Ammonoid growth rhythms



Determination of ammonite growth rate and longevity is an intriguing but difficult problem. Doguzhaeva (1982) made an exciting contribution through her study of ‘growth rings’ and their distribution on a variety of ammonite shells. She postulated a fortnightly or lunar monthly periodicity for chamber formation and a daily periodicity for growth line secretion on the outer shell. Before these suppositions become doctrine, however, a measure of skepticism seems warranted. Some of the relevant remarks run over familiar ground and were raised in response to Kahn & Pompea (1978), who assumed that growth increments on the outer shell of Nautilus are daily and septal deposition lunar monthly. Criticisms of their work focused on the nature of their data and the lack of a basis for the initial time assumptions (Jones & Thompson 1979; Saunders & Ward 1979). These criticisms apply to Doguzhaeva's study because her approach is similar and because Nautilus is the best living analog for ammonite growth.