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.