Conch geometry, ontogeny and dimorphism in the Early Bajocian ammonoid Stephanoceras from Normandy, France

A large suite of specimens belonging to the Early Bajocian Stephanoceras from Évrecy (Normandy, France) displays wide variation in conch and sculpture. The quantitative analysis of the conch geometry and its ontogeny on the basis of more than 50 sectioned specimens reveals major problems in the separation of species within the assemblage; clear morphological boundaries between possible species cannot be established. The material shows wide variation in sculpture and conch parameters throughout ontogeny, and correlation between the characters is weak. With the study of conch ontogeny, it is possible to separate dimorphs, as microconchs show a sudden change in the conch parameters (umbilical egression, change in the shape of the whorl profile). The onset of these changes varies in the assemblage between 40 and 75 mm conch diameter.