A history of land mammals in the western hemisphere

by William Berryman Scott

The recorded history of the edentates was developed almost entirely in South America. In the Casa Mayor formation there were numerous armadillos, but as only scutes of the carapace have been found, little is known of them. The †ground-sloths (†Protobradys) have been reported, but from such imperfect material that the reference is uncertain. The first assuredly determinable members of this suborder were in the Astraponotus beds and, associated with them, the most ancient known †glyptodonts. In the Deseado stage were many armadillos, some of them extremely peculiar, several †glyptodonts and †ground-sloths, some species of the latter very large. Edentates were far more numerous and varied in the Santa Cruz than in any of the preceding stages. Tree-sloths and anteaters have both been reported, but the evidence is insufficient, though there can be little doubt that these suborders had begun their separate existence in some part of South America other than Patagonia. The three families of †ground-sloths were already distinguishable, though much less clearly separated than they afterwards became; none of them were large animals, smaller even than some of the Deseado species and veritable pygmies in comparison with the giants of the Pliocene and Pleistocene. The †glyptodonts were likewise far smaller than their Pliocene and Pleistocene successors and in several respects more primitive, approximating the armadillos more closely; nor was there any such variety of forms as in the later stages. The armadillos were extremely numerous and varied; they all belonged to extinct genera and most of them apparently have no descendants at the present day. The tropical forests of Brazil and the Guianas must then, as now, have swarmed with mammals which did not extend their range to Patagonia and of which we consequently have no record. No doubt, it was in these forests that the ancestors of most modern armadillos, as well as of the tree-sloths and anteaters, lived in Miocene times.

Pliocene edentates were of the same suborders as those of the Santa Cruz, but far larger in size. Most of them are known only from very incomplete specimens, but the Pleistocene has yielded an enormous mass of beautifully preserved material. Of the tree-sloths and anteaters, only questionable remains have been found. That these tropical and forest-loving animals should not have occurred in the open Pampas of Argentina is not surprising, but it is difficult to account for their absence from the extremely rich cave-faunas of Brazil. Nearly all the existing genera of armadillos have been obtained, and with these were associated several extinct genera, some of them (†Chlamydotherium, †Eutatus) relatively huge, as large as tapirs. There was a wonderful variety of †glyptodonts, most of them enormous creatures, of which no less than five genera have been collected in Argentina and Brazil, and the †ground-sloths were even more numerous and varied. Nine genera, with many species, of these great beasts, which ranged in size from an elephant to a tapir, are already known and no doubt the list is still incomplete. These †glyptodonts and †ground-sloths must have been among the most conspicuous elements of the Pleistocene fauna.

Aside from certain problematical Eocene forms, the first North American edentates, which were immigrants from the southern continent, appeared probably in the middle Miocene of Oregon in the form of †ground-sloths, but the specimen, as well as a similar one from the lower Pliocene of Nebraska, is not sufficiently complete for positive reference. In the middle Pliocene the †ground-sloths and †glyptodonts were unquestionably present, and in the Pleistocene these two suborders were numerously and conspicuously represented. Three or four genera of the huge, elephantine †ground-sloths coexisted in Pleistocene North America. †Megalonyx was abundant in the forested regions east of the Mississippi, from Pennsylvania southward, and on the Pacific coast; †Mylodon was transcontinental in distribution; while †Megatherium was apparently confined to the southern states. While all three genera undoubtedly originated in South America, †Megalonyx has not yet been found in that continent.

This genus was originally named by President Jefferson in 1805 from an ungual phalanx found in a cave in Virginia, and he imagined that it belonged to a colossal lion which must still be living in the mountains of western Virginia. This was deduced from the assumption that no species could become extinct, and the passage is of interest as showing the prevalent belief of the time, although Cuvier had already demonstrated that many species had actually been extinguished. The passage is as follows: “The movements of nature are in a never ending circle. The animal species which has once been put into a train of motion is still probably moving in that train. For, if one link in nature’s chain might be lost, another and another might be lost, till this whole system of things should evanish by piecemeal.”

The †glyptodonts were also southern in distribution, and only very imperfect remains of them have yet been recovered from the North American Pleistocene, quite sufficient, however, to make the identification certain.

There were several genera of rather small †ground-sloths in the Pleistocene of Cuba. The best known of these, †Megalocnus, had several peculiarities of structure, but was plainly a member of the †Megalonychidæ. The ancestors of this genus probably invaded Cuba in the Pliocene, when the island was joined to Central America.

Suborder †Gravigrada. †Ground-sloths

As the †ground-sloths would appear to have had a more central position within the order than any of the other groups, our study of development may well begin with them. In the Pleistocene there were three families of these gigantic brutes, which ranged through the western hemisphere from Pennsylvania and California to Patagonia. Unfortunately our knowledge of the developmental stages within the different families is very unequal, and it is therefore impracticable to do more than sketch the changes of the suborder as a whole and in a general way. In the successive geological stages the proportionate representation of the different phyla varied greatly; in the South American Pliocene and Pleistocene the †Mylodontidæ and †Megatheriidæ were the abundant forms, while the †Megalonychidæ were but scantily represented. In the Santa Cruz Miocene, on the other hand, the overwhelming preponderance was with the †Megalonychidæ, the other two families being comparatively rare and incompletely known. From the still more ancient formations, the material so far collected is so fragmentary that family distinctions have little meaning. After all, there was no very wide range of variation among the contemporary members of the three families, and the differences were principally in size, in the form and number of the teeth, the shape of the skull and the number of digits; in essentials they were all much alike.

The genus †Megatherium ([Fig. 122, p. 220]) included the largest and most massive members of the suborder, †M. americanum being as large as an elephant, but very differently proportioned, as it was much longer and lower in stature, owing to the shortness of the extraordinarily heavy limbs; some of the skeletons measure 20 feet or more in length. The teeth, which were 5/4 in number, formed an uninterrupted series on each side; all had the same quadrate form and by abrasion were worn into two transverse ridges, formed by the meeting of the harder dentine with the thick coating of cement. The result was a form of tooth which much resembled the lower molars of a tapir, but it was not a tooth-pattern in any proper sense of the word, being due entirely to the mode of wear.

The skull was very small in proportion to the huge body and was low and narrow in shape; the cranium had a broad, flat roof, without sagittal crest; the orbit was completely encircled in bone, and the descending process of the zygomatic arch beneath the eye was very long and conspicuous. The nasals were short, and the slender, toothless premaxillaries projected far in front of them, which makes the presence of some sort of a proboscis likely. The lower jaw had a long, narrow, spout-like symphysis, which was abruptly rounded at the free end, not pointed; below the teeth, the lower margin of the jaw was very strongly convex, descending in a great flange. The neck was short, the body very long and enormously heavy, as was also the tail. The immense shoulder-blade had a very long acromion, which curved forward and inward, fusing with the coracoid and forming a bony loop or bridge. The hip-bones had the anterior element (ilium) enormously expanded transversely, so as to support the huge mass of viscera in the semi-erect position which the animal, it is believed, frequently assumed in feeding. Collar-bones were present.