An Exceptionally Large Upper Jaw of Lytoceratoidea from the Upper Bajocian (Middle Jurassic) of the Northern Caucasus

Abstract

A large-sized upper jaw of an ammonite from the Niortense Zone of the Upper Bajocian of the Kuban basin (Karachay-Cherkessia) is described. In its anterior part a pointed calcified structure (rhyncholite) is located. Judging by its shape and size, the upper jaw belonged to a species of the genus Lytoceras (family Lytoceratidae), the lower jaw of which was already described from the same area immediately below in the section. The presence of a rhyncholite allows us to attribute the new finding to the rhynchaptychus type of ammonoid jaws. This is the earliest known complete jaw of this type and, to date, the only known upper jaw of Lytoceratina. Probably, calcification of the anterior part of the jaw apparatus of ammonoids of the suborders Lytoceratina and Phylloceratina was a normal process occurring as the mollusk matured.