Radiolitid shells (Hippuritacea, Bivalvia) collected from the Gredilla de Sedaño section (Upper Campanian-Lower Maastrichtian, Northern Burgos, (Spain) have been studied with regard to their shell microstructure.
The distribution of organic matter in the skeletal walls and growth lines follows a characteristic sequence and seems to have been a control upon further diagenetic evolution of the shells. In fact, the organic matter content increases just in the vertical transition from the upper part of the walls to the lower of the immediately upper growth line. Slight truncations between sets of laminae are thought to have originated as a result of reproductive episodes and/or environmental crisis. On the contrary, some strong interruptions and truncations in the shell development can be seen as very complex intergrowth textures indicating collonization of younger individuals upon older ones. In these cases, the younger rudists start to grow by compact layers that precede the further characteristic «honeycomb» textures. The internal skeletal cells («honeycomb») acted as scenario for such diagenetic processes as pyrite formation, early dolomite and «dogtooth» calóte and later «blocky» calcite precipitation
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