A synopsis of the phylogeny and paleobiology of Amphipithecidae, South Asian middle and late Eocene primates

Amphipithecidae of late middle Eocene to late Eocene of Myanmar and Thailand is a phylogenetically enigmatic group that some place with Anthropoidea and others with Adapoidea. A linkage with adapoids is hard to demonstrate because it relies largely on a series of similarities that are arguably symplesiomorphies of Primates as a whole. The possibility that amphipithecids are specially related to crown anthropoids (e.g. Aegyptopithecus) is suggested by some shared-derived dental and gnathic anatomy. The postcranial anatomy indicates that the amphipithecids, if they are anthropoids, are probably a distantly related stem group outside the clade of African late Eocene-to-Recent anthropoids. Even the stem-group anthropoid status of amphipithecids is not supported by the absence of postorbital closure and enlarged olfactory bulbs, since postorbital closure and reduced olfactory bulbs characterize a more inclusive crown haplorhine clade of Tarsius plus Anthropoidea. An appealing possibility is that amphipithecids are basal haplorhines whose divergence would have predated the Tarsius-Anthropoidea split. Larger amphipithecids equal or exceed the body size of the largest known Eocene primates. Dental and mandibular anatomy suggests these large-bodied amphipithecids were fruit and hard-object (nut) feeders. A more primitive contemporary amphipithecid, Myanmarpithecus, was smaller, about 1-2 kg, and its cheek teeth suggest a frugivorous diet but do not imply seed eating. The humerus and calcaneus of a large amphipithecid from Myanmar (Pondaungia or Amphipithecus) suggest a slow-moving arboreal quadrupedal locomotion like that of lorises. A talus of an amphipithcid is more suggestive of an active arboreal quadruped.