Article: Bioimmured ctenostomes from the Jurassic and the origin of the cheilostome Bryozoa

Soft-bodied ctenostome bryozoans, preserved as bioimmurations following overgrowth by encrusting organisms with hard skeletons, are described from the Middle and Upper Jurassic of England and Normandy. They include one new genus, Cardoarachnidium, and three new species, C. bantai, C. voigti and Arachnoidella abusensis. These and new bioimmured specimens of Arachnidium smithii (Phillips) show fine-scale details of original morphology as well as artefacts caused by partial collapse of zooids during overgrowth. The D-shaped zooidal orifices present in Cardoarachnidium probably indicate that the zooids were operculate. Opercula are one of the two apomorphies of the closely-related cheilostomes, and therefore the Oxfordian Cardoarachnidium is placed in the stem-group of the Cheilostomata, which first appear in the Tithonian.