MEGATHERIUM (properly Megalotherium), a huge extinct edentate mammal from the Pleistocene deposits of Buenos Aires, typifying the family Megatheriidae (or Megalotheriidae), and by far the largest representative of the Edentata. Except, indeed, for its relatively shorter limbs Megatherium americanum rivalled an elephant in bulk, the total length of the skeleton being 18 feet, five of which are taken up by the tail. The Megatheriidae, which include a number of genera, are collectively known as ground-sloths, and occupy a position intermediate between the sloths and the ant-eater: their skulls being of the type of the former, while their limbs and vertebrae conform in structure to those of the latter. As in the other typical South American edentates, there are no teeth in the front of the jaws, while those of the cheek-series usually comprise five pairs in the upper and four in the lower. In nearly all the other Pleistocene forms these teeth were subcylindrical in shape, with the summit of the crown (except sometimes in the first pair) forming a cup-like depression; enamel being in all cases absent. From all these Megatherium differs in the form and structure of the teeth.
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Fig. 1.—Skeleton of the Megatherium, from the specimen in the Museum of the Royal College of Surgeons of England. |
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| (From Owen.) |
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Fig. 2.—Lower Jaw and Teeth of Megatherium. |
In form, as shown in fig. 2, the teeth are quadrangular prisms, each of which is surmounted by a pair of transverse ridges. They grew apparently throughout life, and were implanted to a great depth in the jaws, being 7 or 8 in. in length, with a cross-section of at least an inch and a half. The ridges on the crown are due to the arrangement of the vertical layers of hard dentine (fig. 3, d), softer vasodentine (v) and cement (c). The skull is relatively small, with the lower jaw very deep in its central portion, and produced in part into a long snout-like symphysis for the reception, doubtless, of a large and fleshy tongue (fig. 2). Unlike sloths, the megatherium has seven cervical vertebrae; and the spines of all the trunk-vertebrae incline backwards. The pelvis and hind-limbs are much more powerful than the fore-quarters; thereby enabling these animals, in all probability, to rear themselves on their hind-quarters, and thus pull down the branches of trees: if not, indeed, in some cases to bodily uproot the trees themselves. Large chevron-bones are suspended to the vertebrae of the tail, which was massive, and probably afforded a support when the monster was sitting up. The humerus has no foramen, and the whole fore-limb was very mobile. The first front toe was rudimentary, having no phalanges, but the fifth was rather less aborted, although clawless; the other three carried enormous claws, protected by reflected sheaths. The hind-foot is remarkable for the great backward projection of the calcaneum, and likewise for the peculiar shape of the astragalus; the middle toe alone carries a claw, this being of huge size, and ensheathed like those of the fore foot. No trace of a bony armour in the skin has been detected; but, from the evidence of other genera, it may be assumed that the body was clothed in a coat of long, coarse hair. Although similar teeth occur in the phosphorite beds of South Carolina, which may have been transported from elsewhere, no undoubted remains of Megatherium are known from North America.
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| (From Owen.) Fig. 3.—Section of Upper Molar Teeth of Megatherium. |
The typical species ranged from Argentina and Chili to Brazil. For certain small ground-sloths from Patagonia with Megatherium-like teeth, see Mylodon. (R. L.*)