Mandibular ramus morphology and species identification in Australopithecus sediba

• Published in: South African Journal of Science 2022-03, Vol.118 (3), p.95-97
• Main Authors: Hawks, John, Berger, Lee R
• Format: Article
• Language: English
• Subjects: Australopithecus; Australopithecus sediba; Fragments; Hominids; Homo; Hypotheses; Malapa; Mandible; Morphology; Species
• ISSN: 0038-2353; 1996-7489
• DOI: 10.17159/sajs.2022/12544

The site of Malapa, South Africa, has produced fossil evidence from multiple individuals of Australopithecus sediba including the par tial skeletons designated as MH1 (holotype) and MH2 (paratype). A recent ar ticle in this Journal presented the hypothesis that MH1 and MH2 are not one species but instead represent two different genera: Australopithecus and Homo, respectively. Here we briefly evaluate this claim. We review the evidence from across the skeleton that demonstrates that MH1 and MH2 represent a single species, and we highlight other fossil samples that show the same pattern of mandibular ramus variation as observed in MH1 and MH2. The evidence shows that there is no reason to separate MH1 and MH2 into different species or genera based upon mandibular ramus morphology. This case illustrates how misleading small fragments of anatomy can be, why researchers should not use such fragments par ticularly for species and genus-level diagnoses, and why it is essential to use all available evidence. This study shows that the mandibular variation that is present in fossils from Malapa attributed to Australopithecus sediba has parallels in both Australopithecus africanus and in Homo. This helps to demonstrate that mandibular form is not sufficient to provide evidence of species diagnosis, but also that the development and adaptations to diet in Au. sediba were overlapping with those present in other related species of hominins. The MH1 and MH2 skeletons are among the most complete known for Australopithecus, dating to approximately 1.977 million years ago.1,2 The preserved elements of each skeleton include portions of upper and lower limb, thorax, pelvis, mandible, dentition and, for MH1, the face and cranial vault.1,3 These remains are among the most studied of any early hominin specimens. Excavation at Malapa has recovered substantial evidence of the burial position of each skeleton, including joints found in articulation or in close anatomical proximity, with all recovered parts showing a low degree of post-mortem dispersion.4 Additional context comes from the different ontogenetic stages and biological sex of the two skeletons. MH2 is adult and MH1 is juvenile with postcranial and dental elements consistent with a maturational age of between 9 and 11 years when compared to a chimpanzee maturational pattern.5 The pelvic remains of MH1 and MH2 are closely similar in size, and similar in most aspects of morphology, but differ in features related to sex, su