Fig. 248.—Skull of †Diadiaphorus, Santa Cruz. American Museum.
The limbs were slender and of moderate length; there was no coössification between the bones of the fore-arm or the lower leg. The feet were three-toed, except in one genus (†Thoatherium) in which they were single-toed, and nearly or quite the whole weight was carried upon the median digit, the laterals being mere dew-claws. The shape of the hoofs and the whole appearance of the foot were surprisingly like those of the three-toed horses, but there were certain structural differences of such great importance as, in my judgment, to forbid the reference of these animals, not merely to the horses, but even to the perissodactyls. In studying the †Litopterna, one is continually surprised to note the persistence of archaic and primitive characters in association with a high degree of specialization.
Fig. 249.—Three-toed †proterothere (†Diadiaphorus majusculus), Santa Cruz. Restored by C. Knight, from skeletons in the American Museum and Princeton University.
The largest Santa Cruz representatives of the family were the species of †Diadiaphorus, animals considerably taller than a sheep and of heavier build. Their appearance was not unlike that of a short-necked, hornless antelope, but with the feet of the three-toed horses! These feet were, however, merely superficially like those of the horses, differing in points of fundamental significance. In the horses, the reduction of the digits was accompanied by a readjustment of the carpal and tarsal articulations, so that, in proportion as the median toe was enlarged and the laterals reduced, the weight was shifted more entirely upon the former. This is the method of digital reduction which Kowalevsky called “adaptive” and is exemplified in all existing artiodactyls and perissodactyls and by none more perfectly than by the monodactyl horses. In “inadaptive reduction,” the method followed by †Diadiaphorus and the other genera of this family, there was no readjustment, or a very imperfect one, of the articulations, the lateral digits, however small and rudimentary, retaining the connections which they had when they were of full size and function. This distinction may seem to be unimportant, but its significance is shown by the fact that not a single ungulate with inadaptively reduced feet has survived to the present time.
