The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Sliding semi-landmarks on symmetric structures in three dimensions

STEFAN SCHLAGER.

Anthropology, University of Freiburg, Germany

Saturday All day, Plaza Level Add to calendar

Analysing the symmetry/asymmetry of biologcial structures has entered the morphometric toolbox more than a decade ago. It allows the quantification and evaluation of directional and fluctuating asymmetry. Additionally, the application of semi-landmarks makes geometric analysis of structures possible where homologous landmarks can only be placed very sparsely. But how can both methods be combined and what has to be thought of beforehand? Usually semi-landmarks are placed semi-automatically by estimating the positions on the target surface through projection from a reference configuration. The implications for asymmetry are quite clear: All configurations will show asymmetry inferred both by asymmetry biologically inherent in the reference and asymmetry caused by manual definition of the semi-landmarks on the reference’s surface. To circumvent these problem, a regularisation is needed. This can be achieved by using sliding semi-landmarks that are relaxed against a symmetrised average. The result are configurations containing semi-landmarks unbiased by their initial reference and “as symmetric as possible”. To test the effects of this method, the amount of directional and fluctuating asymmetry are estimated from the sample containing only “real” landmarks and then compared to the results of tests containing semi-landmarks both relaxed against the ordinary sample’s average and a symmetrised one: While our method and the classical one showed only mild signs of directional asymmetry, the one using non-regularised semi-landmarks, showed strong directional asymmetry, resembling the asymmetry found in the reference configuration.

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