New Additions to the Paranthropus boisei Mandibular Hypodigm from Koobi Fora, Kenya

After teeth, mandibles and mandibular fragments are the best represented element in the early hominin fossil record. Among these, the mandibular hypodigm of Paranthropus boisei is the largest ascribed to a single early hominin species, counting 39 published fossil specimens. Fossils of P. boisei originate from eight fossil localities spread over 1,800 km in a North-South axis across eastern Africa – from the site of Malema in Malawi to that of Konso in Ethiopia. However, the vast majority of the remains originate from Koobi Fora in northern Kenya. Most of the Koobi Fora hominins were discovered during the first decade of exploration of the site (1969-1979), and (besides those singularly important fossils that were published in scientific journals) were described in a major monograph by B. Wood in 1991. Some of the fossils discovered since have yet to be described and analyzed. Here, we describe two previously unpublished hominin mandibles found by the Koobi Fora Research Project led by Meave and Louise Leakey. The fossils - KNM-ER 42709 and KNM-ER 42801– were discovered at Koobi Fora within areas 6A and 8B respectively. KNM-ER 42709 is a left edentulous mandibular corpus with roots of M3 to I1 present, and derives from a stratigraphic context consistent with an age of c. 1.4 million years (Ma), making this the last appearance of P. boisei in Kenya. KNM-ER 42801 is a mandibular corpus fragment with the crown of the M3 present, and the stratigraphic context suggest a date of c. 1.55 Ma. Both fossils show mandibular and dental dimensions, as well as autapomorphic traits, typical of P. boisei, as identified at the time of discovery. We explore quantitatively the placing of these two fossils within the diversity of P. boisei mandibles, and discuss their implications for the evolutionary history of the species.