Symbiosis between gastropods and bryozoans in the late Ordovician of Cumbria, England

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Abstract
Five examples of symbiosis between gastropods and trepostome bryozoans are described from the Ashgill (late Ordovician) of Cumbria (England). The gastropods are invariably found associated with the bryozoans, whereas the bryozoans may be free-living. Encrustation is considered to have taken place, in most cases, on mature, living gastropod shells, resulting in the trepostome utilizing the shell as a surrogate basal disc. Three specimens show encrustation on an empty gastropod shell, causing the trepostome to develop a basal disc similar to non-encrusting forms. The bryozoan colony probably benefited from the symbiotic relationship by increased water flow over the colony, caused by gastropod locomation, whereas the gastropod was afforded protection from predators. The new species Spiroecus nidhoeggi, Diplotrypa hvergelmi and Monotrypa fontinalis are described.