The Palaeopropithecidae are thought to include four extinct lemur genera, which were present at the time of the first human presence and mignt have been seen by Europeans travellers. It has been suggested that these lemurs survived at least until the 19th century in Madagascar. Mysteriously, a bone of one of these lemurs showing signs of butchering was dated to 257 to 417 BC - before the recognised date of human colonisation of Madagascar.

Their arms are long and their legs are short, in contrast to their living relatives among the Indriidae. They are unusual because of their locomotion, with the most specialised having very hooked hands and feet unsuited to movement on land. It is suggested that these lemurs evolved from smaller lemurs that were also leapers.

These lemurs are called "sloth lemurs" because they all show similarities to sloths, to some degree. Madagascan folklore mentions an "ogre with the body of an animal but the face of a human that could be rendered helpless on smooth rock outcrops because it was unable to move on flat surfaces.