and range depended on geography and
climate."
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http://www.flmnh.ufl.edu/fhc/Stratmap1.htm |
| The modern
horse appears to have evolved over three million years ago in North
America. Crossing land bridges, it crossed to Asia and Europe.
It disappeared from this western hemisphere around 10,000 years ago.
Around 3,500 years ago, horses were domesticated. and by 1,000 years
ago they were widely used throughout Asia, Europe and North Africa.
They were reintroduced to America by Spanish and English colonists.
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About 3,500 years ago,
somewhere in the steppe region of Asia, horses were domesticate. The
spotted horses were especially eye-catching and so they frequently
became models for the local artisans. Some of those relics have
endured to present day leaving us some record of horses during
ancient times. |
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The modern horse evolved over three million years
ago and then disappeared from this hemisphere 10,000 years ago. The
horse returned to North America when explorers Cortes and DeSoto
came mounted on magnificent Barbs from Morocco, Sorraia from
Portugal and Andalusians from Spain. |
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The oldest species of "true" horse, Equus
stenonis, was discovered in Italy,
and is believed to have evolved from Plesippus-like animals
at the end of the Tertiary or beginning of the Quaternary periods.
Equus stenonis proliferated into two branches, one lighter in
body mass and one heavier.
Equus stenonis crossed into
North America, where similar forms known as
Equus scotti are common; some types (Equus scotti
var. giganteus) exceeded the modern horse in size. However,
all the horses in North America ultimately became extinct,
approximately 11,000 years ago, perhaps due to
climate change or some
pandemic. It has also been suggested that humans hunted horses
to extinction, as the appearance of humans in the Americas occurred
at about the same time as the extinction of most large mammals in
the Americas. However, there are no known kill sites of Pleistocene
horses in North America, and so this scenario remains unsupported.
Recent studies by a team of geneticists headed by C. Vila
indicate that the horse line split from the zebra/donkey line
between 4 and 2 million years ago.
Equus ferus, ancestor species to
Equus caballus, appeared 630,000 to 320,000 years bp.
Equus caballus was formed from several subspecies of Equus
ferus by selective breeding widely over Eurasia for an extended
time. The details of this process are currently a target of research
by
archaeologists and
geneticists. |
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Hyracotherium /
Eohippus *
also known as Eohippus. This small forest-dwelling browser, which
arose in North America roughly 50 million years ago, was the
ancestor of all modern equids (including the common domesticated
horse). |
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The horse originated in North
America. Its earliest known ancestor was the
Hyracotherium, sometimes called Eohippus or "dawn horse." Thousands of complete, fossilized skeletons
have been found,
mainly in the Wind River basin in Wyoming. It was the size of
a fox and lived about 55 million years ago. |
The
earliest animal to bear recognizably horse-like anatomy was the
Hyracotherium ("hyrax-like beast"). Its scientific name is
derived from initial confusion over early partial fossils'
relationship with living species: Richard Owen likened early
Hyracotherium fossils "to a hare in one passage and to something
between a hog and a hyrax in another". A later name for the Hyracotherium, "eohippus"
("dawn horse"), is also popular, though the earlier name takes
precedence due to scientific naming conventions.
Hyracotherium evolved
in the early Eocene (54–34 million years ago). It was an animal
approximately the size of a fox (250–450 mm in height), with a
relatively short head and neck and a springy, arched back. It had 44
low-crowned teeth, in the typical arrangement of an omnivorous,
browsing mammal: 3 incisors, 1 canine, 4 premolars, and 3 molars on
each side of the jaw. Its molars were uneven, dull, and bumpy, and
used primarily for grinding foliage. The cusps of the molars were
slightly connected in low crests. The Hyracotherium browsed
on soft foliage and fruit, probably scampering between thickets in
the mode of a modern muntjac: the Hyracotherium had a small
brain, and possessed especially small frontal lobes.
Thousands of complete, fossilized skeletons of these animals
have been found in the Eocene layers of North American strata,
mainly in the Wind River basin in Wyoming
This small dog-sized animal is the oldest found horse
ancestor that lived about 55 million years ago. It had a primitive
short face, with eye sockets in the middle and a short diastema (the
space between the front teeth and the cheek teeth). Although it has
low-crowned teeth, we see the beginnings of the characteristic
horse-like ridges on the molars.
