Aptychi, calcitic or phosphatic plates are part of the ammonoid jaw apparatus. Sometimes paleontologists separate paired aptychi (aptychi sensu stricto) and unpaired aptychi (anaptihi). For the first time, paired aptychi appeared in the Carboniferous period, but after a very short time they disappeared. Their affiliation is unknown, it is possible that they did not belong to ammonoids but to nautiloids. In the Early Jurassic, paired aptychi reappeared, and persisted up to the extinction of ammonites at the end of the Cretaceous. Unpaired aptychi (anaptihi) were present in ammonoids from the beginning of their history i.e. the early Devonian and continued to exist in some groups up to the end of Cretaceous.
The majority of aptychi were double-layered: the outer layer was calcitic, the inner - organic. There was a wide variation of layer thickness in different types of aptychi: Boreal and Subboreal ammonoid taxa usually had aptychi with a very thin outer calcitic layer whereas aptychi of many warm-water taxa had a thick outer layer. In the fossil state, these layers can be found preserved either together or, depending on conditions, separately (most commonly calcitic).
Aptychi are now considered as a part of the ammonoid jaw apparatus, more precisely as a lower jaw. Initially, aptychi were considered as opercula which protected the ammonite body chamber. This hypothesis was supported by the size and shape of aptychi, more or less coinciding with the corresponding parameters of the aperture, and by the findings of ammonite shells, whose apertures were closed by the aptychi. However, paleontologists were forced to revise the outlook on aptychi function and recognize them as lower jaws due to the findings of aptychi together with upper jaws and radula, the absence of other candidates for the role of the lower jaws in ammonites and a marked resemblance of aptychi to the lower jaws of other cephalopods.
At the same time aptychi were likely used as opercula in times of danger. Aptychi (especially paired) combined the functions of the lower jaw and operculum. In the case of a predator attack, an ammonite could have turned in the body chamber in such a way that the aptychi blocked the opening of the aperture. The surface of many aptychi is decorated with sculptural elements (ribs, spikes, etc.) which is absolutely not needed by the jaw, but could have been important to the protective operculum.
Since not all aptychi can be strictly related to ammonites, paleontologists made a separate classification for aptychi with artificial genera and species.