Deltidial Coverings of Middle Ordovician Clitambonitidines (Clitambonitidina, Brachiopoda) and Their Significance for Attachment to the Substrate

The growth and microstructure of deltidial covering plates of Lower and Middle Ordovician Clitambonitidines from Estonia and Leningrad Region are reviewed. In the genera Anchigonites, Antigonambonites, Vellamo, and Estlandia, the fusion of deltidial plates was followed by the formation of both the symphytium and the tube covering the pedicle. Subsequently, the end of the tube was sealed with the apical disc, which supposedly provided cementation to the substrate.