The origin of
equines can be traced to the Eocene period, between 60 and 50
million years ago. Eohippus, or Dawn Horse, was about the size of a
Cocker Spaniel - 14 inches at the shoulder - and is thought to have
weighed about twelve pounds. He had four toes on the front legs and
three on the back, which were padded like those of a dog and allowed
easy movement over wet ground. These toes and pads are now the
ergots and splint bones found on the legs of the modern horse.
Eohippus was a browsing animal that lived on soft leaves growing on
low shrubs. He was well equipped to survive in what were then the
semi-tropical forests of the U.S. Midwest. |
Orohippus
Approximately 50 million years ago, in the
early-to-middle Eocene, Hyracotherium smoothly transitioned
into Orohippus over a gradual series of changes.
Although its name means "mountain horse", Orohippus did not
live in the mountains. It resembled Hyracotherium is size,
but had a slimmer body, an elongated head, and slimmer forelimbs and
longer hind legs, all of which are characteristics of a good jumper.
Although Orohippus was still pad-footed, the vestigial outer
toes of Hyracotherium were not present in the Orohippus;
there were four toes on each forelimb, and three on each hind leg.
The most dramatic change between Hyracotherium and
Orohippus was in their teeth: the first of the premolar teeth
were dwarfed, the last premolar shifted in shape and function into a
molar, and the crests on the teeth became more pronounced. Both of
these factors gave the teeth of Orohippus greater grinding
ability, suggesting that Orohippus was subsisting on tougher
plant material.
The earliest evidence of this “little horse” is found in the
middle Eocene of Wyoming, about 2 million years after the first
appearance of Hyracotherium. The two genera coexisted during
the Eocene, although Orohippus fossils are not as numerous or
as geographically widespread as those of Hyracotherium.
Fossils of Orohippus have been found in Eocene sediments in
Wyoming and Oregon, dating from about 52-45 million years ago. |
Epihippus
In the
mid-Eocene, about 47 million years ago, Epihippus, a species
which continued the evolutionary trend of increasingly efficient
grinding teeth, evolved from Orohippus. Epihippus had
five grinding, low-crowned cheek teeth with well-formed crests. A
late form of Epihippus, sometimes called Duchesnehippus,
had teeth similar to Oligocene equids, although slightly less
developed. Whether Duchesnehippus was a subspecies of
Epihippus or a single species is disputed. |
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Mesohippus *
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Mesohippus
Beginning about 32 million years ago, the climate became
drier and early grasses began to evolve. Several million on
years later, Mesohippus or "middle horse" appeared and was one of
the most widespread mammals in North America. . It had six
grinding cheek teeth suitable for chewing the grasses.
Longer legs permitting to run faster, thus avoiding predators.
Fossils have been found throughout the Great Plains. |
In the late Eocene and the early stages of the
Oligocene epoch (32–24 million years ago), the climate of North
America became drier, and the earliest grasses began to evolve. The
forests were yielding to flatlands, home to grasses and various
kinds of brush. In a few areas these plains were covered in sand,
creating the type of environment resembling the present-day
prairies.
In
response to the changing environment, equids, too, began to change.
In the late Eocene, they began developing tougher teeth and becoming
slightly larger and leggier, allowing for faster running speeds in
open areas, and thus for evading predators in non-wooded areas.
About 40 million years ago, the Mesohippus ("middle horse")
suddenly developed in response to strong new selective pressures to
adapt, beginning with the species Mesohippus celer and soon
followed by Mesohippus westoni.
In the early Oligocene, Mesohippus was one of the more
widespread mammals in North America. It walked on three toes on each
of its front and hind feet (the first and fifth toes remained, but
were small and not used in walking). The third toe was stronger than
the outer ones, and thus more weighted; the fourth front toe was
diminished to a vestigial nub. Judging by its longer and slimmer
limbs, Mesohippus was an agile animal.
Mesohippus was slightly larger
than Epihippus, about 610 mm (24") at the shoulder. Its back
was less arched, and its face, snout, and neck were somewhat longer.
It had significantly larger
cerebral hemispheres, and had a small, shallow depression on its
skull called a
fossa,
which in later horses became quite detailed, and serves as a useful
marker for identifying an equine fossil's species. Mesohippus
had six grinding "cheek teeth", with a single premolar in front—a
trait all later equid species would retain. Mesohippus also
had the sharp tooth crests of Epihippus, improving its
ability to grind down tough vegetation.
