Developmental polymorphism in Oxfordian ammonites

Abstract

A new interpretation of the variation in final diameter of the ammonite shell is presented and exemplified by several forms from the superfamilies Stephanocerataceae and Haplocerataceae, chiefly Oxfordian in age. These examples demonstrate occurrence, of more than just two morphs differentiated by their final shell size, as well as occurrence of only one morph in other species. The new interpretation differs from the theory of sexual dimorphism in that the variation in final size of the shell is regarded as a result of intraspecific variation in time of the achievement of sexual maturity by individual specimens. This interpretation is supported by evidence from extant cephalopods.