First articulating os coxae, femur, and tibia of a small adult Paranthropus robustus from Member 1 (Hanging Remnant) of the Swartkrans Formation, South Africa

Abstract

Since paleontological work began there in 1948, Swartkrans (South Africa) has yielded hundreds of Early Pleistocene hominin fossils, currently attributed to (in ascending order of quantity) cf. Australopithecus africanus, Homo spp., and Paranthropus robustus. The bulk of that large sample comprises craniodental remains, with (mostly fragmentary) postcranial materials being much less abundant at the site. In that context, our announcement here of the first articulating partial os coxae, nearly complete femur, and complete tibia of a young adult hominin (SWT1/HR-2), excavated from the <2.3 to >1.7-million-year-old Hanging Remnant (Member 1) of the Swartkrans Formation, represents an important addition to the understanding of hominin postural and locomotor behavior in Early Pleistocene South Africa. We provide qualitative and quantitative descriptions and initial functional morphological interpretations of the fossils, based mostly on external bone morphology. Epiphyseal fusion data, element dimensions, the crural index, and live body stature and mass estimates that we provide all indicate that SWT1/HR-2 is one of the smallest known adult hominins in the fossil record. We discuss the paleobiological implications of these findings in relation to our taxonomic diagnosis of SWT1/HR-2 as representing P. robustus.