†Trigonostylops, Casa Mayor. †Edvardocopeia, Astraponotus Beds.
The genus †Astrapotherium, which was the only well-defined representative of its family and order in the Santa Cruz stage, contained several species, some of them the largest animals of their time, as well as the most grotesque in appearance. The dentition differed in some important respects from that of all the other South American ungulates, the formula being: i 0/3, c 1/1, p 2/1, m 3/3, × 2 = 28. The upper incisors had completely disappeared, but the lower ones were large and, what was an exceptional character, they were partially divided into two lobes, somewhat as in the Eocene †uintatheres of North America ([p. 446]). The canines were very large and formidable tusks, which grew throughout life and apparently formed no root; the upper tusk was nearly straight and was obliquely truncated by the strongly curved and sharp-pointed lower tusk. This arrangement was very unusual among South American hoofed mammals, many of which had no tusks at all; and in those which possessed them, such as the †toxodonts ([p. 468]), they were mostly incisors. Only in the †astrapotheres and †homalodotheres were there canine tusks, and in the latter group they were small and of limited growth. All the teeth, except the canines, were brachyodont and, though rather high-crowned, formed roots before coming into use. The premolars were small and greatly reduced in number (2/1), and in pattern were simpler than the molars. The upper molars were constructed on essentially the same plan as in the †Toxodonta; indeed, the first specimen of this genus collected was referred to a large species of †Nesodon by Owen. On the other hand, the resemblance to the rhinoceros teeth is very decided, and has led several writers to postulate a relationship between the †astrapotheres and the rhinoceroses. The lower molars were of the bicrescentic pattern so frequently met with already; these teeth were very narrow in proportion to their length and strongly suggest those of †Metamynodon, the supposedly aquatic rhinoceros of the North American Oligocene ([p. 346]). It may be confidently inferred that so small a number of premolars was due to reduction from a full series, and this is confirmed by the milk-dentition, in which the premolars were 4/3.
Fig. 254.—Head of †Astrapotherium magnum. Santa Cruz. Restored from a skull in the museum of Princeton University.
The skull was extremely peculiar, more so than in any other of the contemporary genera of hoofed animals. The toothless premaxillaries were quite small, but thick, and must have supported an elastic pad, against which the lower incisors could effectively bite in cropping herbage. The nasal bones were extremely short and there must have been a proboscis or greatly inflated snout, probably the former; the immense development of sinuses in the frontal bones elevated the whole forehead into a great, dome-like convexity, a feature which is not equalled in any other known mammal. The orbits were open behind and the brain chamber was small, so that the sagittal and occipital crests were very high and strong, to afford sufficient surface for the attachment of the great temporal muscles. The horizontal portion of the lower jaw was shallow vertically, but very thick and massive, and the symphyseal region was broad and depressed.
Unfortunately, the skeleton is still very incompletely known. Of the vertebræ, only the atlas and axis have been recovered, and these resembled those of the Santa Cruz †toxodont †Nesodon, on a larger scale. The scapula had a very thick spine, without the projections which were found in most of the Santa Cruz ungulates. The limb-bones were long and comparatively slender, and the processes for muscular attachment were singularly small and weak; the bones of the fore-arm and lower leg did not coössify and were proportionately elongate, the tibia being but little shorter than the femur. The latter had the flattened shaft which recurs in nearly all of the very heavy ungulates, but retained a remnant of the third trochanter. If the feet found isolated in the Santa Cruz and Deseado stages have been correctly referred to this order, then the genus was five-toed and the feet were broad, short and heavy, quite elephantine in appearance, especially the fore foot. The ankle-joint was very peculiar and the calcaneum had no articulation with the fibula, which it had in all the other indigenous South American ungulates.
Incomplete as the material is, it is yet possible to form some general conception of this extraordinary animal when in life. The head was short, broad and deep, rounded and very probably furnished with a proboscis; the neck was of moderate length, so that the mouth could not reach the ground without a straddling of the fore legs. The body was no doubt long, the limbs long and rather slender, giving the animal a stilted appearance, the feet very short, broad and columnar. Several species of the genus are known, which differed much in size, the largest (†A. giganteum) probably exceeding any modern rhinoceros in height and length, and the smallest (†A. nanum) not much larger than a Wild Boar.
†Astrapothericulus, of the Patagonian stage, was smaller than the average species of the Santa Cruz genus, and had teeth of the same number, but the canines were not capable of indefinite growth, and the lower molars had the pillar in the posterior crescent so characteristic of the South American hoofed animals. In the Deseado stage, on the contrary, the †astrapotheres were of larger size, and in the commonest genus, †Parastrapotherium, the grinding teeth had lower crowns and the premolars were more numerous, at least 3/2. In the still more ancient †Astraponotus, which gives its name to the upper Eocene (or lower Oligocene) of Patagonia, the premolars were present in full series. In the Casa Mayor the order was abundantly represented by still more primitive genera, which assuredly had an undiminished number of teeth, though this has not been proved. One of these genera, †Albertogaudrya, was the largest animal of its time and the highly probable ancestor of the series leading to the Santa Cruz †Astrapotherium.