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A juvenile of Pterocnemia pennata (MACN 14780) from the Bonaerian (early Late Pleistocene) locality of Salto (Salto district), in the north of Buenos Aires Province, Argentina is presented. This is the northernmost record for the species. The specimen consists of the hind limbs and fragments of fore limbs, phalanxes and vertebrae. The aim of this work is to describe and analyze those features of the hind limb that could indicate the postnatal ontogenetic stage (juvenile, subadult and adult) of the fossil. Characteristics related to the osseous maturation of the epiphyses as well as proximal and distal regions of the bones and to the periosteum texture were analyzed. The specimen hind limbs showed features which could be attributed to an immature age: the femur lacks the intercondylar region and the proximal epiphyses of the tibiotarsus and tarsometatarsus are absent. In addition, the proximal region of the tarsometatarsus is expanded and bulky and the metatarsals are not completely fussed among each other The texture of the periosteum is characterized by numerous porous in the femur and striations in the tibiotarsus and tarsometatarsus, which are indicative of immatureness. The assignment to the postnatal ontogenetic stage could be misinterpreted if the fragmentary remain belongs to those parts that do not show immaturity features in a juvenile (e.g. proximal region of femur; distal region of tibiotarsus and/or tarsometatarsus). The presence of a new specimen of Pterocnemia pennata in a territory outside its current geographical distribution is another evidence that reinforces the hypothesis of arid conditions for this region during the early Late Pleistocene.