|Year of Publication:
|P. S. Balson, Taylor P. D.
Bioclastic sands of the Coralline Crag are characterized by abundant bryozoans including large colonies of four cyclostomes: Blumenbachium globosum Koenig, Meandropora aurantium (Milne Edwards in Lyell), M. tubipora (Busk), and Multifascigera debenensis sp. nov. These species are systematically described and the relationships investigated between colony growth pattern, form, and inferred ecology. Colonies of each species are composed of numerous subcolonies bounded by exterior walls. Times of autonomous subcolony growth were punctuated by periods of subcolony anastomosis. Most colonies acquired a roughly hemispherical shape but others developed a subspheroidal form. The former apparently retained a stable attitude during growth whereas the latter either enveloped an unstable substrate (circumrotatory growth) or were attached to a perishable substrate which supported the colony above the sea-bed. Colonies are absent from the turbulent sandwave facies of the Coralline Crag but are present in other facies where mobile animals rather than currents may have been responsible for overturning circumrotatory colonies. Evolution of exterior wall-bounded subcolonies, known in many post-Palaeozoic cyclostomes, was possible because interzooidal pores allowed soft tissue connection between subcolonies beneath the colony surface when soft tissue was absent above it. The localization of coelomic damage may have been a factor in the success of this type of organization.