Megalonychidae is a group of
sloths including the extinct
Megalonyx and the living
two toed sloths. Megalonychids first appeared in the early
Oligocene, about 35 million years ago, in southern Argentina (
Patagonia), and spread as far as the
Antilles by the early
Miocene. Megalonychids first reached North America by
island-hopping, about 9 million years ago, prior to the formation of the
Isthmus of Panama. Some lineages of megalonychids increased in size as time passed. The first species of these were small and may have been partly tree-dwelling, whereas the
Pliocene (about 5 to 2 million years ago) species were already approximately half the size of the huge
Late Pleistocene Megalonyx jeffersonii from the
last ice age. Some
West Indian island species were as small as a large cat; their dwarf condition typified both tropical adaptation and their restricted island environment. This small size also enabled them a degree of arboreality.