Unusual pubic bone morphology in A.L. 288-1 (Australopithecus afarensis) and MH2 (Australopithecus sediba)

Objectives: We describe a novel pelvic feature, the “ventral sulcus,” located on the pubic bone ventrolateral to the pubic symphysis, which is present in A.L. 288-1 (Australopithecus afarensis) and MH2 (Australopithecus sediba). We determine how widespread the appearance of the ventral sulcus is in fossil hominins, modern humans, and other extant hominoids. Materials and Methods: We examine all known hominin fossils that preserve the medial portion of the pubic bone, including A.L. 288-1 and MH2. We also study 217 humans derived from global osteological collections and all genera of extant apes. Results: We illustrate, describe, and define the ventral sulcus and distinguish it from age- and sex-related morphologies that are sometimes present in extant humans. We do not find evidence for the ventral sulcus in our comparative human and ape samples, but find equivocal evidence for its presence in the fossils Sts 14 (Australopithecus africanus) and StW 573 (A. africanus/Australopithecus prometheus). Discussion: Ventral sulci are potentially present in all four known adult Australopithecus fossils that preserve the medial portion of the pubic bone, but not present in MH1, a subadult Australopithecus sediba. This raises questions about whether ventral sulci are widespread in Australopithecus, confined to adults, or, as A.L. 288-1, MH2, StW 573, and Sts 14 have all been estimated as female, whether it is a sex-associated trait. Alternatively, ventral sulci could be taphonomic artifacts or idiosyncratic pathologies. Further research is needed to determine if ventral sulci are functional in origin and how widespread they are in the genus Australopithecus.