Sloth lemurs are extinct species that are the sister clade to the Indiids. They have the widest range of body sizes of any of the Madagascar species. They are called sloth lemurs because of some convergent features they display with sloths – including suspensory behavior.
Earlier, extremely fanciful, interpretations had them as aquatic. For example:
Italian paleontologist Guiseppe Sera embraced Standing’s aquatic theory of giant sloth lemur locomotion and carried it further. Beginningin 1935 with a confused assemblage that included a humerus and radius of Megaladapis, a fibula of Megaladapis misidentified as a clavicle, and the astrago- navicular of a crocodile, Sera… reconstructed Palaeopropithecus as an arboreal-aquatic acrobat with a locomotor repertoire combining climbing, diving, and swimming. In 1938 he broadened his aquatic theory to encompass other extinct lemurs… Megaladapis, for example, became a dorsoventrally flattened skate- or ray-like swimmer, its underwater concealment while feeding on aquatic mollusks and crustaceans facilitated by the varus orientation of its knee and rotation of the iliac blade into the frontal plane! (Godfrey and Jungers 2003 – afarensis)
I mention this because Science Daily mentions that a new species of Palaeopropithecus has been discovered:
Recent discoveries by the MAPPM… on sites in northwest Madagascar have established the existence of a third species of Palaeopropithecus, which has been dubbed P. kelyus. Scientists have suspected the existence of this species for more than 20 years. P. kelyus, whose weight is estimated around 35 kg, is smaller than the two known Palaeopropithecus species, but is very large in comparison with the largest living lemur, the Indri, which weighs only 10 kg.
The paper announcing the find is available here – but requires a subscription.
Additional Literature on Sloth Lemurs in General
Ancient DNA from giant extinct lemurs confirms single origin of Malagasy primates Karanth et al2005 PNAS 102(14):5090-5095
The Extinct Sloth Lemurs of Madagascar Godfrey and Jungers, 2003 Evolutionary Anthropology 12:252–263