The wrinkle layer is a special layer in ammonoid and nautiloid shells, having a more or less rough surface covered with wrinkles, folds and tubercles. The wrinkle layer likely was present in all ammonoids, but it is clearly visible mostly in Paleozoic shells. It is subdivided into dorsal (Runzelschicht) and ventro-lateral (Ritzstreifen) parts. In addition, the dorsal part of the wrinkle layer protrudes in front of the ammonite aperture on the surface of the previous shell whorl (Supracephalic attachment area). Probably, both parts of the wrinkle layer are homologous to the corresponding layers in Recent Nautilus and Allonautilus shells: the dorsal part corresponds to the black Nautilus layer and the ventro-lateral part to the black stripe which is located along the edge of the aperture (it appears in adult nautilus shells, whereas in ammonoidsthe wrinkly layer was present at all stages of development). The wrinkle layer was needed for strong, temporary attachment of the soft tissues (mantle) to the shell walls and for adhesion of the mantle to the shell walls during forward movement of the ammonite body from the inside of body chamber. The layer was also present in the shells of many nautiloids (Orthocerida, Tarphycerida, etc.)

The text was added and/or edited by: Alex