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Megalonychoidea (ground sloths; suborder Xenarthra, infra-order Pilosa) An extinct superfamily of ground-dwelling edentates which are known first from the Oligocene but which thrived during the Pleistocene in N., Central, and S. America, and which may be broadly ancestral to modern tree sloths. Early forms attained a length of little more than a metre; but later much larger forms appeared, including Megatherium, which was more than 6 m long, and more massive than an elephant. Ground sloths had claws on all five digits on each limb, the claws in some species being so large as to require the animal to walk with its feet turned on their sides, as do modern New World ant-eaters. The teeth were reduced and simple and some may have possessed horny plates used in cropping vegetation. Later ground sloths were contemporaries of early humans: a skeleton of Nothrotherium (an animal about the size of a tapir) has been found that shows signs of having been killed by humans. In the W. Indies ground sloths evolved into dwarf forms, some no larger than a cat.