Abstract
As one of the essential components of the fossil record, shells of molluscs provide crucial data for taxonomic, phylogenetic or evolutionary studies. The mollusc shell has often very few discrete morphological characters, and its most important character is its three-dimensional (3-D) geometry, which is well diversified, continuous and highly integrated. However, standard morphometric methods remain mostly two-dimensional (2-D) and do not account for the ontogenetic changes of the shell, which are preserved thanks to its accretionary mode of growth. This study proposes a new non-destructive method enabling acquisition of three-dimensional quantitative morphometric parameters that thoroughly describe the geometry of coiled mollusc shells throughout their ontogeny. First, digital three-dimensional data of a shell is acquired by means of microcomputed tomography, which produces a series of grey-scaled, two-dimensional images. Second, all these stacked images are processed to obtain a three-dimensional reconstruction of the shell, from which a centreline is extracted. Finally, the geometry of shell aperture through ontogeny is extracted by successive cross-sectioning of the shell with a succession of planes, each of these being perpendicular to this centreline. The resulting outlines of the successive apertures can be quantified by elliptic Fourier analysis. These geometric parameters, coupled with the displacement vector of the successive cross-sectioning planes, constitute an n-dimensional morphometric space, in which ontogenetic trajectories of different individuals and species can be compared. The approach proposed in this study provides a basis for the quantitative analysis of growth patterns within and across species.
Monnet, C