The "middle horse" earned its name. Mesohippus
is intermediate between the eohippus-like horses of the Eocene,
(which don't look much like our familiar "horse") and more "modern"
horses. Fossils of Mesohippus are found at many Oligocene
localities in Colorado and the Great Plains of the US (like Nebraska
and the Dakotas) and Canada. This genus lived about 37-32 million
years ago.
By the Oligocene period,
about 38 million years ago, Eohippus had evolved into Mesohippus and
Miohippus and had achieved the size of a German Shepherd. Both these
evolutions were taller and heavier, with teeth that allowed them to
eat a wider variety of plants. They were still browsers living in
forests and swamps. Their front feet were reduced to three toes,
still padded, but the middle toe carried most of the weight. |
Miohippus
*
Miohippus was a genus of prehistoric horse that lived in what is
now North America during the Oligocene Period some 25 to 40 million
years ago. It is believed to have branched off from Mesohippus,
and the two coexisted for about four-eight million years.
Around 36 million years ago, Soon after the development
of the Mesohippus, the Miohippus ("lesser horse")
emerged, the earliest species being Miohippus assiniboiensis.
Like Mesohippus, Miohippus's evolution was relatively
abrupt, though a few transitional fossils linking the two genera
have been found. It was once believed that the Mesohippus had
anagenetically evolved into the Miohippus by a gradual series
of progressions, but new evidence has shown that Miohippus's
evolution was cladogenetic: a Miohippus population split off
from the main Mesohippus genus, coexisted with Mesohippus
for around 4 million years, and then over time came to replace
Mesohippus.>
The Miohippus was significantly larger
than its predecessors, and its ankle joints had subtly changed. Its
facial fossa was larger and deeper, and it also began to show a
variable extra crest in its upper cheek teeth, a trait that became a
characteristic feature of later equid teeth.
The Miohippus ushered in a major new period of
equid diversification. While Mesohippus died out in the
mid-Oligocene, Miohippus continued to thrive, and in the
early Miocene (24–5.3 million years ago), it began to rapidly
diversify and speciate. It branched out into two major groups, one
of which adjusted to the life in forests once again, while the other
remained suited to life on the prairies.
Species of Miohippus gave rise to the first burst of
diversity in the horse family. Until Miohippus, there were
few side branches, but the descendants of Miohippus were
numerous and distinct. During the Miocene, over a dozen genera
existed. Fossils of Miohippus are found at many Oligocene
localities in the Great Plains, the western US and a few places in
Florida. Species in this genus lived from about 32-25 million years
ago. |
Miocene
and Pliocene: True equines
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Kalobatippus
The forest-suited form was
Kalobatippus (or Miohippus intermedius, depending on
whether it was a new genus or species), whose second and fourth
front toes were long, well-suited travel on the soft forest floors.
Kalobatippus traveled to Asia via the Bering Strait land
bridge, and from there to Europe, where its fossils were formerly
described under the name Anchitherium. Kalobatippus is
believed to be ancestral to another European species known as
Hyohippus, which became extinct near the beginning of the
Pliocene. |
Parahippus
The Miohippus population that
remained on the steppes is believed to be ancestral to Parahippus,
a North American animal about the size of a small pony, with a
prolonged skull and a facial structure resembling the horses of
today. Its third toe was stronger and larger, and carried the main
weight of the body. Its four premolars resembled the molar teeth and
the first were small and almost nonexistent. The incisive teeth of
Parahippus, like those of its predecessors, had a crown as
humans do; however, the top incisors had a trace of a shallow crease
marking the beginning of the core/cup. =D
Parahippus
appears to be the evolutionary “link” between the old
forest-dwelling horses and the modern plains-dwelling grazers. It
has 3 toes, like primitive horses, but the side toes are smaller.
They are "horse-faced," or long-headed with the eye socket well back
from the middle of the skull.
Fossils of Parahippus are found at many early Miocene
localities in the Great Plains and Florida. Species in this genus
lived from 24-17 million years ago.
