The past history of horses is well known from an abundance of fossil
material, ranging in age from the Eocene down to the present. Modern
horses have only one toe in each foot, but there are remnants of two
additional toes which may be seen only in the bony structure underlying
the skin. Most of their ancestral relatives were three-toed as far back
as the Oligocene period. During Eocene time, however, there was a stage
which may be regarded as four-toed although it was evidently a temporary
condition, linking known horses with more remote forms having five toes.
[Illustration: Oligocene Mammals From Weld County, Colorado
The giant pigs (_Archaeotherium mortoni_) at the left of the group,
and the rhinoceros (_Trigonias osborni_) were common animals of the
western plains region at one time.]
_Eohippus_, the “dawn horse” as it has been called, is one of the oldest
and best known of the American horses. Its relation to existing members
of the family can be traced by means of changes in tooth structure as
well as in the gradual reduction in the number of toes that is seen
among intermediate forms. Its ancestors some day may be positively
identified in that group of generalized, primitive, five-toed, hoofed
mammals which are known to have lived at the beginning of the mammalian
era, but such identification has not yet been established. Even
_Eohippus_ bore little resemblance to the familiar horse of today. Its
height was only eleven inches, and in body form it had much of the
appearance of a modern dog. There were four toes on the front foot, one
of them decidedly shorter than the others but complete in all its parts,
and evidently capable of service in carrying a portion of the animal’s
weight. The hind foot had three complete toes and a tiny remnant of a
fourth which could not have been apparent externally.
As changes in the structure of the feet progressed, the central toe of
the original five continued to increase in size while the adjacent
digits became relatively shorter and eventually so reduced in length
that they could touch the ground no longer. The smaller bones at the
extremities, corresponding to the joints of our fingers and toes,
eventually disappeared from the side toes. Then the longer bones of the
outer digits lost the broadened supporting surface, where the missing
toes had been attached, and became reduced to pointed remnants known as
splints. Extreme shortening of the splint bones eventually leaves only a
small knob which is often referred to as a rudimentary toe. In the
skeleton of a large horse the splints are readily seen, but in some of
the earlier species they are so small that they may easily be destroyed
or overlooked by the collector who removes the fossilized material from
the surrounding rocks. Even then, the bones of the wrist and ankle may
indicate in an unmistakable manner that an additional toe once was
present, for each bone is supported by another, and at the point of
attachment there is a characteristic surface whose purpose is usually
obvious.
Throughout the Cenozoic era the changes continued. Among the horses of
the North American Oligocene were _Mesohippus_, approximately the size
of a collie dog, and _Miohippus_ which was slightly larger. Both were
three-toed, but the rudimentary splint of a fourth toe was still present
in the front foot. _Parahippus_ and _Merychippus_ carried on during the
Miocene period, the latter being characteristic of the time, and
showing, in addition to other progress, a decided trend toward the
modern structure of molar teeth. There was some increase in size but the
largest horse of that period was hardly more than a small pony.
[Illustration: A Pleistocene Horse of the Texas Plains (_Equus
scotti_)]
_Hipparion_ and _Protohippus_, living during Upper Miocene and Pliocene
time, represent later stages of the three-toed condition. The side toes
were completely formed but greatly shortened, only the central toe
touching the ground. In some of the species the outer toes had also
become very slender, approaching the splint condition. By this time the
molar teeth were longer and better adapted for feeding on grasses which
were becoming sufficiently abundant to attract some of the forest
dwellers into the open country.
During the Pliocene period, in the genus _Pliohippus_ and also in
_Hipparion_, the feet were far advanced in structure, with most of the
species single-toed, the side digits having reached the splint stage.
Pleistocene horses of the genus _Equus_, like living species of that
genus, were strictly one-toed animals, ranging over grassy areas and
highly specialized for a life in that kind of environment.
Specialization is to be noted partly in the foot and leg structures
where the modifications have contributed to greater speed and travelling
ability. This is of great service to an animal of the plains where food
and water are often scarce, and great distances frequently have to be
covered in order to obtain sustenance. The horse, as we know it, is
built for speed, its limbs and feet being elongated to permit a greater
stride, and also modified to decrease the weight without loss of
strength. The ordinary ball-and-socket joint is replaced by a
pulley-like construction which limits the direction of movement but
provides an excellent mechanism for locomotion, especially over flat,
open ground. Flexibility in other directions is sacrificed for greater
strength, and the foot incidentally becomes less suited for other
purposes.
This is what is meant by “specialization”—a departure from
“generalization.” The study of fossils provides numerous illustrations
of specialized development which contributes greatly to an interest in
prehistoric life. Any specialized structure or habit which increases
fitness for a particular way of living is also known as an “adaptation.”
Quite in line with the idea of specialization and adaptation is the
change which occurred in the construction of the horses’ teeth, for the
dental equipment of the modern grazing animals differs widely from that
of the browsing creatures which lived on the soft leaves and other plant
substances of the forests.
[Illustration: The Structure of Molar Teeth
The large lower molar of a long-jawed mastodon shows worn and unworn
cusps, with the enamel layer forming a heavy border around the
central dentine where the surface covering has been worn through. In
the grinding teeth of rhinoceroses (illustrated at the right) the
crown pattern is quite different, but both types are adapted for
softer foods and are similar in having the protective enamel on the
outside only. The central tooth shows the condition after the
shallow surface depressions have been removed by wear.]
