Mylodon

Mylodon (Gr. for "mill-tooth" from µuXwv and Mobs), a genus of extinct American edentate mammals, typified by a species (M. harlani) from the Pleistocene of Kentucky and other parts of the United States, but more abundantly represented in the corresponding formations of South America, especially Argentina and Brazil. The mylodons belong to the group of ground-sloths, and are generally included in the family Megatheriidae, although sometimes made the type of a separate family. From Megatherium these animals, which rivalled the Indian rhinoceros in bulk, differ in the shape of their cheek-teeth; these (five above and four below) being much smaller, with an ovate section, and a cupped instead of a ridged crown-surface, thus resembling those of the true sloths. In certain species of mylodon the front pair of teeth in each jaw is placed some distance in front of the rest and has the crown surface obliquely bevelled by From Owen.

Skeleton of Mylodon robustus (Pleistocene, South America).

wearing against the corresponding teeth in the opposite jaw. On this account such species have been referred to a second genus, under the name of Lestodon, but the distinction scarcely seems. necessary. The skull is shorter and lower than in Megatherium, without any vertical expansion of the middle of the lower jaw, and the teeth also extend nearly to the front of the jaws; both these features being sloth-like. In the fore feet the three inner toes have large claws, while the two outer ones are rudimentary and clawless; in the hind-limbs the first toe is wanting, as in Megatherium, but the second and third are clawed. The skin was strengthened by a number of small deeply-embedded bony nodules.

Although the typical M. harlani is North American, the mylodons are essentially a South American group, a few of the representatives of which effected an entrance into North America. when that continent became finally connected with South America. Special interest attaches to the recent discovery in the cavern of Ultima Esperanza, South Patagonia, of remains of the genus Glossotherium, or Grypotherium, a near relative of Mylodon, but differing from it in having a bony arch connecting the nasal bones of the skull with the premaxillae; these include a considerable portion of the skin with the hair attached. Ossicles somewhat resembling large coffee-berries had been previously found in association with the bones of Mylodon, and in Glossotherium nearly similar ossicles occur embedded on the inner side of the thick hide. The coarse and shaggy hair is somewhat like that of the sloths. The remains, which include not only the skeleton and skin, but likewise the droppings, were found buried in grass which appears to have been chopped up by man, and it thus seems not only evident that these ground-sloths dwelt in the cave, but that there is a considerable probability of their having been kept there in a semi-domesticated state by the early human inhabitants of Patagonia. The extremely fresh condition of the remains has given rise to the idea that Glossotherium may still be living in the wilds of Patagonia.

Scelidotherium is another genus of large South American Pleistocene ground-sloths, characterized, among other features, by the elongation and slenderness of the skull, which thus makes a decided approximation to the anteater type, although retaining the full series of cheek-teeth, which were, of course, essential to an herbivorous animal. The feet resemble those of Megatherium. A much smaller South American species represents the genus Nothrotherium.  In North America Mylodon was accompanied by another gigantic species typifying the genus Megalonyx, in which the fore part of the skull was usually wide, and the third and fourth front toes carried claws. Another genus has been described from the Pleistocene of Nebraska, as Paramylodon; it has only four pairs of teeth, and an elongate skull with an inflated muzzle. All the above genera differ from Megatherium in having a foramen on the inner side of the lower end of the humerus. A presumed large ground-sloth from Madagascar has been described, on the evidence of a limb-bone, as Bradytherium, but it is suggested by Dr F. Ameghino that the specimen really belongs to a lemuroid. Be this as it may, the North American mammals described as Moropus and Morotherium, in the belief that they were ground-sloths, are really referable to the ungulate group Ancylopoda.

Although a few of the Pleistocene ground-sloths, such as Nothropus and Nothrotherium (= Coelodon), were of comparatively small size, in the Santa Cruz beds of Patagonia few of the representatives of the family much exceeded a modern sloth in size. The bestknown generic types are Eucholoeops, Hapalops and Pseudahapalops, of which considerable portions of the skeleton have been disinterred. In these diminutive ground-sloths the crowns of the cheek-teeth approached the prismatic form characteristic of Mega[lo]therium, as distinct from the subcylindrical type occurring in Mylodon, Glossotherium, &c.

By many palaeontologists a group of North American Lower Tertiary mammals, known as Ganodonta, has been regarded as representing the ancestral stock of the ground-sloths and those of other South American edentates; but according to Professor W. B. Scott this view is incorrect and there is no affinity between the two groups. If this be so, we are still in complete darkness as to the stock from which the South American edentates are derived.

See W. B. Scott, Mammalia of the Santa Cruz Beds, Edentata, Rep., Princeton Exped. to Patagonia, vol. v. (1903-1904); B. Brown A New Genus of Ground-Sloth from the Pleistocene of Nebraska, Bull. Amer. Mus. Nat. Hist., xix, 569 (1903). (R. L.*)