3D morphometry of polyconitid rudist bivalves based on grinding tomography

Previous taxonomic studies have shown that polyconitid rudists have a characteristic arrangement of the myocardinal system with an ectomyophoral cavity on the posterior side of the left valve. The specific arrangement of the myophores and associated cavities defines the different genera. However, there has been little research on the three-dimensional spatial distribution and size of the internal features, for want of a technique that is suitable for large and low density-contrast specimens. The tomographic technique described herein is based on automatic serial grinding and serial scanning; the resulting images are treated with biomedical image software. The technique has been applied to a pair of well-preserved specimens of Polyconites verneuili from the Upper Aptian of Spain. Fifteen quantitative characters have been obtained using multiplanar virtual cuts, volume-rendering, and isosurfaces reconstructions. The study revealed the shape, size, and distribution of the ectomyophoral, body and accessory cavities, the lengths and volumes of the teeth, and the arrangement of the myophores. We conclude that this technique facilitates the description of rudist bivalves and is suitable for other fossils; moreover, it has the potential to be used in other fields of geology.