Cyprian Kulicki

Ammonoid Shell Microstructure

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.

Development of the Embryonic Shell Structure of Mesozoic Ammonoids

Abstract Exceptionally well-preserved embryonic shells (ammonitellae) of the early Aptian ammonoid Aconeceras cf. trautscholdi Sinzov, 1870, preserved as coprolite remains from Symbirsk, Russia, were examined with scanning electron microscopy (SEM) to investigate the developmental sequence of the embryonic shell structure. Our SEM observations reveal that these shells can be classified into the following three groups with different wall microstructure: Group 1, with a thin (ca. 5 µm), double-layered shell wall, consisting of inner prismatic and outer homogeneous layers, the former of which is absent in the adapical portion and becomes thicker adorally; Group 2, with a three-layered shell wall that consists of inner prismatic, middle homogeneous, and outer prismatic layers, with tubercles on the outer layer; and Group 3, with a thick nacreous swelling (primary varix) on the anteroventral side near the aperture. The middle homogeneous layer of the embryonic shells of Group 2 is the same as the outer homogeneous layer in shells of Group 1 and may be composed of amorphous calcium carbonate (ACC). In embryonic shells of Group 3, the middle homogeneous layer is absent and there are voids instead. It may have been transformed into the inner prismatic layer or else dissolved during diagenesis. In modern Nautilus and gastropods, embryonic or larval shell development is initiated by the secretion of a cap-shaped, fully organic shell prior to the deposition of calcium carbonate. This stage is not preserved in the material examined, but probably existed in the Ammonoidea. Based on our observations and data from extant Nautilus and gastropods, we propose a model for the development of the embryonic shell structure of Mesozoic ammonoids, starting from secretion of an organic primary shell, followed by deposition of ACC and its transformation into the inner prismatic layer, and terminating in the deposition of a primary varix on the inside of the ventral and ventrolateral position of the shell just adapical of the aperture.