Fossil platyrrhine forelimb bones from the early miocene of Argentina

Primate scapula and ulna fragments of uncertain taxonomic affinity (MACN-SC 101) have been recovered from the Pinturas deposits at Arroyo Feo, Santa Cruz, Argentina in association with Santacrucian (Early Miocene) land mammals. Least-squares regression of body weight on surface area and on height of the glenoid fossa of the scapula indicates an estimated mean weight of 3.6 kg for this individual. On the basis of qualitative and several metric features, the fossil scapula and ulna most closely resemble living platyrrhine monkeys. In estimated body weight and relative height of the coronoid process, the fossil is similar to arboreal quadrupeds, such as Cebus apella and Chiropotes. However, spinoglenoid, axilloglenoid, and axillospinal angles, length of lever arm, and length and breadth of the sigmoid notch imply behavioral similarity with larger species that also use their forelimbs extensively in climbing, such as Alouatta and Lagothrix. MACN-SC 101 may represent the incipient divergence of a generalized platyrrhine arboreal quadruped toward a more suspensory form.