3D CHARACTERIZATION OF MOLLUSCAN SHELL MICROSCULPTURE USING SURFACE METROLOGY

Many mollusks have distinctive patterns of shell sculpture. These are important in taxonomy and systematics and have also been used in ecological and evolutionary studies. While many conchological terms exist to describe these features, these are often inadequate for characterizing and discriminating the observed diversity of sculpture except in a very general way.

Surface metrology offers a potentially robust means of explicitly and quantitatively describing shell sculpture by making use of the amplitude, spacing, and texture direction of surface features. While other methods exist for analyzing 3D data, surface metrology parameters have a number of unique benefits. They are designed to summarize statistically any surface or length that is mathematically continuous at any scale. A region of interest, whether it represents the whole surface of an organism or a patch within it, can be sampled with thousands of digitized points, thereby enabling a more comprehensive analysis of morphology. Comparing different surfaces metrologically within and among taxa could enable a more precise characterization of morphological features at multiple resolutional scales.

We examine the use of surface metrology parameters for quantifying patterns of shell microsculpture (microns to tens of microns). Gastropod and bivalve shells are digitized using digital close-range photogrammetry and laser confocal microscope to generate dense 3D point clouds of sculptural features. Microsculptural patterns are characterized using a combination of parameters. Clear ontogenetic changes in surface texture are apparent in several of the taxa examined. Analyzing spatially sequential surface patches of these shells permits the quantification and comparison of ontogenetic trajectories. To assess the sensitivity of different parameters, computer generated surfaces are analyzed metrologically.

Surface metrological evaluations of microsculpture are a useful complement to qualitative descriptions and will enhance future taxononomic and systematic studies. The inclusion of surface metrology parameters in morphological analyses should lead to a greater understanding of the diversity, ontogeny, and evolution of shell textures in fossil and recent taxa.