The aptychi of ammonites combined the functions of lower jaws and protective opercula. They consist of two parts: an inner organic layer and an outer calcitic lamella. In different evolutionary lineages of ammonites, the shape of aptychi, the sculpture of their surface and the microstructure of the calcitic layer vary greatly. However, the structure of the aptychi is not known for all evolutionary lineages of ammonites. Although numerous aptychi have been described for the Jurassic family Aspidoceratidae, almost all of them belong to only one evolutionary branch of this family – the Aspidoceratinae (sensu lato). For the second branch – the Peltoceratinae, only one aptychus had been described to date and the structure of its calcitic layer remained unknown. In this article, for the first time, the structure of the aptychus of the Peltoceratinae (upper Callovian Peltoceras) is described. The surface of this aptychus is covered with rough ribs and the calcitic part consists of only one layer of dense calcite. The thickness of the aptychus is much greater than that of the aptychi of supposed ancestors of the Peltoceratinae. The increase in the thickness of the aptychi in both the Aspidoceratinae and the Peltoceratinae, contemporaneously with the appearance of spines on their shells, is most likely related to increasing the protective function of the aptychi of these ammonites in the late Callovian.
The structure of aptychi - elements of the lower jaw of ammonites, which were also used as protective opercula - has been unevenly studied in terms of the level of detail in different evolutionary lineages of Jurassic and Cretaceous ammonites. For example, in the family Aspidoceratidae, which includes two subfamilies (Aspidoceratinae and Peltoceratinae), the aptychi have been known for more than a century and are very well studied in Aspidoceratinae, but almost nothing had been known about the aptychi of Peltoceratinae and about the structure of their external calcitic layer. This article is devoted to the discovery of the aptychus of the ammonite genus Peltoceras, which has a well-preserved calcitic layer, in the Upper Callovian sediments of the Ryazan region in Russia. The study of the Peltoceras aptychus showed that the microstructure of its calcitic layer and surface sculpture fundamentally differ from that in the representatives of the closely related subfamily Aspidoceratinae. The aptychus of Peltoceras lacks a spongious tubular layer, characteristic of the aptychi of Aspidoceratinae macroconchs. The calcite in the aptychus of Peltoceras is dense, similar to the calcite of the aptychi of ammonites of the Perisphinctidae family, the ancestors of aspidoceratids. However, in perisphinctids, the calcitic layer is very thin, whereas in the aptychus of Peltoceras it is thick, at least five times thicker. The surface of the Peltoceras aptychus is covered with rough rectangular ribs, such ribs are absent both in the ancestral aptychi of perisphinctids, and in the aptychi of aspidoceratins. These differences in the structure of the aptychi of aspidoceratins and peltoceratins confirm the previously proposed hypothesis about the independent origin of these ammonite lineages from perisphinctid ancestors. Since the aptychi with a thick calcitic layer appeared in both evolutionary lineages of aspidoceratids simultaneously with the appearance of large thorns on their shells, the increase in the thickness of the aptychi must have been associated with an increase in their protective function.
Aspidoceratidae, Peltoceratinae, Peltoceras, aptychi, ammonites, Jurassic.