Variation in mandible shape in Thrichomys apereoides (Mammalia : Rodentia): Geometric analysis of a complex morphological structure

The model of development and evolution of complex morphological structures conceived by Atchley and Hall in 1991 (Biol. Rev. 66:101-157), which establ ishes that changes at the macroscopic, morphogenetic level can be statistic ally detected as variation in skeletal units at distinct scales, was applie d in combination with the formalism of geometric morphometrics to study var iation in mandible share among populations of the rodent species Thrichomys apereoides. The thin-plate spline technique produced geometric descriptors of shape derived from anatomical landmarks in the mandible, which we used with graphical and inferential approaches to partition the contribution of global and localized components to the observed differentiation in mandible shape. A major pattern of morphological differentiation in T. apereoides i s attributable to localized components of shape at smaller geometric scales associated with specific morphogenetic units of the mandible. On the other hand, a clinal trend of variation is associated primarily with localized c omponents of shape at larger geometric scales. Morphogenetic mechanisms ass umed Lobe operating to produce the observed differentiation in the specific units of the mandible include mesenchymal condensation differentiation, mu scle hypertrophy, and tooth growth. Perspectives for the application of mod els of morphological evolution and geometric morphometrics to morphological ly based systematic biology are considered.