On the lower jaws of Kosmoceratidae (Ammonoidea: Stephanoceratoidea) from the Middle Jurassic of Central Russia

Aptychi are the lower jaws of Jurassic and Cretaceous ammonites, which consist of a pair of mirror-symmetrical valves. In various evolutionary lineages of ammonites, aptychi differ in shape as well as the structure of the outer calcitic layer and the sculpture of its surface. These differences are the basis of the parataxonomic classification of aptychi. Different types of aptychi are typical of different evolutionary lineages of ammonites, however, these types are very unevenly studied. The aptychi of the family Kosmoceratidae are among the poorest studied aptychi. Previous authors previously attributed these aptychi to three different parataxa: Praestriaptychus, Granulaptychus, and Kosmogranulaptychus. We are describing herein the findings of the Upper Bathonian and Lower Callovian aptychi from three localities of Central Russia, which belong to sexual dimorphs of the genus Kepplerites. Kepplerites aptychi have a rounded subtriangular shape, they are wide, with gentle and non-prominent ribs. The surface of their calcitic layer bears rows of tubercles, which indicates their belonging to the formal genus Granulaptychus. The presence of Granulaptychus-type aptychi in the oldest subfamily of Kosmoceratidae gave ground for the assumption, that the Granulaptychi had likely appeared in the Bajocian in Stephanoceratidae and later were independently inherited by both the Keppleritinae and Garantianinae. It is important to note that tubercles are present only on specimens from clay, but not from sandy nodules. The granulaptychi from sandy nodules, devoid of tubercles, are indistinguishable from praestriaptychi. This taphonomic difference suggests that some of the aptychi, which originated from sandy sediments and had been described as praestriaptychi, could likely belong to other types of aptychi.