The traditional point of view that fossil ground sloths (Xenarthra) were a relatively uniform, ecologically little diverse group has been recently challenged. Marine habits have been ascribed to Thalassocnus natans of the Pliocene of Peru. Also, a more diverse diet has been proposed by one of us (R.A.F.) for some Lujanian (late Pleistocene-early Holocene of South America genera of ground sloths. In this paper, an aspect of this latter hypothesis is tested, i.e. that Megatherium americanum had morphological features that are better explained by its having had carnivorous habits rather than by solely herbivorous ones. Specifically, the question of its forearms having been designed for optimizing speed rather than strength of extension is addressed. Such a trait might have been associated with a potentially aggressive use of the animal large claws, whereas a strong extension would be more proper for tearing branches out. On the other hand the high mechanical advantage of the biceps might have made it possible for the animal to have lifted and carried heavy weights. This in turn, suggests the possibility that the animal could have manipulated large prey (for instance, turning dorsally armoured preys or carcasses upside down to expose softer parts and cached large food pieces in a safer place. By this view, Megatherium americanum would be the largest land mammal hunter to have existed.
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