FRACTURAS “FÓSILES” EN ESQUELETOS DE EQUINOIDEOS DEL CRETÁCICO SUPERIOR - “FOSSIL” FRACTURES IN UPPER-CRETACEOUS EQUINOIDS TESTS

Many calcitic skeletons of the fossil echinoid Micraster coranguinum show small triangular holes on their surface originated by the loss of a pyramidal fragment. The geometry of the holes, at first sight surprising, is simply explained by the intersection of fractures of the single crystalline skeleton plates along the three easy cleavage planes of calcite. The base of the pyramidal fragments is often close to an equilateral triangle; the morphology of such fractures is explained by the preferred orientation of the Micraster plates, whose surface is parallel to the basal calcite plane (0001). The fracture process leading to such fractures is most probably of thermo-elastic origin.