Popular Science Monthly/Volume 13/June 1878/The Gigantic Extinct Armadillos and their Peculiarities, with a Restoration
|THE GIGANTIC EXTINCT ARMADILLOS AND THEIR PECULIARITIES, WITH A RESTORATION.|||
THE general principle that, with increased size, there is an increase in the thickness and strength of the skin and its protective appendages, is in no instance better illustrated than in the extinct and living armadillos; in the former the thickness attained by the bony armor sometimes exceeds an inch, in the latter it is usually less than one-eighth of that thickness. Note, also, the corresponding contrast in size. The smallest living species is less than a foot in length, whereas the largest known fossil form measured between twelve and thirteen feet in length, or quite as much as the largest rhinoceros. In structure or make-up these giants were sufficiently different from their living relatives to characterize a distinct family, appropriately named by Huxley the Hoplophoridæ, or armor-bearers. Unlike the living armadillos, the back and sides of the body were covered with an inflexible carapace, or coat-of-mail, which, like the same in living forms, was made up of numerous more or less nearly hexagonal tesseræ or plates. In recent forms the armor is divided into two parts: a forward part, covering the scapular or region of the shoulder-blades, and a posterior, covering the pelvic or region of the hips and flanks; between the two a series of mobile bands or zones of plates are interposed transversely, so as to enable the animal to bend its covering upon itself, and thus envelop all the soft parts, and thereby protect itself from enemies almost as effectually as the hedgehog can with its spines. The Hoplophoridæ were provided with an additional rigid, pear-shaped armor-plate or buckler upon the under side of the body, hence they have also been called Biloricata, or two-shielded, in contradistinction to the living Loricata (armadillos), which are shielded only on the back. Head-shields covering the upper part of the head are characters common to both living and fossil forms. The extinct species, with their carapace and plastron, or belly-shield, resembled the snapping turtles in not having the belly-shield to cover more than one-half of the area of the lower side of the body left uncovered by the carapace of the back. The reason why this belly-shield was smaller than the area it partially covered was to allow free and unimpeded movement of the limbs. All that remains of this rigid belly-shield in even the best-armored living species are numerous separated plates, which do not interfere with that flexibility of the walls of the abdomen which is necessary in bending the body when the animal covers itself with its dorsal armor or carapace.
Living species are mostly burrowing in habit. Whether the Hoplophoridæ were burrowers cannot be affirmed, but it is extremely doubtful; though, from the great resemblance of the fore-limbs and claws to those of living species, it is likely that they were able, upon occasion, to dig with great rapidity and dexterity.
I have seen the tongue protruded nearly two inches with great quickness by a young six-banded armadillo; it is tapering and very flexible, and is no doubt used to advantage in capturing insects which
fall in its way, as well as to convey other food to the mouth with more readiness. The elongated, protrusible tongue of recent species, as well as of the remaining representatives of the order, gives a clew to the nature of the tongue of the extinct giant armadillos, which was probably used as a herbage-grasping organ, as in the giraffe, and as Prof. Owen finds reason to believe must have been the case with the great extinct sloths, Megatherium, and its allies. It is highly probable, nay, almost certain, that the prehensile powers of the tongues of the Edentata are intimately associated with their want of incisors, or cutting teeth. Similar disappearance or loss of function of the incisors by ruminants, proboscidians, and rhinoceroses, is similarly correlated with a grasping tongue, trunk, or lips.
