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.[[120]] 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.[[121]] 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."[[122]]
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.