Solenomeris: from biomineralization patterns to diagenesis

Microstructural and diagenetic patterns in acervulinid foraminifers are investigated through detailed petrographical, SEM, and geochemical analyses of Recent and fossil Acervulina and Eocene Solenomeris. The acervulinid test is composed of an optically radial hyaline magnesian calcite. The chessboard-chamber arrangement represents an efficient way for minimizing the volume accretion rate of cytoplasm, this being probably an important requirement for foraminifera with no predesigned external shape, such as the encrusting forms. Several newly described skeletal features reflect directly adaptation and responses to environmental constraints, such as microborer pressure, water energy, adaptation to substrate shape, skeleton repair, and compartmentalization of cytoplasm. The Eocene Solenomeris from the Pyrenees have recorded in their skeletons several diagenetic processes comprising two different phases of cementation followed by recrystallization. These events are accompanied by a marked loss of magnesium content and preceded or followed by an alteration of the Primary Organic Membrane. These diagenetic processes, in particular the early cementation, alter the overall layered pattern typical of foraminifera and produce a pseudo-file arrangement, mimicking an algal structure.