Ontogenetic Trajectories of the 3-D Morphology of Gastropods and Their Phylogenetic Signal

The fossil record provides evidence for evolutionary changes and may contribute to our understanding of the tempo and mode of evolution. Marine organisms with a broad paleogeographical distribution, excellent stratigraphic record, high diversity/disparity, and fast evolutionary rates such as ammonoids (cephalopods) constitute an invaluable material for evolutionary studies. However, the strong developmental integration of the mollusk shell morphogenesis, combined with an accretionary mode of growth, does not easily lend itself to standard phylogenetic methods. Although the long-recognized relationships between phylogeny and ontogeny are complex, this project addresses the problem of how to reconstruct the phylogeny of mollusks by means of morphometric analyses of shell ontogeny. A set of non-destructive methods have been implemented to allow quantification of the entire ontogeny of shell shape of single individuals by means of micro-computed tomography and mesh analyses. Micro-computed tomography enables acquisition of digital three-dimensional data of a shell as a series of grey-scaled images. These stacked images are processed to obtain a model of the shell as a connected array of triangles. Finally, shell shape is extracted through ontogeny by means of a curve-skeleton. Ontogenetic trajectories of shell shape are quantified by extending Raup's morphogenetic shell space from its classical 4-dimensions into n-dimensions by reconstructing shell aperture with elliptic Fourier analysis. The phylogeny of studied organisms is reconstructed by comparison of their ontogenetic trajectories in this n-dimensional morphologic space. These ontogenetic trajectories are used as continuous morphometric traits in classic tree-based phylogenetic analyses. This approach is validated by analyzing a group of extant mollusks with known molecular phylogeny (Helicidae, pulmonate Gastropoda). The approach proposed in this project is also expected to yield a more refined understanding of the respective roles of developmental variation and environmental change in the evolution of ammonoids.