The internal structure of varices and constrictions in Jurassic and Cretaceous ammonoid shells

Abstract

The structure of pristine preserved shells of Jurassic and Cretaceous ammonoids was examined to identify whether the character states of varices and constrictions are homologous or homoplasic traits. The internal structure of varices and constrictions of phylloceratids, lytoceratids, perisphinctids and desmoceratids shows no interruptions in shell growth which implies that varices and constrictions were formed by continuous secretion. Their internal thickenings are formed by the nacre layer, while the inner prismatic layer thins to compensate the internal relief. The corresponding surface relief, e.g. furrows or ridges, is formed by undulation of a continuous outer prismatic layer, similar to the way common ribs are formed. The absence of an interruption in shell secretion is different from the well-examined nepionic constriction of the ammonitella. However, a delayed secretion of inner nacreous portions which are responsible for the actual thickening of varices and constrictions is characterized by “internal interruptions”; only the most inner nacreous portions seem to continue shell growth. The nacreous thickening is secreted subsequent to the formation of a distinct peristome, e.g. furrows, ridges or flares, i.e. not at the same time the peristome is formed. The thickening of varices and constrictions does not narrow an (interimistic) aperture and does not represent a halt in growth. However, certain areas of the shell (e.g. furrows, ridges, flares) are reinforced by this process. Morphological transition from smooth shelled varices to ridged and constricted habitus within phylloceratids and desmoceratids prove to be a homology. Owing to the general structural conformity of varices and constrictions in shells of Jurassic and Cretaceous taxa, we support the theory that they are homologous traits.