As in the case of the †ground-sloths, the fossil armadillos so far available are insufficient for tracing the history of the various phyla, or for doing more than making a very brief sketch of the development of the suborder as a whole. Nearly all of the modern genera have been found in the Pleistocene together with several that are extinct, some of the latter of very large size. One of these, †Eutatus, had a carapace without bucklers and made up of 33 movable, transverse bands. Another, †Chlamydotherium, as large as a rhinoceros and the largest known armadillo, had anterior and posterior bucklers, with several movable bands between; it was especially characterized by the teeth, which were divided by a vertical groove into pillars or lobes, thus approximating the teeth of the †glyptodonts. The genus went far back into the Pliocene, and the more ancient species were successively smaller.
Though remains of armadillos abound in the formations between the Pampean and the Santa Cruz, they are for the most part so fragmentary as to be of no service in deciphering the history of the group. In the Santa Cruz beds also they are very abundant and varied, and several of the genera are very completely known. As a whole, this assemblage of armadillos was very different from that of the Pleistocene, and only a few direct ancestors of the latter have been found in the Miocene of Patagonia; no doubt, like the ancestral tree-sloths and anteaters, they were then living in the warmer regions of the north. Most of the Santa Cruz armadillos belonged to aberrant types, of which no descendants have survived; but, nevertheless, they throw welcome light upon the developmental stages of the suborder.
Fig. 292.—Skull of †Peltephilus, Santa Cruz. Ameghino collection.
These armadillos had the complete armour of head-shield, carapace and tail-sheath, but the carapace had no anterior buckler in any of the Santa Cruz genera, and in some there was no posterior buckler, the carapace consisting entirely of transverse, movable bands, as in the Pleistocene †Eutatus. In one especially peculiar genus, †Peltephilus, the head-shield was remarkable; it was made up of large, polygonal plates, the two anterior pairs of which were elevated into high, sharp points, which must have supported horns, that were quite large in proportion to the size of the animal. A 4-horned armadillo, like a tiny rhinoceros in armour, must have been a sufficiently bizarre object.
As a rule, the teeth of the Santa Cruz armadillos were of the same simple, cylindrical form as in the modern genera and arranged in the same way, but there were some exceptions. In the horned †Peltephilus, the teeth of each jaw were so inserted as to form a continuous series around the sides and front of the mouth; and, at first sight, it would seem that this genus differed from all other known edentates in having a full set of incisors, but actually it had but one on each side above and below, as has the modern Dasypus, with the difference that, in the latter, the incisors of the opposite sides are widely separated and in †Peltephilus were brought close together. The anterior upper teeth were long and sharp and passed outside of the lower ones, when the jaws were closed, and all the teeth had an external layer of hard and shining dentine, which had almost the appearance of enamel. Another variant in dentition was †Proeutatus, which was the largest of Santa Cruz armadillos and larger than any existing forms except Priodontes and Cabassous. It had teeth like those of the huge Pliocene and Pleistocene †Chlamydotherium, of which it was a probable ancestor; the five posterior ones in each jaw were of trihedral shape, and the two kinds of dentine, of which they were composed, were so arranged as to form a rough grinding surface. Probably this animal subsisted largely upon vegetable food; at all events, the food was of such a nature as to keep the teeth worn down more than in any of the associated genera. A fourth type of dentition was displayed by †Stegotherium ([Fig. 243, p. 480]); the teeth were so few and small that they can have had no functional value and were merely minute points almost level with the gums. In all probability, †Stegotherium was more exclusively insectivorous than the other genera.
Among the Santa Cruz armadillos may be distinguished four well-marked types of skull. (1) That which agrees closely with the modern form, especially as exemplified by the genus Dasypus. (2) †Proeutatus had a higher and less flattened cranium and a very long, cylindrical muzzle. (3) In the horned †Peltephilus the face was very short and broad, and the lower jaw was horseshoe-shaped, the two halves coössified at the symphysis, which is not true of any other armadillo. (4) Quite the opposite extreme was displayed by †Stegotherium, in which the face was drawn out into a very long, slender and tapering muzzle; the lower jaw was extremely weak and thin, the posterior, ascending portion low and ill-defined, the condyle and coronoid process much reduced. No other known armadillo has such fragile jaws, and there was a distinct likeness in the skull to that of the Ant-Bear.
Fig. 293.—Skull of †Proeutatus, Santa Cruz. Princeton University Museum.