In living armadillos the grinding-teeth vary in number from twenty-six to thirty-eight, and are in the form of cylindrical or oval columns. All the Hoplophoridæ have thirty-two grinders, sixteen above and the same number below, without enamel, as in recent forms. Two deep grooves run vertically up and down on both the inside and outside of each tooth, causing the appearance of two deep bays on each side in transverse section, Which is not quite twice as long as wide. Unlike living allied forms, these giants had strong descending processes directed downward from the zygomatic arches (cheekbones), similar to those of the great extinct and small modern sloths; and, like the first of the last mentioned, the bones of the pelvis, hind-limbs, and tail, were relatively more massive than in their existing representatives, showing in these features strong resemblances to the sloth-like division of the order. From the deep implantation of the grinding-teeth in the giant armadillos, one cannot resist the inference that, like the great sloths, they were herbivorous. This idea is further countenanced by their size, which, in terrestrial mammals, is usually an accompaniment of herbivorous habits. The features, however, which unmistakably ally them to living armadillos, are the presence of a third trochanter on the femur, and the union of the tibia and fibula and the annular and tubular armor covering the tail. The animal of our figure has the basal part of the tail surrounded and covered with eight slightly mobile rings of armor plates; each one of these rings is supported on the inside by five strong processes of bone which arise radially or like the spokes of a wheel from each of the first seven caudal vertebræ. The first caudal vertebra supports two of the armor-rings. The last fourteen joints of the tail are inclosed by rigid armor, much the same as the end of the finger is covered by a thimble. These terminal joints, confined within this inflexible bony case, become united into a continuous bony rod. Other species have been described which have the tail covered throughout with rings of armor-plates, within which each joint of the tail is separate, as usual in the tails of other vertebrates. In one living species the tail is almost
naked, whence its name, Dasypus gymnurus. The little three-banded species has the upper face of the tail covered with relatively thick plates. In living specimens I have also noticed that the ears, though very thin, were covered with thin and minute (usually polygonal) scales, both inside and out.
Studies of the feet of the Hoplophoridæ show that they can be divided into two well-defined groups: the first, to which the animal represented in the figure belongs, has four claw-bearing toes on the fore-legs, and four hoof-bearing toes on the hind-legs; the second group has four claw-bearing toes on the fore-legs, and five hoof-bearing toes on the hind ones. As the figure shows, and which is fully supported by the osteology, the hinder extremities are proportionally more massive and longer than the fore ones, which fact, together with the enormously expanded pelvic bones, shows that the creature perhaps raised the fore-part of the body into a more or less nearly vertical position with the help of its tail to reach the leaves of plants upon which it fed, as did its huge congeners, the extinct sloths. This view is favored by the flattened condition of the tail-case or armor toward its extremity, perhaps from the pressure to which it was often subjected from below while in the bipedal position. This also explains the use of the belly-shield to have been to afford protection against enemies from below while in such an attitude, as the animal, because so well protected otherwise, was probably less favored in respect to sight and hearing.
The carapace was supported for nearly half its length upon the haunch-bones (ilia and ischia), as well as by the strong, longitudinal, median, bony crest rising from the lumbar and sacral vertebræ, consisting of their united neural or spinous processes. The carapace rested directly on these bones, and was joined to them by suture, as the roughened and expanded surfaces for such juncture clearly show. The
entire union of the lumbar and sacral vertebræ into a hollow bony bar, and the union of this to the lateral elements of the pelvic arch, together with the union of both by suture with carapace, rendered any lateral bending of the trunk impossible, so that an almost universal union of the trunk or body segments ensued, owing to this structurally enforced loss of mobility between the vertebral elements. As a consequence, the centra or bodies of the segments disappeared, or were atrophied, leaving only their trough-like plates about one-fourth of an inch thick, formed of the degenerate and united central bodies. This trough, with the united rib-bearing arches that arose from its edges, formed a tube for the lodgment and protection of the spinal or nervous cord. Unlike all vertebrates, except turtles, this tube in that portion over the lungs is perforated at intervals on each side at points about midway of the length of each one of the several united segments to give egress to the spinal nerves. The points of egress for the spinal nerves are usually between the spinous processes in other orders of vertebrates.
In living armadillos the centra of the trunk vertebræ still remain as more or less depressed cylinders of bone, or at least they are distinguishable as centra, from which arise the rib-bearing arches, which do not completely unite, leaving lateral inter-spinous openings so as not to entirely close over the nervous cord, as happens in fossil forms. The reason why the vertebræ remained separated in recent species is undoubtedly because of the mechanical conditions to which these parts of their skeleton were subjected. Here the carapace was jointed and flexible; hence the need of flexibility in the spinal column. In the extinct species, as in turtles, the degeneration of the centra into mere conduits for the nervous cord is one of the many contrivances the origin and ideological significance of which can only be explained by a mechanical theory. The vertebral column in both was similarly conditioned with respect to strains, mostly transverse—hence the similarity of structure; which it must be borne in mind is, however, no indication of zoological affinity.
