The Shetland Pony

by Charles Douglas; A. I. Douglas

During the Miocene period the horse passed through the most interesting phases of his evolution, his elaborate dental battery was almost brought to perfection, and the second and fourth toes were gradually dwarfed and hidden out of sight, not even a trace of the hoofs being left, as in sheep, to suggest polydactylous ancestors. As already hinted, though Europe was the birthplace of the remote ancestor of the Equidæ, it is in the Miocene deposits of North America that we have a record of the most important phases of their evolution. The remarkable progress made in Miocene times was doubtless necessary to enable horses and other grass-eaters to keep abreast of the profound changes in the environment—the great increase of prairies in some areas, and the upheavals which resulted in the appearance of extensive mountain ranges in others.

The Oligocene species which proved sufficiently plastic to respond to the new conditions varied in different directions, and gave rise to, amongst other types, one well adapted for a forest life, and one highly specialised for ranging far and wide over boundless prairies.

In Hypohippus we have an example of a “forest” horse, in the American Hipparion (Neohipparion) we have a horse more specialised for a desert life than the fleetest Arab, while in Merychippus we have a link with Oligocene species deserving attention, because, on the one hand, it gave rise through Neohipparion to the Hipparions, now extinct, but once common in Europe and Asia; and because, on the other hand, through Protohippus it seems to be the ancestor of the slender-limbed species of the “desert” or plateau type, now best represented by Celtic and Mexican ponies.

In Merychippus the orbit is complete, and the crowns of the permanent molars are cemented as in recent horses, but the hoof bone has a cleft at the apex (fig. 19), and in some cases there is a minute vestige of the “splint” bone of the fifth or outer digit of the fore-foot. In fig. 11a, the fore-foot of Neohipparion, the Miocene race-horse, is represented. Fig. 19 gives the fore-foot, and fig. 16 is a restoration of Merychippus.

Hitherto Merychippus, through Protohippus and Pliohippus, has been by many regarded as the progenitor of all the modern horses, as well as of the extinct Hipparions. That slender-limbed horses with short-pillared molars are mainly descended from one or more varieties of Merychippus is possible, but it seems to me that modern breeds with short broad cannon-bones and long-pillared molars are probably mainly descended from browsing ancestors with limbs of the Hypohippus type.

Hypohippus, like Eohippus, but unlike all the known Oligocene horses, had in the fore limb, in addition to three complete and functional toes, a distinct vestige of the first metacarpal—i.e., of the bone which in man carries the thumb. No vestige of a first metacarpal has ever been found in slender-limbed breeds, but once and again a vestige of the first metacarpal occurs in coarse-limbed breeds. The vestigial first metacarpal, taken along with other facts, suggests, as already said, that coarse-limbed breeds include a browsing race with limbs of the Hypohippus type amongst their ancestors. A few years ago it was assumed that Hypohippus, the 40-inch forest horse of Dakota and Montana, became “extinct during the Miocene, leaving no descendants.”[F13] Now, however, it is admitted that browsing horses possibly “identical with Hypohippus of the Miocene of America”[F14] lived in China at the beginning of the Pliocene. Though it is inconceivable that a species with the short-crowned teeth of Hypohippus could give rise to any of our modern breeds, it is possible that in the Pliocene of Eastern Asia a race (with Hypohippus-like limbs but long-crowned molars) may be found bearing the same relation to long, low, big-boned modern “forest” horses that Merychippus bears to fine-boned modern plateau or desert horses.

[F13] Lull, loc. cit., p. 177.

[F14] Osborn, ‘Age of Mammals,’ p. 333.

The fore-foot of Hypohippus is represented in fig. 20. When a toe corresponding to the second digit of Hypohippus (II. fig. 20) appears in a modern horse it has sometimes, as in Hypohippus and Eohippus (I. fig. 10), a vestige of the first metacarpal at its upper. Fig. 15 is a restoration of Hypohippus.