A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 295.—Pampean †glyptodonts, †Dœdicurus clavicaudatus and †Glyptodon clavipes. Restored from skeletons in the museums of La Plata and Buenos Aires.

The skull was remarkably short, broad and high, the facial region being especially abbreviated; the cranium, though forming the greater part of the skull, was yet small in comparison with the size of the animal; it had a distinct, though not prominent, sagittal crest. The occipital surface was inclined forward and had a very elevated position, the condyles being near the top of the head and raised very far above the level of the teeth. The orbits were relatively small, more or less completely encircled with bone and as near to the top of the head as they could be brought; this was to make room for the extremely high teeth, which required a great depth of jaw; the elevation of the whole cranium left unlimited space for the jaws beneath it. The zygomatic arches were strong and curved out widely from the sides of the skull; beneath each eye was given off a very long descending process which projected downward, outside of the lower jaw. In most of the species the upper profile of the skull was nearly straight, but in †Panochthus it descended very steeply from the forehead to the nose. The forehead was dome-like and the nasals extremely short. Sinuses were extensively developed, especially in the frontals, and in †Sclerocalyptus the bones around the nostrils were grotesquely inflated. The two halves of the lower jaw were fused together, and the symphysis was prolonged into a short, wide spout, which projected considerably in advance of the upper jaw, showing that the soft parts of the muzzle must have had a corresponding extension. The horizontal portion of the lower jaw, carrying the teeth, was short and very deep; the posterior, ascending portion had a forward inclination and was very high.

The skeleton of the Pleistocene †glyptodonts was unique among mammals, though evidently a modification of the armadillo type. The extreme modification was conditioned by the enormous weight of the carapace, which the skeleton had to support. The neck was very short, made up of short vertebræ, which were extensively coössified; the atlas was always free, but the axis was fused with a varying number of the succeeding vertebræ; usually, the axis and the third to the sixth formed one mass, while the seventh was fused with the dorsals. The joint between the sixth and seventh vertebræ was such as to permit at least a partial downward bending of the head beneath the carapace, closing its anterior opening with the head-shield. The seventh neck vertebra and all the dorsals, except the last one, were coössified into a heavy curved rod, the “dorsal tube”; the conjoined neural arches formed a tunnel for the spinal cord and the spines made a continuous ridge. As the hind legs were very much longer than the fore, the back was strongly arched upward from the neck to the hips. The last dorsal, the lumbars and the sacrum were all fused together to form the “lumbo-sacral tube,” of which the coössified neural spines made a very prominent ridge, the principal support of the carapace in the median line; the anterior half of the trunk skeleton, comprising the short, deep thorax, was free from the carapace, which in that region must have rested upon the muscles of the back and shoulders. The number of neck and trunk vertebræ combined varied in the different genera from 26 to 28, but fusion had reduced the number of separate parts to 4, or at most 5. Such greatly diminished flexibility of the back was rather an advantage. The tail differed much in length in the various genera, but was always massive; the anterior vertebræ, usually 7 in number, were free, the others were fused into a heavy, tapering rod; but for nearly its whole length the processes of the vertebræ were very prominent, each vertebra touching the tail-sheath at five points and thus giving it very effective support. In †Glyptodon the tail-vertebræ were all free.

In most of the genera the scapula was very broad and had the very long acromion common to all the edentates; there were no clavicles. The hip-bones were very peculiar; the anterior element (ilium) stood almost vertically, at right angles to the backbone, and formed a broad plate, facing forward, the top of which was roughened and thickened to support the carapace. The posterior element (ischium) was also much expanded, but faced outward, and its hinder end, curved upward and thickened, was another point of strong support for the carapace. The two elements together formed an inverted arch, the crown of which rested on the head of the femur.

Though less massive than those of the hind leg, the bones of the fore limb were yet very heavy. The humerus was short and had reduced deltoid and supinator ridges and no epicondylar foramen; the short fore-arm bones were separate and heavy, the ulna especially so. The femur was much the longest of the limb-bones and was extremely strong, especially in its great breadth, the antero-posterior flattening, common to nearly all very heavy mammals, being well marked. A very unusual feature was the position of the third trochanter near the lower end of the shaft. The tibia and fibula were much shorter than the femur, extremely heavy and coössified at both ends. The very short and broad feet retained five digits; in the manus the claws were sometimes comparatively long and sharp, sometimes blunt and hoof-like; those of the hind foot were always broad hoofs.

Among all the many strange and grotesque mammals which the study of fossils has brought to light, none can have been more remarkable than the Pleistocene †glyptodonts; slow-moving hillocks they must have seemed, the larger species 12 to 14 feet long and 5 feet or more in height. Those that had claws on the fore feet probably used them to dig for roots and tubers, but all were plant-feeders. When attacked by the †sabre-tooth tigers (†Smilodon) or the great bears (†Arctotherium) they needed only to squat down, bringing the edges of the carapace to the ground, and draw in the head, to be perfectly protected, while a sweep of the spiny or club-like and horned tail would have been fatal to anything in its path.

As in the case of so many other groups, little has yet been learned regarding the history of the †glyptodonts during the interval between the later Pliocene and the Santa Cruz; the intermediate formations have yielded many †glyptodonts, but not in such preservation as to be of any service in this connection. We find, as might be expected, many and very great differences between the Pampean and the Santa Cruz representatives of the suborder, the latter being in all respects less modified and less widely removed from the armadillos.

(1) The most obvious and striking distinction was in size, the Santa Cruz forms being all small and some of them very small.