It is interesting to note that the existing Perissodactyles and Artiodactyles are to be distinguished by their unpaired or paired horns. But while there are no Artiodactyles with unpaired horns (save occasional sports) the Perissodactyles have more than once tried, so to speak, paired horns, which ultimately proved fatal to them. The Rhinoceros Diceratherium apparently inherited and improved upon the small paired horns of Aceratherium, but it has left no descendant. The paired horned Titanotheria offer another instance of the same apparent incompatibility between the Perissodactyle structure and the persistence of paired horns.
Sub-Order 1. CONDYLARTHRA.
This group is characterised by the following assemblage of characters. Extinct, often plantigrade Ungulates, with five-toed limbs. Bones of carpus and tarsus not always interlocking, but sometimes lying above each other in corresponding positions. The humerus has an entepicondylar foramen. Dental formula quite complete; the molars brachyodont and bunodont. The premolars are simpler than the molars. The canines are small. As with other early types, the zygapophyses are flat and do not interlock. The astragalus is like that of the Creodonta. This group was American and European in range, the remains of its rather numerous genera being of Eocene time. The best-known genus is Phenacodus, of which some account will be given before discussing the, in many cases, more fragmentary remains of other allied forms.
The genus Phenacodus was first described so long ago as 1872, from a few scattered teeth. Since then several nearly complete skeletons have been obtained, and we are in full possession of
the details of its osteology. It was not a large creature (see Fig. 110, p. [196]), about 6 feet in length, with a small head. The feet were more or less plantigrade, and five-toed. The last phalanges of the toes show that they carried hoofs and not claws; yet the fore-feet look a little as if they could be used as grasping organs. The third digit of both hind- and fore-feet exceeds the others, and thus a Perissodactyle-like foot characterised this Eocene creature. The tail is exceedingly long, and must have reached the ground as the animal walked. This is of course by no means an Ungulate character. Still, in the totality of its organisation the animal was decidedly Ungulate, though Professor Cope spoke of Phenacodus as not merely an ancestral Ungulate but as the parent form of Insectivores, Carnivores, Lemurs, Monkeys, and Man himself! The scapula indeed is from its breadth and oval contour rather like that of a Carnivore. The clavicles as in other Ungulates are absent. The femur is Perissodactyle rather than Artiodactyle in the presence of a third trochanter. The creature had fifteen pairs of ribs and five or six lumbar vertebrae. The two bones of the leg which lie below the femur are perfectly distinct and separate. A cast of the brain-case shows that the cerebral hemispheres were smooth and small, the cerebellum of course completely uncovered and nearly as large as the cerebrum. The olfactory lobes were also large. The complete skeleton of Phenacodus has lately been excavated more fully from the enveloping matrix by Professor Osborn,[[117]] and mounted in what is regarded as the natural position of the beast. It appears that though five-toed it went upon the three middle toes only, and furthermore that of these the middle one was the more prevailing, so that Phenacodus was distinctly "Perissodactyle," at least in habit. Moreover its "long hind-quarters, the long powerful tail ... are reminiscent of Creodont ancestry." The genus was European and American in range.
Meniscotherium ( = Hyracops[[118]]) comprises several forms of about the size of a fox; they are both European and American in range. The teeth are more distinctly Ungulate in form than those of Phenacodus, with a
-shaped outer wall. The skull is described as possessing "indifferent, primitive characters," permitting a comparison with those of Opossums, Insectivores, and
Creodonta. It has, as in Phenacodus, no orbital ring. The humerus resembles that of a Carnivore rather than that of an Ungulate. The carpus and tarsus are serial. The fibula articulates with both the calcaneum and the astragalus, which is not the case with Phenacodus. It is suggested that these animals are ancestral forms of the Chalicotheres. In the brain the hemispheres do not cover the cerebellum.
More primitive apparently than Phenacodus was the less-known genus Euprotogonia, or Protogonia[[119]] as it has been called. The best-known species is E. puercensis, so called from its occurrence in the Puerco beds of the American Eocene. It was a slender, long-limbed creature, smaller than Phenacodus, with a long and heavy tail as in that animal. Like Phenacodus it was semiplantigrade, and shows more likenesses to the Creodonta. The skull is only known by a part of the lower jaw with teeth, and by the teeth of the upper jaw. The vertebrae are not entirely preserved, but enough remain to show that the animal had a tail of 16 or 17 inches, which is a considerable length when compared to its height, about a foot at the rump. In the fore-limb the most noteworthy point is that the ulna has a convex posterior border as in the Creodonts, the same border in Phenacodus being concave. The humerus is slender, with less-marked tuberosities. The fifth digit seems to have been less reduced. The phalanges seem to have borne horny sheaths somewhat intermediate between hoofs and claws. The pelvis is described as being, as is also that of Phenacodus, rather like that of the Creodonta. The right hind-limb is known in all its details. It appears that the bones are not serial but interlocking; this, however, on the views with regard to the relations of these two forms of tarsus mentioned on p. [198], does not militate against regarding Euprotogonia as the ancestor of the genus Phenacodus. The third toe is the pre-eminent one, the animal thus being Perissodactyle. The lateral digits are larger than in Phenacodus, and the metatarsals and the phalanges are slightly curved, which is again a Creodont character as compared to the perfectly straight corresponding bones of Phenacodus. It seems evident that this animal is to be looked upon as a more ancient type than Phenacodus, even if not as its actual ancestor.