Aragonitic shell structures in the rudist Biradiolites, and some palaeobiological inferences

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Abstract
A dark blue Santonian marl, from the Massif de la Sainte Baume, Provence, France, contains many poorly sorted fossils showing aragonite preservation. Biradiolites angulosissimus, here common in all life stages, is shown to have three shell layers, (from the outside, inwards) : — (1). Calcitic prisms. This is cellular in the right valve (RV) and compact in the left (LV). (2). Crossed-lamellar (aragonite). This also forms the teeth and their grooves. (3). Complex crossed-lamellar (aragonite). This also forms the myophores, and surrounds the teeth. In addition, myostracal prisms form pads on the myophores (LV), a thin palliai attachment layer and adductor traces in the RV, and also a thick lens near the centre of the LV, from which the body possibly hung. An initial growth stage has a spirogyrate RV, and an operculi-form LV. The calcite layer is an ontogenetic addition. These three shell layers seem typical of rudists.

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