Mesopropithecus

Mesopropithecus is an extinct genus of small to medium-sized lemur, or strepsirrhine primate, from Madagascar that includes three species, M. dolichobrachion, M. globiceps, and M. pithecoides. Together with Palaeopropithecus, Archaeoindris, and Babakotia, it is part of the sloth lemur family (Palaeopropithecidae). Once thought to be an indriid because its skull is similar to that of living sifakas, a recently discovered postcranial skeleton shows Mesopropithecus had longer forelimbs than hindlimbs—a distinctive trait shared by sloth lemurs but not by indriids. However, as it had the shortest forelimbs of all sloth lemurs, it is thought that Mesopropithecus was more quadrupedal and did not use suspension as much as the other sloth lemurs. All three species ate leaves, fruits, and seeds, but the proportions were different. M. pithecoides was primarily a leaf-eater (folivores), but also ate fruit and occasionally seeds. M. globiceps ate a mix of fruits and leaves, as well as a larger quantity of seeds than M. pithecoides. M. dolichobrachion also consumed a mixed diet of fruits and leaves, but analysis of its teeth suggests that it was more of a seed predator than the other two species. Although rare, the three species were widely distributed across the island yet allopatric to each other, with M. dolichobrachion in the north, M. pithecoides in the south and west, and M. globiceps in the center of the island. M. dolichobrachion was the most distinct of the three species due to its longer arms. Mesopropithecus was one of the smallest of the known extinct subfossil lemurs, but was still slightly larger than the largest living lemurs. Known only from subfossil remains, it died out after the arrival of humans on the island, probably due to hunting pressure and habitat destruction. Mesopropithecus is a genus within the sloth lemur family (Palaeopropithecidae), which includes three other genera: Palaeopropithecus, Archaeoindris, and Babakotia. This family in turn belongs to the infraorder Lemuriformes, which includes all the Malagasy lemurs. Mesopropithecus was named in 1905 by Herbert F. Standing using four skulls found at Ampasambazimba. He noted that the animal had characteristics of both Palaeopropithecus and the living sifakas (Propithecus). In 1936, Charles Lamberton defined Neopropithecus globiceps (based on one skull from Tsirave) and N. platyfrons (based on two skulls from Anavoha). He thought that Neopropithecus was a separate, intermediate genus between Mesopropithecus and Propithecus. In 1971, paleoanthropologist Ian Tattersall merged N. platyfrons into N. globiceps and Neopropithecus into Mesopropithecus. Until 1986, Mesopropithecus was only known from cranial (skull) remains from central and southern Madagascar, and because these are similar to teeth and skulls of living indriids, particularly those of Verreaux's sifaka (Propithecus verreauxi), Mesopropithecus was often assigned to the family Indriidae. For example, in 1974, Tattersall and Schwartz labeled Mesopropithecus as a sister group to sifakas. With the discovery of an associated skeleton of M. dolichobrachion near Ankarana in 1986, it became clear that Mesopropithecus shared distinct traits with sloth lemurs. Unlike the indriids, but like the sloth lemurs, they had elongated forelimbs and other adaptations for arboreal suspension (hanging in trees), linking them most closely to family Paleaeopropithecidae. A comparison of these morphological traits between the sloth lemurs and indriids suggest that Mesopropithecus was the first genus to diverge within the sloth lemur family. Three species are recognized within Mesopropithecus: The genus Mesopropithecus includes some of the smallest of the recently extinct subfossil lemurs, but all species were still noticeably larger than all living (extant) lemurs. They ranged in weight from 10 to 14 kg (22 to 31 lb). They were also the least specialized of the sloth lemurs, more closely resembling living indriids in both skull and postcranial characteristics. Skull length ranged from 93.4 to 105.5 mm (3.68 to 4.15 in). The dentition and cranial proportions, however, more closely resembled those of the sifakas. The dental formula of Mesopropithecus was the same as in the other sloth lemur and indriids: either 2.1.2.31.1.2.3 or 2.1.2.32.0.2.3 × 2 = 30. Mesopropithecus had a four-toothed toothcomb, like all indriids and most other sloth lemurs. It is unclear whether one of the permanent teeth in the toothcomb is an incisor or canine, resulting in the two conflicting dental formulae. Like other sloth lemurs and indriids, Mesopropithecus had rapid tooth development.