but one of them—the giant aye-aye—are assumed to have been diurnal (due to their body size and small orbits), whereas many small lemurs are nocturnal and medium-sized are cathemeral. Their skeletons suggest that most subfossil lemurs were tree-dwellers, adapted for living in forests and possibly limited to such habitats. Unlike some of the living species, the subfossil lemurs lacked adaptations for leaping. Instead, suspension, used by some indriids and ruffed lemurs, was extensively used in some lineages. Living lemurs are known to visit the ground to varying extents, but only the extinct archaeolemurids exhibit adaptations for semiterrestrial locomotion. Due to the size of the extinct subfossil lemurs, all were likely to travel on the ground between trees. They had shorter, more robust limbs, heavily built axial skeletons (trunks), and large heads and are thought to have shared the common lemur trait of low basal metabolic rates, making them slow-moving. Studies of their semicircular canals confirm this assumption, showing that koala lemurs moved slower than orangutans, monkey lemurs were less agile than Old World monkeys, and sloth lemurs exhibited slow movements like those of lorises and sloths. Types Sloth lemurs The sloth lemurs (family Palaeopropithecidae) were the most species-rich group of the subfossil lemurs, with four genera and eight species. The common name is due to strong similarities in morphology with arboreal sloths, or in the case of Archaeoindris, with giant ground sloths. They ranged in size from some of the smallest of the subfossil lemurs, such as Mesopropithecus, weighing as little as , to the largest, Archaeoindris, weighing approximately . Their characteristic curved finger and toe bones (phalanges) suggest slow suspensory movement, similar to that of an orangutan or a loris, making them some of the most specialized mammals for suspension. Their day vision was very poor, and they had relatively small brains and short tails. Their diet consisted mostly of leaves, seeds, and fruit; dental wear analysis suggests they were primarily folivorous seed-predators. Koala lemurs The koala lemurs of the family Megaladapidae most closely resemble marsupial koalas from Australia. According to genetic evidence they were most closely related to the family Lemuridae, although for many years they were paired with the sportive lemurs of the family Lepilemuridae due to similarities in their skulls and molar teeth. They were slow climbers and had long forelimbs and powerful grasping feet, possibly using them for suspension. Koala lemurs ranged in size from