Diapositive 1 - de l'Université libre de Bruxelles

Chapitre IIIINTRODUCTIONEchinoderms are abundant, widely distributed benthic invertebrates and theirendoskeletons are generally well preserved in the geological records. This skeleton ismade of ossicles located in the dermis. Each ossicle consists in a three-dimensionalnetwork of mineralized trabeculae, the stereom. Each ossicle is composed of a singlecrystal of high-magnesium calcite in which Sr is the main trace element. It also contains0.1% organic material (the intrastereomic organic matrix). The skeleton is formedintracellularly in cell processes of the skeleton-forming cells (for a review, see Duboisand Chen 1989). The high-magnesium calcite phase results from the progressivecrystallization of a transient amorphous calcium carbonate phase (ACC) presumablydelivered in the calcifying vacuole in the form of ACC-containing vesicles (Wilt 2002,Beniash et al 1997, 1999, Politi et al 2004).The Mg/Ca ratio of the skeleton is related to temperature (Clarke & Wheeler 1922, Chave1954, Weber 1969) and seawater Mg/Ca ratio (Ries 2004). Because the Mg/Ca ratio inseawater is spatially constant and unlikely to change on time scales of