Geometric morphometric analysis of mandibular shape diversity in Pan

Area of Science:

• Paleoanthropology
• Comparative Anatomy
• Biomechanics

Background:

• The mandibular corpus is frequently found in the hominin fossil record.
• Previous studies have not deeply explored 3D mandibular corpus shape differences between adult bonobos (Pan paniscus) and chimpanzees (Pan troglodytes) using geometric morphometrics (GM).
• GM offers quantitative analysis and visualization of 3D skeletal shape changes, complementing traditional 2D methods.

Purpose of the Study:

• To determine if GM techniques can differentiate mandibular corpus shape between bonobos and chimpanzees.
• To explore the implications of these shape differences for understanding hominin evolution.
• To assess the utility of mandibular shape for taxonomic analyses of fossil hominoids.

Main Methods:

• Collected 18 mandibular landmarks from adult bonobos and chimpanzees using a Microscribe 3DX digitizer.
• Applied Generalized Procrustes analysis for landmark superimposition and multivariate statistics for shape analysis.
• Analyzed size-adjusted shapes and residuals from regression on centroid size.

Main Results:

• Size-adjusted mandibular corpus shapes distinguished bonobos and chimpanzees with ~93% accuracy.
• Significant Procrustes distances were found between the two species.
• Shape differences were largely size-related, but the mandibular symphysis proved particularly useful for differentiating Pan species.
• Chimpanzee subspecies and Australopithecus specimens showed lower identification accuracy (≤75%).

Conclusions:

• Geometric morphometrics effectively distinguish bonobo and chimpanzee mandibular corpus shapes.
• The mandibular symphysis is a key region for taxonomic identification in fossil hominoids.
• These findings have implications for hominoid phylogeny and the interpretation of fossil hominin mandibles.