The watershed in the development of the horse occurred in the
Miocene period, about 26 million years ago, when he moved out of the
forests and swamps and onto the plains. As he adapted to changing
conditions, his neck and head became longer, the incisors moved
forward in the skull and the form and position of the eyes altered
to allow the horse to view the horizon while grazing. His legs
became longer, giving him speed to escape from predators. These
horses, Parahippus and Merychippus, stood firmly on a single toe
with semi-functional side toes, and were about 10.5 hands (42")
high. |
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Merychippus |
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Merychippus
was a
milestone in horse evolution. Though it retained the primitive
character of 3 toes, it looked like a modern horse. It had
high-crowned cheek teeth, making it the first known grazing horse
and the ancestor of all later horse lineages.
Fossils have been found
throughout the United States. The species in this genus lived from
17-11 million years ago. |
In the middle of the Miocene epoch,
an animal called Merychippus was alive. Merychippus
had wider molars than its predecessors, which are believed to have
been used for crunching the hard grasses of the steppes. The hind
legs, which were relatively short, had side toes equipped with small
hooves, but they probably only touched the ground when running
Merychippus
represents a milestone in the evolution of horses. Though it
retained the primitive character of 3 toes, it looked like a modern
horse. Merychippus had a long face. Its long legs allowed
it to escape from predators and migrate long distances to feed. It
had high-crowned cheek teeth, making it the first known grazing
horse and the ancestor of all later horse lineages.
Fossils of Merychippus
are found at many late Miocene localities throughout the United
States. Species in this genus lived from 17-11 million years ago. |
Hipparion
Three new
equids are believed to be descended from the numerous varieties of
Merychippus: Hipparion, Protohippus and
Pliohippus. The most different from Merychippus was
Hipparion. The main difference was in the structure of tooth
enamel: in comparison with other equids, the inside, or tongue side,
had a completely isolated parapet. A complete and well-preserved
skeleton of the North American Hipparion shows an animal the
size of a small pony. They were very slim, rather like antelopes,
and were adapted to life on dry prairies. On its slim legs,
Hipparion had three toes equipped with small hooves, but the
side toes did not touch the ground.
An American form of Hipparion, also known as
Neohipparion, proliferated into many kinds of equids several of
which managed to migrate to Asia and Europe during the Pliocene
epoch. (The European Hipperia differs from the American
Hipparion in the smaller body size – the best-known discovery
of these fossils was near Athens.) Recent research suggests
that Hipparion is an ancestor of the zebra and the donkey,
rather than the horse. |
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Pliohippus,
This was the first truly single-hoofed horse-like
animal evolved about seven million years ago. The side toes
became the splint bones found in modern horses. For many years, it
was believed to be the "grandfather" of the horse, but recent
studies indicate that it was from a parallel line that died out.
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Pliohippus
Pliohippus arose from
Calippus in the middle Miocene, around 15 million years ago. It
was very similar in appearance to Equus, though it had two
long extra toes on both sides of the hoof, externally barely visible
as callused stubs. The long and slim limbs of Pliohippus
reveal a quick-footed steppe animal.
Until recently, Pliohippus was believed to be the
ancestor of present-day horses because of its many anatomical
similarities. However, though Pliohippus was clearly a close
relative of Equus, its skull had deep facial fossae,
whereas Equus had no fossae at all. Additionally, its
teeth was strongly curved, unlike the very straight teeth of modern
horses. Consequently, it is unlikely to be the ancestor of the
modern horse; instead, it is a likely candidate for the ancestor of
Astrohippus.
"Grandfather" to the modern horse, Pliohippus appears
to be the source of the latest radiation in the horse family. It is
believed to have given rise to Hippidion and Onohippidion,
genera that thrived for a time in South American, and to
Dinohippus which in turn led to Equus. Fossils of
Pliohippus are found at many late Miocene localities in
Colorado, the Great Plains of the US (Nebraska and the Dakotas) and
Canada. Species in this genus lived from 12-6 million years ago.
The first truly
single-hoofed horse was Pliohippus, which evolved about seven
million years ago in the Pliocene period. The side toes became the
splint bones found in modern horses. This small, lightly built horse
was the prototype for the Equus caballus, the first true horse,
which evolved during the Pleistocene period, almost two million
years ago. Equus had a rigid spine, with short, powerful and
well-muscled bones in the upper limbs and long, slender unmuscled
lower limbs. He was well equipped for life on the open plain and had
a well developed defense system. The foot pad of earlier evolutions
became the frog of modern horses. |
Dinohippus
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Dinohippus was the most common horse in North America during
the late Pliocene. It was originally thought that Dinohippus was a
monodactyl horse, but a 1981 fossil find in Nebraska shows that some
were tridactyl.
