Article: The spiral bryozoan Terebellaria from the Jurassic of southern England and Normandy
Morphological study of the monospecific genus Terebellaria has determined its unusual pattern of colony growth. Erect, almost cylindrical branches possessed an apical growth tip from which endozonal zooids were budded to extend the branch distally. A helico-spiral growth margin or, less commonly, a succession of annular growth margins, arose from the growth tip and progressed towards the colony base by budding exozonal zooids which were directed proximally so that their distal ends were nearer to the colony base than were their proximal ends. Multilamellar overgrowth of earlier-formed parts of the zoarium was thus achieved. Ontogenetic zones of feeding zooids, with open zooecial apertures and peristomes directed obliquely towards the colony base, occupied bands on the branch apex side of each whorl of the helico-spiral growth margin or each annular growth margin. The creation of a colonial water current system is suggested by this distribution of feeding zooids. Branch proliferation occurred by dichotomy at growth tips and also by 'adventitious' branch formation at exozonal growth margins proximal to the growth tip. By successively re-encrusting its erect branches, growth was relatively efficient, the zoarium was continually strengthened, and the establishment of an exogenous epifauna was hindered.