In the lower Eocene (Wasatch stage) lived a tiny creature, †Trigonolestes (family †Trigonolestidæ), smaller even than †Homacodon of the Bridger, and one of the most ancient and primitive of known artiodactyls, but, unfortunately, still represented only by very imperfect specimens, so that much which it would be highly desirable to learn must await the finding of better material. The upper molars were triangular and tritubercular, i.e. with three principal cusps arranged in a triangle, and are hardly to be distinguished from those of other early mammalian orders. From the teeth alone the artiodactyl nature of the animal would not have been suspected, and, in fact, they were, when first discovered, referred to primitive monkeys. The feet probably had five toes each, but this is not certain, and the femur had the third trochanter, the only known artiodactyl of which this is true. As this little Wasatch genus is so imperfectly known, it would be premature to claim it as the starting point of the camel family, and yet it may very well have been so. Better material of this genus and the links of the chain which belong in the upper Bridger and the Wind River respectively must be recovered before this earliest portion of the family history can be written in more than tentative fashion.
Fig. 213.—Right manus of camels. A, †Protylopus, Uinta. B, †Poëbrotherium, White River. C, †Procamelus, upper Miocene. (After Cope.) D, Recent Guanaco.
Fig. 214.—Right pes of camels. A, †Protylopus. B, †Poëbrotherium. C, †Procamelus. (After Cope.) D, Guanaco.
The mode of evolution displayed by the camels does not differ in any significant respect from that seen in the horses. There was the same increase in bodily stature and in the relative lengths of the limbs and feet, the same kind of diminution in the number of digits from the original five, the same reduction of the ulna and its coalescence with the radius and the loss of the fibula save for its two ends. There was also a similar development of the high-crowned, or hypsodont, grinding teeth, from the low-crowned, or brachyodont, type. In still another respect there was a similarity in the mode of development of the two families, namely, in the way in which the several phyla of each originated. For the earlier portion of their history there was in each but a single distinguishable series, though it is very possible that fuller knowledge and more complete material would enable us to distinguish more than one. This monophyletic condition continued through the Eocene and most of the Oligocene, but in the upper portion of the latter and, more markedly in the lower Miocene, the two families branched out, each in its own fashion.
Of course, there were differences in the development of the camels and horses, some conditioned by the fundamental distinction between artiodactyl and perissodactyl, such as the didactyl foot as the possible minimum and the formation of cannon-bones in the camels. Other differences are characteristic of the latter family, such as the great elongation of the neck and the peculiar structure of its vertebræ, the formation of pads on the feet and concomitant reduction of the hoofs. In a general way, the two families kept quite an even pace in their advance from the more primitive to the more specialized condition and, though the camels were the first to acquire certain modifications, the horses ultimately surpassed them.
Even more close was the parallelism in evolution between the camels and the true ruminants (suborder Pecora), and this case is of particular importance as clearly demonstrating the development, in two independent but related lines, of similar structures not derived from a common ancestry. This comparison must naturally await the description of the Pecora.
7. †Hypertragulidæ. †Hypertragulids
This was a very peculiar family, of exclusively North American distribution and of doubtful systematic position, the known history of which extended from the upper Eocene into the lowest Miocene and then abruptly terminated. None of its members attained to considerable size, the largest hardly surpassing a sheep, and some were extremely small. In view of its comparatively brief career, this family was surprisingly ramified, and no less than four phyla may be distinguished within its limits.