- 化石 (ISSN:00229202)
- no.94, pp.19-31, 2013-09
To understand the inter- and intraspecies variation and ontogenetic change in shell ornamentation of Polyptychoceras (Cretaceous diplomoceratid), reproduction of shell morphologies was carried out based on the creeping soft-part model and the lazy messenger effect. As a result, we found several constructional rules on this ammonite as follows. (1) The shell surface of Polyptychoceras consists of "striation-" and "smooth-parts", and the former was much more slowly formed than the latter. (2) The changes in shell ornamentation during growth of Polyptychoceras (Polyptychoceras) haradanum were probably caused by change in shell prolongation rate, and were successfully restored if the striation- and smooth-parts were assumed to be formed through gradual expansion and intermittent forward movement of the soft-part, respectively. (3) The formation of the multiple ribs observed in Polyptychoceras (Subptychoceras) yubarense can be attributed to a refractory period for propagation of the morphogenetic signals just after the formation of the continuous smooth-part including ribs. Our conclusion is that the great variability of shell ornamentation developed in Polyptychoceras is the result of compound effect of two factors, i.e. the rate of shell prolongation and the duration of the refractory period for ribbing.