The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

of neck a necessity ; without that the horse could not reach down to its food.

The grinding teeth of a horse are beautifully and elaborately adapted for dealing with the tough grasses that he consumes. They are peculiar in three ways. First, they are all alike, instead of the true molars being more complex than the premolars as is usual. The premolars, we may remark in passing, are those grinding teeth which have predecessors in the milk dentition; the molars have not. Secondly, these quite similar molars and premolars of the horse all have an extremely complicated surface pattern. Before they cut the jaw-bone they are covered with hard, glossy enamel, rising up in ridges round a couple of deep cavities.

Merychippus Mid Miocene

‘Eohippus Orohippus Mesohippus Lower Eocene Mid Eocene Mid Oligocene

a |

Fig. 124. Tooth evolution in horses. During the Cenozoic, the teeth of the horse stock become progressively bigger, especially in height, and their grinding surface becomes more complicated. A series of teeth from upper jaws is here shown to scale, above in surface view, below in side view. Left to right: Eohippus ; Orohippus ; Mesohippus ; Merychippus ; Pliohippus ;

Modern Horse.

These cavities later get filled up with cement, a substance not quite so hard as enamel, which is secreted by special glands as the tooth breaks through the gum. Under the enamel is the softer dentine. Use soon grinds off the top of the teeth, with the result that their tops become nearly flat ; but as the three materials wear down at different rates, sharp edges of enamel stand up a little beyond the cement, and a little higher above the dentine. ‘The whole forms a remarkably effective miniature millstone, with the advantage over our millstones that it keeps its grinding-ridges sharp as it is worn down (Fig. 124).

The third point about the teeth is that they are of remarkable depth, and that during the first eight years of life they have no closed

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THE SCIENCE OF LIFE

Pliohippus Upper Miocene

GHAPTER 2

roots, but go on growing from below, like a rabbit’s front teeth, as they are worn away above. After this roots are formed, new growth ceases, and the teeth are simply pushed up to compensate for the wear at the surface till they are all worn away, and the animal dies because it cannot chew its food. Our own teeth, and most mammalian teeth, are finished and complete as soon as they have erupted, but in a horse completion is delayed for eight years to give a longer working life.

The horse’s teeth, then, are admirably adapted for chewing up grasses; and the size and peculiar shape of his head is due to the need for finding room for these powerful and deep-rooted living millstones, and for the muscles to work them.

Now, when we look back to the fossils of the Eocene (V A), the earliest period of the Cenozoic, we find nothing resem-

Equus caballus Recent

bling a horse. We find no mammal so specialized as a horse. In the earliest Eocene, almost

all the mammals were small, they all had either four or five toes to each foot, and their teeth were short, low-crowned, and provided with more or less rounded or conical cusps, never with grinding ridges. Obvious carnivores with teeth for slicing and cutting, like those of lion or wolf, did not exist, nor obvious herbivores, with grinding and chewing teeth like those of cow or elephant. In the later stages of the Eocene Period, definite carnivores and herbivores can be recognized, together with other well-marked types; but these early forms are all extremely different from any living animals. When Owen in 1856 described Hyracotherium—for that is the first name in our history—he never even guessed at the relationship between it and the horse ; now not only do we know that the one is ancestral to the other, but we can fill up all the gaps between the two.

If we go back stage by stage through the rocks of the whole Cenozoic Period (V), we find that the horse has recorded its pedigree in fossils. There are four main stages. In the last, the fossil horses resemble the living forms in all essentials of teeth and feet, differing