A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 294.—Skull of †Stegotherium, Santa Cruz. Princeton University Museum.

Aside from carapace and skull, the skeleton of the Santa Cruz armadillos was surprisingly modern. The vertebræ of the neck were coössified, those of the lumbar and posterior dorsal regions had the extremely complex articulations and the high processes for the support of the carapace, just as in the Recent genera. The limb-bones did not differ in any significant way from those of the latter, and the feet closely resembled those of the modern Dasypus; none of the genera displayed the specialization of the manus seen in Cabassous, Priodontes or Tolypeutes. Whether these specializations have all been acquired since Santa Cruz times, or whether they had already appeared in some other region of the continent, is a question that remains to be determined.

Little can yet be done in the way of tracing the history of the armadillos through the stages preceding the Santa Cruz times, because of the fragmentary character of the material. The suborder was abundantly represented in the Deseado stage, in which some of the Santa Cruz genera existed. Even in the most ancient of the Patagonian Tertiary formations are found scutes of the carapace essentially like those of the modern armadillos. The group is thus of very high antiquity, older than any other of the suborders is known to be.

In addition to the typical armadillos of South America, there were, in other continents, certain more or less doubtful forms, concerning which a word should be said. In the Bridger Eocene of North America was a genus (†Metacheiromys) of armadillo-like animals, the true relationships of which are far from clear. The teeth were mostly lost, leaving but one on each side of each jaw, and this was covered with enamel, which is not true of any unquestioned edentate. However, this is not an insuperable objection to the inclusion of these animals in the edentates, for there can be no doubt that these were derived from ancestors with enamel-covered teeth. Even in modern armadillos the enamel-organ is formed in the embryo, though it does not perform its functions. The skull of †Metacheiromys had something of the armadillo-shape, but was not especially characteristic. The vertebræ of the neck were all separate, and those of the dorsal and lumbar regions did not have the complex articulations common to all known edentates, fossil and Recent; the sacrum had on each side but one point of contact with the hip-bones, and the sternal ribs were not ossified. The shoulder-blade, hip-bones and humerus were all armadillo-like. The plantigrade feet were five-toed and the metapodials were very edentate in form. No indication of bony armour has been found. While these curious animals may very possibly have been referable to the Edentata and, at all events, had several features suggestive of relationship to that order, it can hardly be maintained that they were unequivocal members of it. In the Oligocene of France have been obtained some very fragmentary fossils which were classified and described as armadillos, but their character is quite problematical. It is thus possible, though far from certain, that in the early Tertiary, armadillo-like edentates were spread all over the northern hemisphere.

Suborder †Glyptodontia. †Glyptodonts

In the Pliocene and Pleistocene these huge armoured creatures ranged from the southern United States to Patagonia. That they were nearly related to the armadillos is clear, but they were so greatly modified and specialized as to demand recognition as a distinct suborder.

Aside from their enormous size, the most striking feature of the †Glyptodontia is the extraordinary development of their defensive armour, which was far more complete and massive than in the armadillos. The top of the head was protected by a thick head-shield, or casque, composed of several coössified plates; the body and much of the limbs were enclosed in the immense carapace of elongate-oval, domed shape, which covered the neck and trunk and on the sides almost reached to the ground. This tortoise-like carapace was composed of very thick, polygonal plates of bone (no doubt covered externally with horny plates) immovably fixed together by their rough edges, and ornamented with an elaborate pattern of sculpture, which varied according to the genus. With one or two exceptions, the plates of the carapace were not arranged in transverse rows, but formed a mosaic without discernible banding. In the exceptions noted, the sides of the carapace were made up of bands, and near the margins were two or three overlapping transverse bands which permitted a minimal degree of flexibility. The tail-sheath was remarkable and differed much in appearance and make-up in the various genera. In †Glyptodon the tail was comparatively short and the tail-sheath was made up of a series of overlapping rings, each ring consisting of two rows of plates; those of the second row were ornamented, on the top and sides of the tail, with very prominent, conical projections, capped, in the living animal, with still longer and sharper spines of horn, so that the tail must have bristled with spikes. A more usual type of tail-sheath was exemplified by †Sclerocalyptus, in which there were several overlapping rings at the root of the tail, but for much the greater part of its length the plates of the sheath were fused together into a long, transversely oval tube, tapering very gently to the free end, where it was bluntly rounded. A modification of this type was the very long tail-sheath of †Panochthus, in which there were seven overlapping rings at the root, followed by a long, massive tube, the sides of which were set with three or more large and heavy, horn-like spines. In †Dœdicurus was reached the maximum specialization of this type; the very long tube had its free end greatly expanded and thickened into a huge, club-shaped mass, on the top and sides of which were fixed long and sharp horns.

The teeth, which in all the known genera numbered 8/8, were all very much alike; each was divided by two broad and deep vertical grooves on each side into three pillars, connected by narrow necks. Harder dentine in the centre and on the periphery of the tooth, with a softer intermediate layer, kept the grinding surface rough through differential wear. Teeth of this character are indicative of a vegetable diet and these great creatures were, no doubt, as harmless and inoffensive as possible.