Wrinkle layer and supracephalic attachment area: Implications for ammonoid paleobiology

Abstract

In many planispiral ammonoid shells, the dorsal wrinkle layer is located not only inside the body chamber, but also protruded beyond the apertural edge, covering the part of the penultimate shell whorl in front of the aperture (similar to the black layer of Recent Nautilida). This external (relative to the body chamber) part of the wrinkle layer was rarely the focus of research and its implications for ammonoid paleobiology have not been studied in detail. This paper deals with this area found on the shells of seven genera of planispiral Jurassic (Callovian and Oxfordian) ammonites from Central Russia. The size and shape of this area as well as the surface structure of well-preserved specimens are described. The proposed name "supracephalic attachment area" (SCAA) represents the location and presumed biological significance of this part of the wrinkle layer. The implications of the existence of this area for the ammonoid mode of life, soft body structure and aperture orientation are discussed. The location of the part of the wrinkle layer outside of the body chamber proves that some part of the ammonoid soft body (likely the cephalic region) moved forward beyond the aperture during active swimming. This forces a review of calculations of ammonite aperture orientation which were based on the postulate that the soft body was located largely within the body chamber.