Ontogenetic shell development of a Cretaceous desmoceratine ammonoid "Tragodesmoceroides subcostatus" Matsumoto, 1942 from Hokkaido, Japan

Ontogenetic shell development of the Turonian desmoceratine ammonoid Tragodesmoceroides subcostatus is examined based on more than 200 specimens collected by detailed biostratigraphic survey in the Tappu and Saku areas, Hokkaido, Japan. After the ammonitella stage, the mode of ribbing on the shell surface becomes coarser with growth. The shell surface ornament changes from stage 1 (almost smooth) to stage 3 (coarse ribbing) via stage 2 (weak ribbing) Stage 1 is subdivided into an earlier substage 1a with desmoceratid-type constrictions and a subsequent substage 1b without constrictions. Appearance order of these four ornament stages and substages seems to be ontogenetically fixed in the present species. Other characters, such as ammonitella and early shell shape and whorl geometry are also stable. On the other hand, the shifting-timing from stages 1 to 2 and from stages 2 to 3 changes chronologically, i.e., it becomes ontogenetically earlier in upward sequence. In the Lower and Middle Turonian, the adult or subadult shells still remain stage 1 or 2. In contrast the strongly ribbed holotype and paratypes showing stage 3 are regarded as the Upper Turonian peramorphic endmembers of the single biospecies T. subcostatus. Taking these features into consideration, T. subcostatus should be taxonomically revised.