Terebella phosphatica Leriche (Polychaeta) associated with phosphatic crusts and particles (Lower Turonian, Bohemian Cretaceous Basin, Czech Republic)

Abstract

The polychaete worm Terebella phosphatica Leriche occurs at the locality of Plaňany (Bohemian Cretaceous Basin) together with remains of the Atreta-Bdelloidina community, attached to the crystalline cobbles and boulders at the base of the Bílá Hora Formation, Whiteinella archaeocretacea Zone, the Lower Turonian. Terebella formed tubes with agglutinated phosphatic particles, mostly faecal pellets, firmly attached to the substrate. The surrounding phosphatic lag containing not only faecal pellets, but also coprolites, various indeterminate phosphatic particles and remains of phosphatized and unphosphatized macrofauna (ichthyolites, bivalves, sponges, crinoids) served as a source of phosphatic particles for the construction of the worm tubes. Formation of the phosphatic lag and the production and phosphatization of its components have been evaluated in relation to repeated phosphogenic episodes during which the phosphatic crusts covering the substrates encrusted by epifauna were formed. Three phosphogenic episodes alternated with episodes of substrate and crust colonization. The tube construction of Terebella shows size- and shape- selective use of particles both during growth and in relation to position in the tube wall. Orientation of faecal pellets normal to the tube axis was found at the bases of the tube walls. High juvenile mortality found in some areas of the substrate was probably due to stress caused by turbulent sedimentation nearby. All the phenomena described probably are part of the unique sequence of short-term events that characterizes the condensed sedimentation of the basal Lower Turonian in the Bohemian Cretaceous Basin. The tubes of the polychaete worm Terebella phosphatica described here are not only the first find of this species in the Bohemian Cretaceous Basin, but also the oldest so far known from any part of the world.