The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


Scaling patterns of talar articular surfaces within Euarchonta

GABRIEL S. YAPUNCICH1,2 and DOUG M. BOYER1,2,3.

1Department of Anthropology, CUNY Graduate Center, 2New York Consortium of Evolutionary Primatology, NYCEP, 3Department of Evolutionary Anthropology, Duke University

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The articular facets of synovial joints must transmit forces while maintaining relatively low stresses. Joints that transmit higher forces should therefore have larger facet areas to prevent overloading. The relative contributions of body mass and muscle-induced forces to joint stress are unclear, but generate opposing hypotheses. If mass-induced forces dominate, facet area should scale with positive allometry to mass. Alternatively, muscle-induced forces should cause facets to scale isometrically with mass. Both scaling patterns have been reported for articular surfaces of the femoral and humeral heads, but more distal elements are less well studied. Additionally, examination of complex articular surfaces has largely been limited to linear measurements, so that “true area” remains poorly assessed. To re-assess these scaling relationships, we examine the relationship between body size and articular surface areas of the talus. Area measurements were taken from microCT scan-generated surfaces of all talar facets from a comprehensive sample of euarchontan taxa. Log-transformed data were regressed on literature-derived log-body mass with both ordinary and phylogenetic least squares regression. Groups representing “all euarchontans”, “all primates” and “anthropoids only” exhibit mainly positive allometry; only the medial tibial facet scales isometrically. Among strepsirrhines, slope estimates tend to be allometric, but confidence intervals include isometry. Scaling coefficients are not correlated with sample size, clade inclusivity, or behavioral diversity of the sample. These results suggest talar facet areas scale proportional to stresses induced by larger body mass, rather than greater muscle forces. Whether this is a difference between proximal and distal joints remains to be addressed.

This research was supported by an NSF grant to DMB (NSF BCS 1125507).

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