Final stages of skeletogenesis and early stages of disintegration for modern polycystine radiolaria

Abstract

The bars and spines that collectively constitute the opal skeleton of Holocene Polycystinea are composed of at least two layers on perfectly preserved speciments. The interior of these skeletal components has a granular periphery, which appears to be of primary origin. For most Spumellarida and Nassellarida in the plankton, this core is enveloped by a glaze-like layer (the epivitrium), 0.10–0.15 μm thick, that produces a smooth surface texture. The epivitrium is formed as the final stage of skeletogenesis and appears to have the same structure as the mantle of Callimitra. Both the epivitrium and mantle are of uniform thickness, and they are composed of a central layer of coarse granules sealed on both surfaces by veneers of much finer particles. Under most conditions, the epivitrium disintegrates rapidly after skeletons are deposited on the sea floor. Specimens from the Orca Basin sediments document this deterioration history, which commences with the development of intricate dendritic crack systems on the surface. The epivitrium first ruptures over swellings or sharp edges of lattice bars and spines by deepening and lateral growth along these minute cracks. As delamination progresses, large sheets of epivitrium peel away from the core.