Functional morphology of the cap-like apparatus in autozooids of a Palaeozoic trepostome bryozoan

A membranous, cap-like structure was discovered in the bryozoan genus Hallopora from upper Ordovician siltstones from Sardinia. The structure is interpreted to be analogous to the perimetrical attachment organ and to the part of the membranous sac sustaining the sphincter muscle in Recent cyclostomes. The progressive mineralization of the cap-like apparatus occurred in later ontogenetic stages of the colony in order to have the function of a terminal diaphragm. This diaphragm is related to a particular degeneration—regeneration cycle very similar to the cyclic growth patterns (intracolony overgrowths) of some post-Palaeozoic cyclostomes. After this phase the colony seems to end its life cycle.