A history of land mammals in the western hemisphere

by William Berryman Scott

These small Santa Cruz †ground-sloths were not so clumsy and slow-moving as their gigantic successors of the Pampean, and must have been inoffensive plant-eaters, some of them perhaps more or less arboreal in habits, but they could defend themselves with their long, sharp claws.

Fig. 291.—Left pes of †Hapalops, Santa Cruz. Princeton University Museum. Letters as in [Fig. 290] and scale of reduction the same.

It would require far too much space and lead us into a labyrinth of anatomical technicalities to point out all the many resemblances to other edentate suborders which are to be noted in the skeleton of the Santa Cruz †Gravigrada, which thus justified their position as the most nearly central group of the entire order. Not only was the skeleton of these Miocene †ground-sloths very much less specialized than in their Pleistocene successors, but they were much closer to the anteaters than were the latter. Aside from the skull, all parts of the skeleton displayed this resemblance in so marked a manner that the common derivation of the two suborders seems hardly open to question. Different as was the skull in the two groups, the differences were not such as to preclude the origin of both from the same type. Even more closely connected were the †ground-sloths and the tree-sloths; the resemblance was most clear in the teeth and skull, but there were also many points of likeness throughout the skeleton. In the tree-sloths the entire bony structure has been profoundly modified in adaptation to their altogether exceptional mode of life, in hanging suspended from the branches of trees; but, despite this modification, there are so many notable resemblances between the Santa Cruz †Gravigrada and the existing Tardigrada as irresistibly to suggest their community of origin, and thus the former served to connect the anteaters, on the one hand, with the tree-sloths, on the other. This must not be construed as meaning that the Miocene †ground-sloths were the ancestors of the other suborders, which were probably already in existence as distinct groups, but merely that all three suborders had a common origin, from which the Santa Cruz †Gravigrada had departed less than have the sloths and anteaters.

There is evidence that at least two of the †ground-sloth families, the †Megalonychidæ and the †Mylodontidæ, were distinguishable in the Deseado stage, but materials are still lacking to give us any real knowledge of the suborder in that or the more ancient stages.

Section Loricata. Armoured Edentates

Suborder Dasypoda. Armadillos

Armadillos are still an important and characteristic element of the Neotropical fauna, ranging from Texas to Patagonia and showing a considerable variety of structure and appearance. Existing species are all of small or moderate size, and the one which is by far the largest (Priodontes gigas) may somewhat exceed three feet in length, exclusive of the tail, and the smallest (Chlamydophorus truncatus) is hardly more than five inches long. In most armadillos the hair is greatly diminished in quantity and the animal is sheathed in a conspicuous armour of bony scutes, covered with horny plates. There is a head-shield which covers the top of the skull, and the tail is enclosed in a sheath; the back and sides are protected by the great carapace and the limbs by irregular scutes and scales, leaving only the under side of the body and the inside of the legs uncovered. In most existing genera, the carapace is in three parts, an anterior and posterior buckler, in which the plates are immovably fixed together by their edges, and between a varying number of transverse, overlapping bands, from 3 to 13, which permit sufficient flexibility of the body. The tail-sheath is made up of a series of rings. One genus (Tolypeutes) has the power of rolling itself into a ball, the head-shield exactly closing the anterior notch of the carapace and the tail-sheath filling the posterior notch. The animal is thus perfectly protected against attack and does not seek refuge by digging, as other armadillos do and with astonishing rapidity. In the little Pichiciago (Chlamydophorus) the dermal ossifications are very thin and the carapace is composed of twenty transverse bands of horny plates, without bucklers; the rump is covered with a broad and heavy shield of bone, overlaid with thin plates of horn, which is attached to the hip-bones and notched below for the short tail. In certain rare and little known genera there is a greater development of hair; in one (Praopus) the whole carapace is covered with a dense coat of hair, and in another (Scleropleura) the middle of the back has only a hairy skin and the carapace is restricted to the sides.

The teeth vary in number and size in the different genera; in some (e.g. Dasypus) there is one upper incisor on each side; the teeth are all simple and of nearly cylindrical form. The skull is low and flattened, with long tapering snout and orbits widely open behind; the zygomatic arches are uninterrupted. Most of the vertebræ of the neck are fused into a single piece; in the lumbar and posterior dorsal regions there are not only the usual highly intricate articulations between the vertebræ, but also high processes on each side for the support of the carapace. The fully ossified sternal ribs have movable joints with the breast-bone, but not the double articulations found in the anteaters and †ground-sloths. The shoulder-blade has a very long acromion, which does not form a bony loop with the coracoid, and the clavicles are complete. The anterior element (ilium) of the hip-bone is narrow, very different from the broad plate of the †Gravigrada. The humerus has prominent deltoid and supinator ridges and an epicondylar foramen, and the femur has the third trochanter. Though the fore-arm bones are separate, the radius has no freedom of rotation; tibia and fibula are coössified at both ends.

In the hind foot there is no great variety of character; it is five-toed and usually has claws, but may have broad, flat nails (e.g. Priodontes), but the manus, which is a burrowing organ, displays different degrees of specialization, which is carried farthest in the Giant Armadillo (Priodontes). Tatu has the fore foot of quite different type. The armadillos feed chiefly upon insects and worms, but they are omnivorous and eat roots and carrion and sometimes even capture and devour small rodents and lizards.