A history of land mammals in the western hemisphere

by William Berryman Scott

(2) In all cases the carapace was made up of transverse bands, which permitted a slight degree of flexibility, and near the anterior end, at the margins of the shell, were two or three overlapping bands. The plates were thin and were but rarely coössified; the ornamentation was made by shallow grooves.

(3) The tail-sheath, which was of very uniform character, consisted of two quite distinct portions; the anterior region consisted of 5 or more freely movable, overlapping rings, each of two rows of plates, and in the posterior region the rings were closely fitted together, less distinctly marked and not movable. This posterior portion was sometimes thick and ended abruptly, sometimes slender and tapering and in one genus (†Asterostemma) it was very armadillo-like. In none of the genera were there any spines or horns, nor were the separate plates ever fused together to form a tube.

(4) There was considerable variety in the head-shield, which was usually made up of many separate plates, but in one genus (†Eucinepeltus) they were coössified into a single heavy casque.

(5) The teeth had a less extreme height and the four anterior ones of each jaw were much simpler than in the Pampean forms. An interesting survival was the retention of two minute incisors in each premaxillary bone, in one genus (†Propalæohoplophorus), but these were of no functional value and were early lost.

(6) The skull was much longer, narrower and lower and had a relatively longer facial portion; the occiput was higher and more erect, and the condyles had no such elevation above the level of the teeth; the orbit was widely open behind and the descending process given off from the zygomatic arch beneath the eye had no such exaggerated length; the bones were not conspicuously inflated by sinuses. The lower jaw was shallower, the symphysis and anterior spout shorter and the ascending portion far lower.

(7) The backbone had a greater number of separate parts; the atlas, as always, was free, the axis was fused with two or three of the following vertebræ; the sixth was free and the seventh fused with the first and second dorsals to form one piece, which was succeeded by two or three separate vertebræ: the other dorsals, except the last one, were united in the dorsal tube, and the lumbo-sacral tube was already complete. Thus, instead of four or five, there were eight or nine distinct parts. None of the tail-vertebræ were fused together.

(8) There was the same disparity in the length of the fore and hind limbs, but the bones were far more slender and armadillo-like; this was especially true of the radius and humerus, the latter having well-developed deltoid and supinator ridges and epicondylar foramen; the ulna was more massive and glyptodont-like. The femur was very much more slender and rounded and the third trochanter was placed higher up the shaft; tibia and fibula were coössified at both ends and resembled those of the Pampean genera, except for their much greater slenderness.

(9) The feet were much as in the latter, but relatively narrower, and the manus had longer claws.

In short, the Santa Cruz †glyptodonts departed much less widely from the armadillos than did the Pliocene and Pleistocene genera, and, to a certain extent, bridged over the gap between the two suborders. Such backward convergence in time is very strong evidence for the community of origin of the two groups.

The †glyptodonts of the more ancient formations, so far as they are known, teach us little concerning the stages of modification in these extraordinary animals, because of their fragmentary condition. The oldest stage in which representatives of the suborder have been detected is the Astraponotus beds, which may be Oligocene or upper Eocene. On the face of the records, therefore, the †glyptodonts had no such antiquity as the armadillos.