Sub-Order 2. Amblypoda

This group of extinct mammals has the following principal characteristics:—

They are large, semiplantigrade Ungulates, of heavy build and apparently elephantine gait. The dentition is for the most part complete as in other ancient groups, and the canines are in the later forms big tusks. The back teeth are brachyodont and ridged (lophodont). Both radius and ulna in the fore-limb, and tibia and fibula in the hind-limb, are well developed. The bones in the carpus are alternating in position. The toes are five in both feet, and are very short. There is a hint of commencing "perissodactylism" in the fore-feet at any rate. The brain is small and the hemispheres smooth.

The Amblypoda, or Amblydactyla, are so called on account of their short and stumpy feet and toes. They were held by Professor Cope to be on the direct line of ancestry of both Perissodactyles and Artiodactyles, a view which is on the whole not accepted at present.

Fig. 114.—Skull of Protolambda bathmodon. × ¾. e.a.m, External auditory meatus; m, mastoid; m.f, mastoid foramen. (After Osborn.)

As is the case with other groups, the Amblypoda commenced existence as a sub-order with relatively small forms such as Pantolambda, the most ancient type known, which is in many respects a transition between the later forms and other groups of mammals such as the Creodonta. The race culminated and ended in the giant Dinoceras and Coryphodon, and spread into the Old World. In spite of their smooth and diminutive brain, these mammals were able to hold their own and to multiply into many species and genera; in this they were perhaps aided by their formidable tusks and by the horns which many of them possessed. The teeth seem to imply an omnivorous diet, which was quite possibly an additional advantage in the struggle for existence. It does not seem to be necessary to divide off the Dinoceratidae into a sub-order equivalent to the Coryphodontidae as was done by Professor Marsh; the numerous points in common possessed by the members of both families forbid their separation more widely than as families.

The earliest types of Amblypoda belong to the genus Pantolambda, of which the species P. bathmodon was about four feet in length. As restored it seems to have had proportionately short fore- and hind-limbs, and it had a long tail. It was apparently plantigrade, and would have had not a little likeness to a carnivorous type. The skull has no air cavities, such as are developed in the later types from the Lower Eocene, e.g. Coryphodon; Pantolambda is from the basal Eocene. The frontal bones show no trace of the horns that are developed in subsequent forms; the nasals are comparatively long; the zygomatic arch is slender. The molar teeth are in the primitive form of trituberculy, and the premolars, as is so often the case with primitive animals, are unlike the molars in form, being less markedly selenodont. As to the vertebral column, the dorsal vertebrae appear to have had short spines, which argues, as it does also in the case of the larger and heavier Coryphodon, a feebleness in the development of ligaments and muscles supporting and moving the head. The scapula seems to have the same peculiar leaf-like form that it has in the later Coryphodon. This primitive type shows an entepicondylar foramen in the humerus. It is interesting to observe that the posterior border of the ulna is convex, as in the Creodonts, and in the early Condylarthrous form Euprotogonia. In the subsequently-developed Amblypoda, as in the later Condylarthra, that bone acquires a concave outer border. In the carpus the os centrale is distinct. In the femur the third trochanter is well formed; it gradually dies out in later Amblypoda. The fibula articulates with the calcaneum. This species, according to Osborn, "typifies the hypothetical Protungulate, being more primitive than either Euprotogonia or Phenacodus."

Fig. 115.—Skeleton of Coryphodon radians. × 1⁄10. (After Osborn.)

The genus Coryphodon is known by a large number of species, of which the first was discovered in this country, and was represented merely by a jaw with some teeth. This was named by Sir R. Owen C. eocaenus, and was dredged up from the bottom of the sea off the Essex coast. A second specimen consisted of a single canine tooth only, and was brought up from a depth of 160 feet during the making of a well at Camberwell. More abundant remains have since been found in North America.

This genus had a large head, and in some specimens traces of the "horn cores," so marked in the related Dinoceras, are to be noticed. The skull is broad behind and narrowed in front; the roofing bones show the cellular spaces so characteristic of the Elephant. The jugal bone, however, is not, as it is in the Elephant, placed in the middle of the somewhat massive zygomatic arch. As in some other primitive Ungulates (e.g. Phenacodus) there are twenty dorso-lumbar vertebrae, of which fifteen bore ribs.

The scapula seems to have possessed a peculiar leaf-like form, swelling in the middle and ending almost in a point above. It has a well-marked spine, and the acromion projects much. The fore- as well as the hind-feet are in a state of transition between plantigradism and digitigradism. It was at one time held that the animal was digitigrade as to the fore-feet and plantigrade as to the hind-feet. Though, as has been pointed out, it is a fact that the hind-feet are often on a different plane of evolution from the fore-feet, it seems that this amount of difference does not characterise any Ungulate, not excepting the genus now under consideration.

The toes are very spreading. The pelvic girdle is of great strength and broadness. The femur, as in the Perissodactyles, has a well-developed third trochanter; but whereas in this particular the hind-limb is Perissodactyle, it is Artiodactyle in the fact that the tibia and the fibula articulate with the astragalus and calcaneum. The ridged teeth have given the name to the genus.

A curious feature in the structure of the genus are the slender spines of the dorsal vertebrae, which contrast with the enormous ones of some other Ungulates—more curious in this genus, which is of heavy build, than in the lighter Pantolambda. The back of the animal is short, and the limbs are very spreading, so that the gait was doubtless shuffling. The large head, and short and heavy limbs and limb girdles added probably to its cumbrous walk or trot. The canines are great tusks, and spread out on both sides of the mouth.

The late Professor Cope, in 1874, described the probable appearance of the Coryphodon in the following words:—"The general appearance of the Coryphodons, as determined by the skeleton probably resembled the bears more than any living animals, with the important exception that in their feet they were much like the elephant. To the general proportions of the bears must be added the tail of medium length. Whether they were covered with hair or not is of course uncertain. Of their nearest living allies, the elephants, some were hairy and others naked.... The movements of the Coryphodons doubtless resembled those of the elephant in its shuffling and ambling gait, and may have been even more awkward from the inflexibility of the ankle."

The most recent members of this sub-order come from the Middle Eocene beds, and are chiefly referable to the genus Dinoceras, with which Tinoceras and Uintatherium are at least very nearly related, if not identical. These creatures were of great size, larger than the earlier types which have been considered. They show a certain superficial resemblance to the Titanotheriidae, on account of the massive horn cores upon the skull. These horn cores are large upon the maxillae and the parietals, and are paired; on the nasals are smaller horns. The bones of the skull have air cavities. The incisors of the upper jaw are absent; the canines are enormous tusks, and the lower jaws are flanged downwards near the symphysis where these tusks border them. Contrary to what is found in the older types, where the position of the condyle of the lower jaw is normal, this prominence faces backwards in the Dinocerata. The same shortness of the spines of the dorsal vertebrae prevails in this group as in the other Amblypoda, though it is perhaps hardly so marked. The scapula has not the peculiar acuminate form that exists in Coryphodon, but is triangular and broad above. The limbs are elephantine, in that the angle between the humerus and the femur respectively, and the bones which follow, is not marked. The hind-limbs are especially straight. The tail is short as compared with that of the primitive Amblypoda. The Dinocerata are purely digitigrade. The entepicondylar foramen has, as in the Coryphodonts, disappeared. The os centrale of the carpus has become fused, and no longer exists as a separate bone. The fibula no longer articulates with the calcaneum, but both that bone and the ulna are well developed. The genus Astrapotherium is placed among the Amblypoda by some authorities.