Мorphological integration of the cranium and axial skeleton in european newts

Abstract

Using micro-CT scanning and 3D geometric morphometrics of newt craniums and axial skeletons (firstthree vertebrae) we explored the pattern of morphological integration. We tested if i) directly connected seriallyhomologous structures are more integrated than separated ones and ii) morphological integration coincides withregional differentiation. We applied a multilevel approach by analyzing patterns of integration at static andevolutionary levels, i.e., within and between species respectively. At the static level we choose the genus Triturusas a representative monophyletic group. We analysed between-individual variation in shape to detect functionalmodules and within-individuals the asymmetric component of variation in shape to detect developmental modules.At the evolutionary level, 17 species from five genera were analysed in phylogenetic context and taking effects ofallometry on modularity and integration into account. We found that allometry is an important integrating factorin serially homologous structures. At the static level and after the correction for allometry, functional integrationbetween the cranium and first vertebrae was weak but statistically significant between all elements, anddevelopmental integration was significant between the cranium and the atlas and first and second trunk vertebrae.At the evolutionary level, the cranium, atlas and trunk vertebrae separate as three different modules. Our resultssuggest that, at the evolutionary level, morphological integration coincide with regional and functionaldifferentiation of the axial skeleton. This allows the relatively independent evolution of the cranial skeleton andthe vertebral column, separate of the significant functional and developmental integration at the static level.