Suborder †Toxodonta. †Toxodonts Proper
Among the remarkable animals which Charles Darwin found in the Pampean deposits of Argentina and took with him to England, was a skull of one which Sir Richard Owen named †Toxodon, or “Bow-Tooth,” from the strongly curved grinding teeth, those of the opposite sides almost meeting in the median line above the hard palate. For many years †Toxodon, of which hardly anything was known, save the skull and teeth, was a zoölogical puzzle and no one was able to reach any satisfactory conclusion as to its systematic position and relationships, as all the attempts made to force it into one of the known ungulate groups were obvious failures. The discovery of complete skeletons, two of which are mounted in the La Plata Museum, showed the necessity of making a new group for its reception, as Owen had originally proposed. Through the exploration of Argentina and its Patagonian provinces, the history of the suborder was followed far back into the Tertiary period and its indigenous character demonstrated. This and all the other subdivisions of the †Toxodontia were exclusively Neotropical in distribution, and none have been found farther north than Nicaragua and there only in the Pleistocene.
The suborder was represented in the Pampean beds by several genera, which differed in size and in the complexity of the grinding teeth, but only of †Toxodon is the skeleton at all fully known. The Pampean species of this genus were massive, elephantine creatures, rivalling the largest rhinoceroses in bulk, but not equalling them in height. The teeth were all thoroughly hypsodont and apparently continued to grow throughout life without forming roots; the dental formula was: i 2/3, c 0/0, p 3/3, m 3/3, × 2 = 34. The first upper incisor was broad and chisel-shaped, the second more tusk-like, but in some species these proportions were reversed; the lower incisors were procumbent, pointing straight forward, and of these the third was the largest. The canines were lost and there was a long, toothless gap behind the incisors. The premolars were smaller and simpler than the molars, and the anterior ones were very small and were frequently shed at an early stage, making the number of these teeth variable in different specimens. The upper molars also were of quite simple pattern; the broad and smooth external wall showed no distinct signs of a division into cusps, and from it arose two oblique transverse ridges; the deep cleft or valley which separated these ridges was divided and made Y-shaped on the grinding surface by a prominent spur from the outer wall between the two principal crests. The lower molars were composed of two crescents, one behind the other, of which the posterior one was very much longer, and both were very narrow transversely.
The skull had shortened nasal bones, an indication that some sort of a proboscis or prehensile upper lip was present. There was no trace of a horn, and the general aspect of the skull was not unlike that of one of the hornless rhinoceroses, except for its great vertical depth; the sagittal crest was very short and had almost disappeared. The auditory apparatus was very extraordinary, though it can hardly be described without an undue employment of anatomical terms; suffice it to say that in addition to the usual outer ear-chamber, formed by the inflated tympanic bone, there was a second chamber in the rear wall of the skull, communicating with the first by a canal. This arrangement would seem to imply an unusual keenness in the sense of hearing. The external entrance to the ear was placed very high up on the side of the head, as in the pigs and in many aquatic mammals, suggesting that †Toxodon was more or less amphibious. The anterior, or symphyseal, region of the lower jaw was very broad, flattened and shovel-like, hardly projecting at all below the plane of the lower incisors.
The neck was short and stout, the body long and extremely bulky, having an immense, almost hippopotamus-like girth; the spines of the anterior dorsal vertebræ were very long, making a high hump at the shoulders. The limbs were short and very heavy, the bones very massive and with large projections for muscular attachments. The fore leg was much shorter than the hind, depressing the neck and head in very curious fashion. The shoulder-blade was rather narrow, the spine without acromion or distinct metacromion; the hip-bones were greatly expanded and turned outward, quite in elephant-like fashion, a character which almost invariably accompanies great increase in bodily mass. The thigh-bone was also very elephantine in appearance, a likeness due to its shape and proportions, to the loss of the third trochanter and the flattening of the shaft, so that the width much exceeded the antero-posterior thickness. All of these characters are, as a rule, associated with greatly augmented weight and have been independently acquired in several series of large and massive animals, elephants, †uintatheres, †titanotheres, and to this list should be added the †toxodonts. In the fore-arm the bones were separate and the ulna was quite unreduced and very stout, but in the lower leg, which was very short in comparison with the thigh, the tibia and fibula were coössified at the upper end, but not at the lower, a most exceptional arrangement. The feet were surprisingly small and had but three digits, the reduction from the original five having proceeded to that extent before the process was arrested by augmenting weight. The heel-bone (calcaneum) was so articulated with the other bones of the tarsus as to project almost straight backward, nearly at a right angle to the position normal in a digitigrade foot, a feature which is not known to occur in any other mammal. The hoof-bones were so small and nodular that the foot must have been of the columnar type, the weight resting upon the usual elastic pad.
The restoration ([Fig. 121, p. 217]) shows †Toxodon as a very heavy, slow-moving, water-loving animal; the aquatic habits are, of course, conjectural, but the general proportions are accurately given by the skeleton.
From the Pleistocene, †Toxodon may be followed back without notable change to the Pliocene, but there it was in association with the last of a curious phylum, the genus †Trigodon ([Fig. 138, p. 263]), as yet known only from the skull. In these animals a very prominent bony knob or boss on the forehead clearly demonstrates the former presence of a large, rhinoceros-like, frontal horn. But very few of the indigenous South American ungulates possessed horns, or horn-like protuberances of the skull, and all of these so far discovered belonged to the suborder †Toxodonta. †Trigodon was, from present knowledge, the only horned creature of its time and region, for the deer and antelopes which had probably arrived in South America had not advanced so far south as Argentina. Another very peculiar feature of this genus was that the lower incisors were present in uneven number, two on each side and one in the middle. Nothing has been found of the skeleton, but it was doubtless that of a smaller and somewhat lighter †Toxodon.
