On some peculiar fractures observed in tests of the Upper-Cretaceous fossil echinoid Micraster

Abstract

Many skeletons (“tests”) of the fossil echinoid Micraster from the marl limestone strata of the Upper Cretaceous formation of western Navarra (Spain) show small triangular holes on their surface, caused by the loss of a pyramidal fragment. The base of the pyramidal borings is often close to an equilateral triangle. The test of echinoids is a mosaic structure constituted of interlocking monocrystalline plates of bio-calcite which possess a preferred crystalline orientation. In the case of the genus Micraster, the surface of the plates is near-parallel to the basal calcite plane (0001). The geometry of the observed holes, at first sight surprising, is simply explained by the intersection of cracks along the three easy 101¯4 cleavage planes of the calcite single crystalline plates constitutive of the skeletons. The fracture process leading to the pyramidal fractures is most likely due to stresses of thermo-elastic origin induced by the heterogeneous dilatational response of the anisotropic tests adhered to the isotropic marly limestone in which they are embedded and which is constitutive of their inner molds.