|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.