The apparent commitment of this family to arboreal suspensory locomotion has earned it the nickname of “sloth lemurs,” with the genus Palaeopropithecus probably being the most specialized of the group (Fig. 3.4). It had a wide distribution in Madagascar, having been found at sites in the south, southwest, center, northwest, and north of the island. For many years, its locomotor adaptations were the subject of heated debate, but this was resolved in the mid-1980s with the discovery of an associated and relatively complete skeleton in the cave of Anjohibe in northwest Madagascar. As in other dedicated suspensory forms, joints throughout the skeleton of this lemur are built for flexibility rather than for stability and strength. The exception is in the digits of the hand and feet, where stability is most needed when hanging below branches. The first digit is missing and the long, curved phalanges of the rest have tongue-in-groove joints that limit movement to one plane and maximize power grasping. Three species of Palaeopropithecus have been described, Palaeopropithecus ingens, P. maximus, and P. kelyus, but some experts do not distinguish between the first two. P. kelyus, on the other hand, which was only recently discovered in northwestern Madagascar, is smaller and quite distinct (W. L. Jungers, unpubl.) (Figs. 3.9 and 3.10). Body weight estimates for the genus range from 35–50 kg (Jungers et al., 2002), and the diet probably consisted largely of fruits and seeds (Godfrey et al., 2004a). Subfossil specimens from Ankilitelo provide a date for Palaeopropithecus of 510–680 years ago (Simons et al., 1995), and relict populations may have survived until as late as A.D. 1300–1620 (Burney et al., 2004).
Archaeoindris fontoynontii was a much larger lemur, closer to the size of a male gorilla, and very possibly tipped the scales at approximately 200 kg (Godfrey and Jungers 2002). It is known only from the site of Ampasambazimba on the central plateau, and even there only from rare fossils. Its very large size argues for more of a terrestrial existence than that of Palaeopropithecus, perhaps more akin to the giant ground sloths of the Americas that became extinct at the end of the last Ice Age (Lamberton, 1934b; Jungers, 1980). Its postcranial anatomy, especially its hip joint mobility, however, does suggest some sort of adaptation to an arboreal lifestyle (Vuillaume-Randriamanantena, 1988), and Godfrey and Jungers (2002) speculated that it is a “capable, but deliberate, scansorial browser, and that it also frequented the ground to feed and travel.” Experts believe that this species was still living on the high plateau of Madagascar when people arrived on the west coast at about B.C. 350 (Burney et al., 2004).
Babakotia radofilai was another suspensory arboreal lemur from northern Madagascar that, at only 16–20 kg, was significantly smaller than either Archaeoindris or Palaeopropithecus (Godfrey and Jungers, 2002; Jungers et al., 2002) (Figs. 3.3 and 3.11). It is believed to have been a fruit and seed eater, an adaptation that fits with its smaller size (Godfrey et al., 2004a). Its skull very much resembles that of Indri, but the forelimbs are much more elongated, and the hands and feet appear to have been more adapted for strong grasping. The hind foot was also comparatively reduced, which is not characteristic of animals that leap. The single radiocarbon date available for this species places it on the Ankarana Massif at roughly BP 5000, well before humans arrived on Madagascar.
The genus Mesopropithecus contained the smallest of the sloth lemurs (Fig. 3.3). Remains of three species, Mesopropithecus dolichobrachion, M. globiceps, and M. pithecoides, have been found in central and southern Madagascar, and their crania most closely resemble those of modern day sifakas (Propithecus). Post-cranial material, however, is very different from that of modern day indriids. Forelimbs, rather than hind limbs, are elongated, favoring suspensory postures and de-emphasizing leaping as a form of locomotion. Estimated body weights range from 10–14 kg (Jungers et al., 2002). The consensus is that M. pithecoides, which inhabited the high plateau, was largely a folivore, while the other two species were mixed feeders, eating mostly fruits and seeds (Godfrey et al., 2004a). As a group, these species were more quadrupedal than other members of the family, but did engage in suspensory behavior to some degree. Subfossil remains indicate that Mesopropithecus survived at least until A.D. 245–429 (Burney et al., 2004).