Sutural variation in the ammonites Oxynoticeras and Cheltonia from the Lower Jurassic of Bishopʹs Cleeve, Gloucestershire, England and its significance for ammonite growth

Large samples of pyritized ammonites of Oxynoticeras and Cheltonia from the Lower Jurassic of Bishopʹs Cleeve, Gloucestershire, England, reveal that a majority retain parts of the body chamber and died as juveniles. Both genera exhibit similar high variation in sutural angle, chamber size and numbers of chambers per whorl, suggesting they are closely related, perhaps sexual dimorphs. Many examples preserve impressions of paired dorsal and unpaired ventral muscle scars, as well as drag bands and (more rarely) pseudosutures. All stages of the chamber formation cycle described in Recent Nautilus can be recognised in both genera. Juvenile ammonites apparently died at random times in the chamber formation cycle. Frequency of incomplete final septa suggests that septal secretion took about 30% of the full cycle. Rare examples from a restricted horizon preserve narrow, radiating darker bands on the internal moulds. Mean chamber angle and mean angle between dark bands correlate strongly in Cheltonia, suggesting formation of dark bands was related to septal secretion. The dark bands are interpreted as false colour bands secreted on the inside of the aperture during pauses in growth, but there is no other evidence for episodic growth in either genus. Spacing of growth lines is similar to that reported for Recent Nautilus and to spacing of pseudosutures. Both genera may have moved their bodies forward slowly during chamber formation, unlike the rapid movement in Nautilus. This may be one way in which ammonite growth differed from that of Nautilus. Irregular chamber sizes in both ammonites suggest that the period of chamber formation varied, perhaps due to variable pauses in apertural growth. At least Oxynoticeras and Cheltonia grew much more irregularly than modern Nautilus.