Article: A model for the morphogenesis of ribs in ammonites inferred from associated microsculptures

Eight morphological types of microsculpture have been recognized on the outer shell surface of well-preserved ammonites. They consist mainly of wrinkles and creases which were developed on a pliant material. They are interpreted as the result of compressive stresses which occurred on a free (i.e. uncalcified) periostracum. This is the first evidence of periostracal development in ammonoids, and its relation to the shell suggests a mode of calcification similar to present-day molluscs. This periostracum was also attached to the soft body along longitudinal lines. All microsculptures were compression structures. This, their concentration within intercostal valleys, and additional evidence, leads to the proposal of a new morphogenetic model for ammonite ribs, in which they represent compression folds developed on the free periostracum which were later calcified. Evidence that ribs coincide with growth halts supports this view. An enigmatic feather-like ornament is interpreted merely as a long-wave variant of wrinkling. The study of shell structure implies that the aperture was never fully calcified except at maturity. Therefore, ribs did not reinforce the apertural edge, which was easily torn allowing the ammonite to scape, but rather avoided the peeling of the shell from some distance to the aperture backwards. Differences in timing and mode of calcification existed between constricted and non-constricted ammonites. In fact, the presence of constrictions could indicate episodic growth, similar to that of some Recent gastropods.