Sutural asymmetry in the ammonites Bifericeras and Leptonotoceras from the Lower Jurassic of Bishopʹs Cleeve, Gloucestershire, England and its significance for ammonite life orientation

Large samples of pyritized ammonites of Bifericeras and Leptonotoceras from the Lower Jurassic of Bishopʹs Cleeve, Gloucestershire, England, reveal that the majority have asymmetrical suture lines due to siphuncular offset from the usual mid-ventral position. Siphuncular offset is most common (95% of specimens) and most extreme (modes 12–23° left, 23–34° right, maximum 93°) in Leptonotoceras, but less frequent and less extreme in Bifericeras (92%, mean 5°, maximum 26° in B. bifer; 70%, mean 5°, maximum 21° in B. sp. 2). In both genera there is a marked preference for offset to the right side of the ammonite, and in Leptonotoceras the modal value is higher to the right. Trochospiral shells are more frequently found in Bifericeras bifer than in either Leptonotoceras or B. sp. 2. All trochospiral shells have siphuncular offset, with the siphuncle always displaced towards the ‘umbilical’ side of the shell. It is suggested that the position of the siphuncle can be used to interpret the life orientation of the ammonites in the water column, but the relationship is not simple. The frequent occurrence of mature individuals suggests that this unusual inferred orientation was not harmful to the ammonites. Differences in sutures and in body chamber length, together with the presence of at least two species of Bifericeras, but only one of Leptonotoceras, call into question Bayerʹs (1972) interpretation that B. bifer and Leptonotoceras are sexual dimorphs of the same biological species.