Stereom microstructures of Cambrian echinoderms revealed by cathodoluminescen

Abstract

Echinoderms possess a skeleton with a unique and distinctive meshlike microstructure called stereom that is underpinned by a specific family of genes. Stereom is thus considered the major echinoderm synapomorphy and is recognized already in some Cambrian echinoderm clades. However, data on the skeletal microstructures of early echinoderms are still sparse and come only from isolated ossicles of limited taxonomic value in which the primary calcium carbonate has been replaced or thinly coated by phosphates, silica or iron oxides. Here, we applied cathodoluminescence (CL) to reveal stereom microstructures of the diagenetically altered calcitic skeletons of some Cambrian echinoderms (in particular Protocinctus, Stromatocystites and Dibrachicystidae). CL not only provides insights into the diagenesis of their skeletons but also reveals primary microstructural details that are not visible under transmitted light or SEM. Different stereom types (resembling labyrinthic, fascicular, galleried, microperforate and imperforate microfabrics) comparable to those observed in extant echinoderms have been recognized for the first time in these Cambrian taxa. Our results show that the stereom microstructures widely occur in various Cambrian echinoderm clades which suggest that they likely evolved the same genetically controlled biomineralization mechanisms as those observed in modern echinoderms. These results underline that the CL technique can be a powerful tool in the detection of the microstructural organization in even severely recrystallized echinoderm specimens. Given the close association between the skeletal microstructure and the investing soft tissues, the method presented here opens new possibilites for revealing skeletal growth and soft tissue palaeoanatomy of fossil echinoderms.