Ammonoid Shell Microstructure

Abstract

This chapter discusses various aspects of ammonoid shell microstructure, presents a description of the structure of the individual layers that compose the ammonoid shell, shows the distribution and relationships of these layers, and depicts their ultrastructure whenever possible. The major limitation in micro-and ultrastructural studies of ammonoids is diagenetic alteration, therefore the best studied ammonoids are those from the Jurassic and Cretaceous, while the data on Paleozoic and Triassic ammonoids are still scarce. At the ultrastructural level, the three main layers of the postembryonic shell of ammonoids do not differ significantly from those known from the shell of Recent nautilids. The same is also true for the septa. However, the embryonic shells of ammonoids, called the ammonitellas, are distinguished from those of modern and fossil nautiloids in their smaller size and the presence of a spherical or barrel-shaped initial chamber.