Talar Morphology, Phylogenetic Affinities, and Locomotor Adaptation of a Large-Bodied Amphipithecid Primate From the Late Middle Eocene of Myanmar

A well-preserved fossil talus [National Museum of Myanmar Primates (NMMP) 82] of a large-bodied primate is described from the late middle Eocene Pondaung Formation of central Myanmar. The specimen was collected at Thandaung Kyitchaung, a well-known amphipithecid primate-bearing locality near the village of Mogaung. NMMP 82 adds to a meager but growing sample of postcranial remains documenting the large-bodied primates of the Pondaung Formation. This new talus exhibits a suite of features that resemble conditions found in living and fossil haplorhine primates, notably anthropoids. As such, the phylogenetic signal deriving from the morphology of NMMP 82 conflicts with that provided by NMMP 20, a partial skeleton (including a fragmentary calcaneus) of a second large-bodied Pondaung primate showing undoubted adapiform affinities. Analysis subtalar joint compatibility in a hypothetical NMMP 82/NMMP 20 combination (talus/calcaneus) reveals a substantial degree of functional mismatch between these two tarsal bones. The functional incongruence in subtalar joint morphology between NMMP 20 and NMMP 82 is consistent with the seemingly divergent phylogenetic affinities of these specimens, indicating that two higher level taxa of relatively large-bodied primates are documented in the Pondaung Formation. On the basis of its size and morphology, we refer the NMMP 82 talus to the large-bodied amphipithecid Pondaungia. The occurrence of anthropoid-like tali in the Pondaung Formation obviates the need to invoke homoplasy to explain the shared, derived dental characters that are common to amphipithecids and undoubted anthropoids. Functionally, the NMMP 82 talus appears to have pertained to a primate that is engaged in active quadrupedalism in an arboreal environment along broad and subhorizontal branches. The primate taxon represented by NMMP 82 was capable of climbing and leaping, although it was not particularly specialized for either of these activities.