A history of land mammals in the western hemisphere

by William Berryman Scott

This third suborder of the †Toxodontia was in some respects the most peculiar of all; no representatives of it have been found in formations later than the Santa Cruz, and the group attained its culmination in the still older Deseado stage, in which there were very large members of it. These most extraordinary beasts are still incompletely known, and little can be done as yet in the way of following out the steps of change which led up to their exceptional characters, though the suborder itself may be traced back to the Eocene by means of jaws and teeth alone.

The Santa Cruz genus labours under the portentous name of †Homalodontotherium, which may be shortened to the vernacular form of †homalodothere. In this genus the dentition was unreduced in number, and the teeth, though having rather high crowns, were all rooted and placed in continuous series, with a gradual transition in shape from the incisors to the molars. The canines were tusks of very moderate size, which projected but little above and below the plane of the other teeth; the premolars, except the last, which was nearly molariform, were smaller and simpler than the molars, which had a pattern fundamentally the same as in the †Toxodonta. Those of the upper jaw were, however, less complicated by spurs and accessory crests, and they had a somewhat stronger resemblance to the rhinoceros pattern, though the resemblance is demonstrably superficial and not indicative of relationship.

The skull was very like that of the Santa Cruz †toxodonts, †Nesodon, etc., and had the same unusual structure of the auditory region as was found throughout the order, but differed in many details, which it is not worth while to enumerate, though it may be said that the nasal bones were so much shortened that some kind of a proboscis or prehensile upper lip was probably present. The head was quite small in proportion to the size of the animal as a whole. Such of the vertebræ as are known were quite similar to those of †Nesodon, but the limbs were far longer and quite stout, though not massive. The humerus was remarkable for the great development of the ridges for the attachment of the deltoid and supinator muscles and for the prominence of the epicondyles, all of which gave to the bone the appearance of the humerus of a huge burrower, yet it is impossible to believe that so large an animal could have had burrowing habits. The fore-arm bones were separate and very long, the ulna almost as heavy as the radius; the latter is not known from a complete specimen, but there would appear to have been some power of rotation, a power which is conditioned by the shape of the upper end of the radius, and its mode of articulation with the humerus in the elbow-joint. The thigh-bone was long and heavy and its shaft was much flattened, having lost the normal cylindrical shape, but retained a small third trochanter. The bones of the lower leg were separate and relatively short, and the fibula was uncommonly heavy.

So far, there was nothing very unusual, save in the shape of the humerus, about the skeletal structure of the †Entelonychia, the remarkable characters having been confined to the feet. Were it not for these, the group might be included in the suborder †Toxodonta without difficulty. The feet, which were five-toed, differed notably in size, the manus being more than twice as long as the pes. In the former the metacarpals were very long and, though actually stout, were slender in proportion to their length; there was also a very unusual feature in an ungulate foot, that the heaviest of the digits was the fifth, or external one. The mode of articulation of the metacarpals with the first row of phalanges was very exceptional, indicating an extraordinary mobility of the toes, and the hoofs had been transformed into large, bluntly pointed claws, somewhat like those of the †chalicotheres, those aberrant perissodactyls (see [p. 354]), but not so large or so sharp. In the pes, the ankle-bone had hardly any groove for the tibia, and its lower end was hemispherical, as in the †Condylarthra and the clawed mammals generally. The toes were quite grotesquely short in comparison with those of the fore foot, and, as in the latter, the fifth was the heaviest of the series. The hind foot was apparently plantigrade, the heel-bone and the entire sole being applied to the ground in walking, while the fore foot was probably digitigrade, the wrist being raised and the metacarpals vertical. The weight was carried upon the metacarpals and one or more pads under the phalanges, as in the digitigrade carnivores, such as dogs and cats. In describing the †chalicotheres, it was pointed out that it was uncertain whether each foot had a single large pad, or whether there was a separate one under the phalanges of each digit, and a larger one, the “ball of the foot,” under the metacarpals collectively. The same doubt applies to the manus of the †homalodotheres.

This is the third instance to be cited of the acquisition of claws by a hoofed mammal and, as in the other two cases, the †chalicotheres and †agriochœrids ([p. 383]), we are confronted by the seemingly incompatible association of teeth which could have masticated only soft vegetable tissues with feet like those of a beast of prey. As in the other two groups, the problem as to the habits and mode of life of the †homalodotheres is an unsolved one, chiefly because no mammal now living is at all like these extraordinary creatures and one can therefore form but vague conjectures as to the use of such feet to herbivorous animals. Possibly they subsisted largely upon roots and tubers and used the great claws for digging up food, the principal employment that bears now make of their claws. This remarkable transformation of hoofs into claws took place in three unrelated groups of hoofed animals and must have occurred independently among the Artiodactyla, the Perissodactyla and the †Toxodontia. By no possibility, so far as we are able to comprehend the course of evolutionary change, could this common characteristic have been due to inheritance from a common ancestry.

The †homalodotheres were among the largest of Santa Cruz mammals, but they were then already approaching extinction, while in the Deseado stage they were more numerous and varied and some of them very much larger. This is an exception to the more common rule, according to which the successive members of a phylum increased in stature until the maximum was reached and this, in many cases, was followed by extinction. The rule is, however, by no means without exceptions and several have already been referred to. The largest of American proboscideans was the †Imperial Elephant (Elephas †imperator) of the upper Pliocene and Pleistocene and in many other phyla the Pleistocene species were much larger than the Recent. So with the †homalodotheres; they reached their culmination in size and importance in the Deseado stage, fewer and smaller forms surviving into the Santa Cruz, after which the entire suborder vanished. The family may be traced back to the Eocene, where it is represented chiefly by a genus (†Thomashuxleya) which had larger canine tusks and much more brachyodont teeth, but there is no way of determining when the transformation of the hoofs took place. The other two families (†Notostylopidæ, †Isotemnidæ) flourished chiefly or exclusively in the Eocene and were small animals still very imperfectly understood.

Suborder †Pyrotheria. †Pyrotheres

This suborder was a remarkable group, still incompletely known, of elephant-like animals, which reached their culmination and died out in the Oligocene, their last appearance being in the Deseado stage. The genus †Pyrotherium from the Deseado (also called the Pyrotherium Beds) was the latest, largest and best known of the suborder. The dentition was much reduced as is shown by the formula: i 2/1, c 0/0, p 3/2, m 3/3, × 2 = 28. The upper incisors were two downwardly directed tusks, the first quite small, the second considerably larger; the single lower incisor of each side was a stout, but not very long, horizontally directed tusk, with the enamel confined to a longitudinal band; the other incisors and the canines had disappeared. The premolars, except the foremost one, had the molar-pattern, which very rarely occurred among the indigenous South American ungulates. The grinding teeth were similar above and below and each had two elevated, transverse crests, which, when quite unworn, carried a row of bead-like tubercles. These teeth are decidedly reminiscent of the dentition of the aberrant proboscidean †Dinotherium, from the Miocene and Pliocene of Europe ([p. 435]), and this resemblance, together with the form of the tusks, has led to the reference of this group to the Proboscidea, but the assignment is undoubtedly erroneous, as is shown by the character of the skull and skeleton.

Fig. 243.—Head of †Pyrotherium, showing the two pairs of upper tusks. Restored from a skull in the museum of Amherst College.