A history of land mammals in the western hemisphere

by William Berryman Scott

III. Didolodidæ.

†Didolodus, Casa Mayor. †Lambdaconus, do. †Notoprogonia, do. †Proectocion, do., etc., etc.

Only one of the families of this suborder survived into the Pampean stage, where it was represented by a single genus, †Macrauchenia. Like all the other large Pampean mammals of distinctly South American type, this was a grotesque creature, from the modern point of view. The genus was first discovered by Darwin, who says of it: “At Port St. Julian, in some red mud capping the gravel on the 90-foot plain, I found half the skeleton of the Macrauchenia Patachonica, a remarkable quadruped, full as large as a camel. It belongs to the same division of the Pachydermata with the rhinoceros, tapir, and palæotherium; but in the structure of the bones of its long neck it shows a clear relation to the camel, or rather to the guanaco and llama.”[12] The views upon classification and relationship here expressed have been superseded, but the passage is an important one in the history of scientific opinion.

†Macrauchenia ([Fig. 120, p. 216]), as Darwin says, was as large as a camel; it had an unreduced dentition of 44 teeth and in each jaw the teeth were arranged in continuous series and were quite decidedly hypsodont. Both in the upper and the lower jaws the incisors formed a nearly straight transverse row and have a “mark,” or enamel pit, like that seen in the horses; the canines were but little larger than the incisors and did not form tusks. The premolars were smaller and simpler than the molars. The upper molars had two concave and crescentic external cusps, connected by a median ridge, as in several families of perissodactyls; two transverse crests and several accessory spurs and enamel-pockets gave to the grinding surface, when somewhat worn, the appearance of considerable complexity. The lower molars had the two crescents, one behind the other, which recurred in almost all the South American types of ungulates; the vertical pillar which so generally in these types arose in the inner concavity of the posterior crescent was wanting in the permanent teeth of †Macrauchenia, but present in the milk-premolars.

No part of this remarkable animal was more curious than the skull, which was quite small in proportion to the rest of the skeleton. It was long, narrow and low, sloping and tapering forward to a blunt point at the end of the muzzle, though there was a slight broadening here to accommodate the transverse row of incisors. The sagittal crest was replaced by a short, narrow and flat area; the cranium was shortened and the face elongated, the orbits, which were completely encircled in bone, having been shifted behind the line of the teeth, as in the modern horses. The nasal bones were reduced to a minimum, a mere vestige of their original length, the anterior nasal opening being directly over the posterior, making the nasal passage vertical. Such an arrangement is an almost positive proof that in life the animal had a flexible proboscis, a conclusion which is confirmed by the presence, on the top of the head and behind the nasal opening, of deep pits for the attachment of the proboscis-muscles. A very curious feature of this skull was that the bones of the upper jaw, the maxillaries and premaxillaries of the opposite sides, united in the median line, making a long, solid, bony rostrum in front of the nasal opening, a character not found in other land mammals.

The neck was almost as long as in a camel and its vertebræ agreed with those of the latter in the very exceptional character of having the canal for the vertebral artery passing longitudinally through the neural arch, instead of perforating the transverse process. As Darwin says in the passage quoted above, “it shows a clear relation to the ... guanaco and llama,” but this is founded on the postulate that such a likeness must, of necessity, imply relationship. As was shown in the chapters on the Artiodactyla and Perissodactyla, it is the general rule among long-necked ungulates that the odontoid process of the axis assumes a spout-like shape, but †Macrauchenia was an exception and had an odontoid which retained its primitive and peg-like shape; it was, however, relatively very short and in cross-section was no longer circular, but oval. This may be regarded as a step toward the assumption of the spout-like form, but the extinction of the family put an end to further changes in that direction.

The body was rather short and the limbs very long, giving the animal a stilted appearance, while the feet were relatively short. The proportionate lengths of the different limb-segments was unusual; the upper arm was short, the fore-arm very long, the thigh long and the lower leg quite short. The humerus was very heavy; the ulna and radius, which were firmly coössified, formed a very long compound bone, which was broad transversely and thin antero-posteriorly. The long femur had only a small and inconspicuous third trochanter and the shaft was broad and thin, being flattened, or “compressed” antero-posteriorly. The tibia and fibula were united at both ends; the former was very heavy at the upper end, but diminished downward in width and thickness, and the fibula articulated with the calcaneum, as in the artiodactyls. The feet were tridactyl and had mesaxonic symmetry; that is to say, the median digit, or third of the original five, was symmetrical in itself and was bisected by the middle line of the foot, while the lateral toes (second and fourth), each of which was asymmetrical, formed a symmetrical pair. It is this perissodactyl character of the foot to which Darwin refers when he says that †Macrauchenia “belongs to the same division of the Pachydermata with the rhinoceros, tapir and palæotherium.” On the other hand, the very significant structure of the ankle-joint was radically different from that of the Perissodactyla; not only did the calcaneum have a special facet for articulation with the fibula, but the lower end of the astragalus was a convex “head,” resting only on the navicular, as in the †Toxodontia, †Condylarthra, Hyracoidea and other very primitive groups of hoofed animals and in clawed mammals generally. Such a combination of characters is not known in any of the perissodactyls and precludes the reference of the †Litopterna to that order, though such a reference is strongly maintained by several authorities. The ungual phalanges were small and appear to suggest the presence of pads on the feet.

The appearance of †Macrauchenia in life must have been sufficiently strange. The small head with its proboscis and the long neck and legs should probably be regarded as indicative of browsing habits, though the hypsodont teeth show that grazing was at least an occasional mode of feeding. The long limbs and short feet gave to the extremities an appearance unlike that of any existing hoofed animal. The form and size of the ears and the character of the hairy coat are, of course, conjectural.

In the later Pliocene the family was represented by forms which differed so little from the Pampean †Macrauchenia as to call for no particular notice, but in the presumably lower Pliocene of the Paraná stage, occurred several genera, all unfortunately but imperfectly known, which are of interest as being less specialized than †Macrauchenia and as showing the way in which some of the peculiarities of the latter were acquired. In †Scalibrinitherium, which may be taken as an example of these genera, the teeth were brachyodont; the upper molars were rather less complex than those of †Macrauchenia, while the lower molars had the pillar in the concavity of the posterior crescent, which the Pampean genus retained only in the milk-teeth. As we have repeatedly found, the milk-dentition is often conservative and retains primitive or archaic features which have been lost in the permanent teeth, and †Macrauchenia is another illustration of the same principle. In the skull of †Scalibrinitherium the nasal bones, though very short, had not suffered such extreme abbreviation as in the succeeding genus, the nasal opening was farther forward and the maxillaries united in the superior median line for only a short distance, while the premaxillaries were fused together for their whole length. The orbit had not been shifted entirely behind the teeth, but was above the third upper molar.