Mandibular ramus shape of Australopithecus sediba suggests a single variable species

Area of Science:

• Paleoanthropology
• Comparative Anatomy
• Hominin Evolution

Background:

• The Malapa cave fossils, Australopithecus sediba (MH1 and MH2), exhibit distinct mandibular ramus morphologies.
• Previous research has proposed various explanations for these differences, including ontogeny, sexual dimorphism, or distinct taxonomic assignments.

Purpose of the Study:

• To investigate the causes of morphological variation between the Australopithecus sediba specimens MH1 and MH2.
• To test hypotheses of ontogenetic variation, sexual dimorphism, and potential multi-taxon representation.

Main Methods:

• Digitization of 2D sliding semilandmarks from MH1, MH2, and comparative samples of extant apes (Gorilla, Pan, Pongo) and humans (Homo).
• Quantitative analysis of shape variation within and between extant species and the fossil individuals.

Main Results:

• Significant intraspecific shape variation was observed in extant species, unrelated to ontogeny or sexual dimorphism.
• Morphological differences between MH1 and MH2 are unlikely to be explained by ontogeny or sexual dimorphism based on comparisons with extant taxa.
• The observed variation between MH1 and MH2, while notable, falls within the range of intraspecific variation for extant species and does not definitively support multi-taxon hypotheses.

Conclusions:

• Australopithecus sediba specimens MH1 and MH2 represent a single species.
• The substantial morphological variation within Australopithecus sediba is not attributable to ontogenetic changes or sexual dimorphism.
• These findings underscore the complexity of hominin variation and the need for careful consideration of intraspecific diversity in taxonomic assessments.