Placenta and fetal membranes of Hoffmann's Two-toed Sloth (Choloepus hoffmanni) from Turner 1872 |
It was confirmed that two-toed and three-toed sloths are not closely related and group with different forms of ground sloths. A third group with a suspensory life style was found to have diverged even earlier. These sloths crossed a land bridge to the Greater Antilles and went extinct just a few thousand years ago.
Interhaemal barrier of the pale-throated three-toed sloth (Bradypus tridactylus) Courtesy of Allen C. Enders |
Given current opinion on the relations between sloths (Folivora), anteaters (Vermilingua) and armadillos (Cingulata), parsimony dictates that their common ancestor would have had a villous haemochorial placenta. Thus the sloth placenta represents a derived state. The most recent common ancestor of two-toed and three-toed sloths lived >25 million years ago whereas sloths diverged from anteaters >50 mya.
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