The cheek teeth or grinding equipment of the horses underwent as
complete a change as the feet. Modification resulted in a new type of
tooth which enabled herbivorous animals to take advantage of a kind of
vegetation which was late in arriving and has since become the principal
diet of the ungulates. The grasses are coarse and harsh as compared with
the leaves of forest shrubbery, requiring more thorough grinding to make
them digestible. In addition they contain minute particles of silica,
which is a highly abrasive mineral that quickly wears down the tooth
substance, especially the softer materials found in tooth construction.
An increase in the length of the tooth would offset the excessive wear
but would not necessarily produce a better mechanism for grinding.
The fulfillment of the new requirements is to be seen in the change from
what is known as the low-crowned, browsing type of molar, to the
high-crowned, grazing type. Details of the changes that may be traced
through millions of years of gradual adjustment become apparent only
from the examination of a great deal of fossil material. As compared
with earlier types of construction, a modern molar tooth may appear
extremely complicated, but the process which brought about the improved
quality is very simple. A little discussion of tooth structure, however,
is required to make this clear.
A tooth, as everyone knows, is partly imbedded in the jaw, partly
exposed outside the gum. In a short-crowned tooth the exposed portion is
known as the crown, and the part imbedded in the jaw consists of one or
more roots which are comparatively long. The crown is nearly always
protected by a thin layer of hard enamel. In a grinding tooth, the
working surface has a number of more or less prominent elevations known
as cusps. The enamel layer completely covers this surface until wear
begins. As the tooth goes into service the signs of use begin to appear;
the enamel is soon worn from the tops of the cusps, and the underlying
substance, called dentine, becomes exposed. This is far less resistant
to wear, and as the enamel continues to be reduced the tooth becomes
less efficient as a grinding device, partly because of the smoothing off
of the surface, partly because of the relative softness of the inner
material which is being exposed in increasing quantity. A very old molar
tooth of the low-crowned type has a smooth surface from which almost the
last trace of the enamel has been removed. In many prehistoric animals
the enamel is of a darker color than the dentine or cement, this
difference in color enabling one to see at a glance how the teeth are
constructed.
[Illustration: Grazing Type of Molar Teeth
The side view of the bison’s molar and premolar equipment
illustrates the elongated construction which is common among grazing
animals. In the pattern of the grinding surface may be seen a cross
section of the enamel layers. One layer surrounds each tooth while
two folded “cylinders” of the same material occupy the interior.]
In a long-crowned tooth the roots are usually very short, for much of
the crown itself is imbedded in jaw bone, and the longer roots are not
required. Growth of the tooth is usually completed after a few years;
then as it is gradually worn away it is continuously moved upward by the
production of new bone under the roots, which slowly fills the bottom of
the socket and continues to provide the necessary support. An equally
important difference between the two types of teeth, however, is to be
seen in the arrangement of the enamel, the long-crowned type being
provided with this durable substance on the inside of the crown instead
of having a mere protective cap on the outside.
The more complicated structure was developed from the simpler form by
the easy method of deepening certain depressions located between cusps
at the top of the tooth. As the crown of the tooth increased its length
these depressions remained tucked in, and eventually became deep pits
roughly cylindrical in shape. In addition to the enamel and dentine, a
third tooth substance, known as the cement, made its appearance at about
this time, and we find that quantities of this new material were
deposited outside the crown enamel and also inside the enamel walls of
the pit, in this way producing a firmly consolidated structure otherwise
weakened by deep channels and hollow pockets. The cement differs only
slightly from the dentine but is deposited while the uncut tooth is in
the gum tissues of the mouth, the enamel and dentine elements being
formed earlier in the embryonic tooth before it emerges from the jaw
bone.
A tooth constructed by such a process, if cross-sectioned through the
crown, will be found to consist of successive layers of hard and softer
materials. In living animals the top of the tooth soon wears off and the
enamel layers stand in higher relief because of their greater resistance
to wear. A roughened surface of excellent grinding quality is thus
provided, and as long as the wear continues there remains the same
relative amount of enamel to retain the roughness, and resist abrasion.
Among the various types of grazing animals there is a marked difference
in the arrangement and form of the enamel layers. Within a species of
genus, however, the complicated enamel patterns of the molar teeth are
consistently similar. In the case of horses especially, these patterns
provide a most helpful key to the identification of extinct forms. The
general pattern, in any of the more modern horses, may be understood
more readily if the wavy enamel layers be regarded as forming a set of
cylinders with deeply crinkled walls. Near the outer border of the
tooth, surrounded by a thin layer of cement, is the enclosing cylinder
which represents the enamel cap of the old-fashioned, low-crowned tooth.
Inside of this is the central mass of dentine which has been penetrated
by two of the deep pits previously mentioned. The original enamel cap
has been depressed into these pits, forming two inner cylinders which
are filled with cement. Instead of being circular in outline, when the
cap is worn through at the grinding surface these inner cylinder walls
are seen to be wrinkled and folded so as to produce a most irregular
pattern. However, if several teeth of the same kind of horse are
compared, it will be found that the edges of these cylinders produce
figures which are remarkably uniform and characteristic for that
species.
[Illustration: American Mastodon (_Mastodon americanus_)
A true mastodon of the short-jawed type.]