The Significance of Septa and Sutures in Jurassic Ammonite Systematics

Abstract

A study of Stephanocerataceae and Perisphinctaceae of the Middle Jurassic demonstrates that the structure of the septa comprises a complex vault and pillar system which is functionally adapted to the dimensions of the tube so as to give maximal stability to the shell. Only through a study of the septum can we interpret the different forms of the suture and discriminate between important and unimportant features. Different septa may have analogous sutures, especially in the external parts. These septal patterns are strikingly consistent within subfamilies or families even where other features suggest overlap between the groups. Some parts of the septum are not adapted, but either retained from earlier ontogeny or inherited, and constitute vestigial (genotypic) characters distributed through taxons. These empirical results have been applied to the taxonomy of some groups of hitherto doubtful systematic position (Cadomitinae, Normannitinae, Sphaeroceratinae, Parkinsoniidae, Spiroceratidae).