Dinohippus is believed to be the closest
relative to Equus, the genus that includes the living horses,
asses and zebras. Dinohippus fossils are found in the Upper
Miocene of North America and date from 13 - 5 million years ago. |
Plesippus
Plesippus is often considered
an intermediary stage between Dinohippus and the present day
horse, Equus. At the end of the Pliocene, the
climate in North America began to cool down significantly and the
animals were forced to move south. One group of the Plesippus
species escaped to South America, and the other moved across the
land bridge around the Bering Strait into Asia and Europe. A portion
also remained in the southern part of North America. The Ice Age
spread five times over Europe and North America and five times again
receded; it is estimated that approximately one million years
elapsed from the Ice Age (the Quaternary period) to our era.
In South America a form named
Hippidium developed from Plesippus. Hippidium was
relatively short-legged with a deeply recessed nasal notch, very
thin and delicate nasals, and long ectoflexids in the lower
premolars. It continued to live on the South American pampas for a
long time, but eventually died out. |
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Equus
These are the horse-like animals, sometimes called the horse
family. It includes horses, the donkey, the four zebra and the
onager. They first appear in the fossil record 4
million years ago. Some of these animals crossed the Bering
land bridge into Asia. Their descendents migrated to Europe.
The horse became extinct in North America about 11,000 years ago.
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Equus
The oldest species of "true" horse,
Equus stenonis, was discovered in Italy, and is believed to
have evolved from Plesippus-like animals at the end of the
Tertiary or beginning of the Quaternary periods. Equus stenonis
proliferated into two branches, one lighter in body mass and one
heavier.
Equus stenonis
crossed into North America, where similar forms known as Equus
scotti are common; some types (Equus scotti var.
giganteus) exceeded the modern horse in size. However, all the
horses in North America ultimately became extinct, approximately
11,000 years ago, perhaps due to climate change or some pandemic. It
has also been suggested that humans hunted horses to extinction, as
the appearance of humans in the Americas occurred at about the same
time as the extinction of most large mammals in the Americas.
However, there are no known kill sites of Pleistocene horses in
North America, and so this scenario remains unsupported.
Recent studies by a team of
geneticists headed by C. Vila indicate that the horse line split
from the zebra/donkey line between 4 and 2 million years ago.
Equus ferus, ancestor species to Equus caballus, appeared
630,000 to 320,000 years bp. Equus caballus was formed from
several subspecies of Equus ferus by selective breeding
widely over Eurasia for an extended time. The details of this
process are currently a target of research by archaeologists and
geneticists.
The Spanish began to import Iberian
horses to breeding ranches on Cuba, Haiti, and other large islands
offshore of the Americas beginning with Columbus' second voyage in
1493. The first Conquistador horses to land on the main continent
were most likely spare stallions and expendable (infertile) mares
from these island ranches. Later, as Spanish missions were founded
on the main land, horses and cattle would eventually be lost, and
would proliferate into large feral herds.
At the end of the 15th century, when
the first Europeans came to America, no native horses were reported
observed by them in the regions where they landed, or as they began
to explore the interior. The natives of post-Colombian Mexico and
Peru did not have a specific name for the animal, calling it in
their language a "sun dog" or "deer". In one incident, a lame horse
was left behind by a Spanish expedition to be cared for by a local
tribe. It was reported that the indigenous people of that area
attempted to feed it meat, and were surprised that it preferred to
graze on vegetation instead. For these reasons, it has long been
assumed that no one, anywhere, on the continent had ever seen a
horse before. Though, to this day, some Native Americans (in
particular those of the northwestern United States, and southwestern
Canada) by way of their oral traditions, disagree.
Equus spread across the
Bering Strait from America to Asia. Primitive man, starting to
evolve in Asia, followed horse herds back across the Bering Strait
into America, some staying to become the first Americans. When the
glaciers retreated about ten thousand years ago, the land bridges
between what is now Alaska and Asia disappeared. Soon after that the
horse became extinct in North America. No one knows why. They were
later re-introduced to the continent by Spanish explorers, and
became the progenitors of the Mustang. |
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