This study reported the body mass (BM) estimates of the Middle Miocene fossil hominoid Nacholapithecus kerioi from Africa. The average BM estimates from all forelimb and hindlimb skeletal elements was 22.7 kg, which is slightly higher than the previously reported estimate of ~22 kg. This study revealed that Nacholapithecus has a unique body proportion with an enlarged forelimb relative to a smaller hindlimb, suggesting an antipronograde posture/locomotion, which may be related to the long clavicle, robust ribs, and some hominoid-like vertebral morphology. Because the BM of Nacholapithecus in this study was estimated to be below 30 kg, Nacholapithecus probably did not have relatively shorter and robust femora, which may result from other mechanical constraints, as seen in extant African hominoids. The BM estimate of Nacholapithecus suggests that full substantial modifications of the trunk and forelimb anatomy for risk avoidance and foraging efficiency, as seen in extant great apes, would not be expected in Nacholapithecus. Because larger monkeys are less arboreal (e.g., Mandrillus sphinx or Papio spp.), and the maximum BM among extant constant arboreal cercopithecoids is ~24 kg (male Nasalis larvatus), Nacholapithecus would be a constant arboreal primate. Although caution should be applied because of targeting only males in this study, arboreal quadrupedalism with upright posture and occasional antipronograde locomotion (e.g., climbing, chambering, descending, arm-swing, and sway) using the powerful grasping capacity of the hand and foot may be assumed for positional behavior of Nacholapithecus.
Keywords: Africa; Miocene; body mass estimation; fossil hominoid; positional behavior.
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