On the Turonian origin of the Goniocamax-Belemnitella stock (Cephalopoda, Coleoidea)

A new subgenus within the genus Goniocamax, Progoniocamax nov. subgen., is described. It includes the Late Turonian-Early Coniacian species “Actinocamax” surensis Naidin and Actinocamax intermedius Arkhangelsky. The new subgenus represents a transition between the genera Praeactinocamax and Goniocamax. It occurs in the Central Russian Subprovince. From a geographical point of view, it indicates the origin of the Goniocamax-Belemnitella stock to be situated in the Russian Platform (Central Russian Subprovince, East European Province) during the Late Turonian. The origin of the Upper Cretaceous belemnitellid Goniocamax Naidin, and consequently the Belemnitella-stock, is explained herein by the deepening of the pseudoalveolus and the gradual calcification of the anterior part of the rostrum. The occurrence of the so-called “bottom [base] of ventral fissure”, a major morphological feature known in the later belemnitellids (Goniocamax, Gonioteuthis, Belemnitella), the enlarging of the ventral fissure, and the calcification of its surrounding area, enabled a gradualistic increase of the rostra within the Progoniocamax-Goniocamax-Belemnitella evolutionary lineage.