Beginning with the homogeneous notochord or continuous rod-like axis of some such form as Amphioxus, Mr. Spencer points out how, as this axis became bony with the assumption of the characters of the higher fishes, the alternate pressure and tension incident to the flexures of this axis during locomotive acts would tend to differentiate the vertebral segments; for it is obvious that, in order to be flexible and at the same time bony, the vertebral axis must become segmented. The mechanical conditions under which vertebral axes are placed would indicate that the segmentation took place from within outward, which is in accordance with observed facts. It is also obvious, in view of the premises, that, in the absence of flexures or bendings of the vertebral axis, we should have a return to the homogeneous structure, such as we actually find to result in the two cases under consideration, and as happens in a few of the posterior trunk-segments (sacral) of birds and mammals. Embryology and phylogeny both bear out these conclusions; not only do the vertebral centra become more rudimentary as the young condition is departed from in the life-history of the individual tortoise, but the centra also become successively more rudimentary as we pass from the less completely armored genera Sphargis and Trionyx, to the more completely armored Testudo and Cistudo.
Like the tortoises, our huge animal had an arrangement of the neck vertebræ whereby he could withdraw his head slightly backward in case of an attack, so as to bring his head-shield to fit closely against his carapace. The atlas or first joint of the neck was separate, the next four were united, the sixth was separate, and a "trivertebral bone," which seems to have taken a share in the neck as well as in the thorax, followed next, and probably was the bone which enabled the creature to retract its head somewhat; next followed ten united rib-bearing trunk vertebræ, which Prof. Burmeister has aptly called the "dorsal tube" (see cross-section, Fig. A). Succeeding the "dorsal tube" are eight lumbar and probably eight sacral vertebræ firmly united together and to the ilia; following these, come twenty-one caudal or tail bones, footing up a total of fifty-six segments in the entire spinal column, which is not far from the number found in living species, though only about one-fourth as many are united together in them as in our fossils. The plates of the carapace were united by suture in the fossil species, rendering the armor as rigid as the carapaces of land-tortoises. In living forms, the plates, in some species at least, are slightly separated by intervening integument, rendering the armor more or less flexible throughout.
The remains of the Hoplophoridæ—better known by Prof. Owen's older name as the Glyptodons—have been found mostly in the bone-caves of Brazil, and in the alluvium and pampean Pliocene of Eastern and temperate South America. The finest collection of their remains in existence is in the Public Museum of Buenos Ayres. They were probably contemporaneous with some of the great Carnivora, whose remains have also been found in the caves. One of these, the sabre-toothed tiger (Machairodus), would, no doubt, have frequently rendered the almost invulnerable armor of the giant (but perhaps harmless) armadillo of great service, within which he could feel himself secure from the attacks of such a well-armed foe.
The restoration is one-eighteenth of the natural size, and is based on the figures in Burmeister's work. It is believed to be approximately correct, since nothing was needed to make the originals assume the appearance of life except to clothe the skull and neck with flesh, and furnish the extremities with claws and hoofs, muscles and tendons. The animal was between nine and ten feet in total length, and stood about four and a half feet high at the highest part of the back. Prof. Burmeister has christened the species Panochthus tuberculatus.
- It is but just to refer to Prof. H. Burmeister's magnificent monograph on these animals in the "Anales del Museo Público" of Buenos Ayres, for 1866-73, from which most of the materials which I have used in my studies and comparisons have been drawn. From the wealth of materials at his command he has been enabled to present a fuller account of the osteology of these creatures than any other hitherto published. The memoirs of Owen, Lund, Nodot, Huxley and others, have also been consulted.
- See Brehm's "Thier-leben," vol. ii., p. 508, for an interesting account of this habit.
- The total length of the tongue in the ant-bear (Myrmecophaga), from its origin at the xiphoid end of the breastbone, is three feet.
- In birds, as, e. g., the common fowl, the first segments of the sacrum, the centra of which are similarly atrophied, are perforated laterally in the same situation.