[Proceedings of the symposium 'Molluscan Palaeontology' : 11th International Malacological Congress, Siena (Italy) 30th August - 5th September 1992 / A.W. Janssen and R. Janssen (editors)]: The bivalve shell as a preservation trap, as illustrated by the Late Jurassic gryphaeid, Deltoideum delta (Smith).

Three processes by which a bivalve shell can preserve soft-bodied organisms during its secretion are described. These are: a) bioimmuration by the attachment area of a cementing bivalve, b) bioimmuration by a growth lamella and, c) deformation of the periostracal sheet. Examples of all three are provided by the Late Jurassic gryphaeid Deltoideum delta (Smith, 1817); Recent examples are also given. Extrapallial cement - here termed the Harper layer - is shown to have been produced throughout growth in this species, which allowed accurate lamellar bioimmuration as well as facultative recementation of the left valve. The importance of bivalve shell secretion for the preservation of unmineralized, hard-substrate-dwelling epibionts in the fossil record is outlined.