A New Ammonoid Zone of the Upper Carnian Substage in Northeastern Russia

Abstract

A review of the biostratigraphic division of the Upper Carnian substage of the Boreal paleobiogeographic Realm by ammonoids is presented. The history of identifying the yakutensis ammonoid zone of the Upper Carnian in Northeastern Russia and the evolution of the views on its volume and paleontological characteristics are considered. The systematic composition and stratigraphic distribution of ammonoids within the yakutensis zone in the sections of Northern Priokhot’e (Sea of Okhotsk region), the Adycha River basin and the right bank of the lower reaches of the Lena River are revised. It is established that the species “Sirenites” (=Orientosirenites) yakutensis Kiparisova is confined to the lower part of the yakutensis zone (in its previous volume), whereas Orientosirenites bytschkovi Konstantinov is bound to its upper part. In this regard, the position of the upper boundary of the yakutensis zone is changed, and it is proposed that a new bytschkovi zone overlapping the yakutensis zone (in the new volume) should be identified. The yakutensis and bytschkovi zones are widespread and are traced in the stratigraphic sequence in a series of the sections on the territory of Northeastern Russia. In the presence of the common genus Orientosirenites, they are correlated with the Jovites borealis beds of Arctic Canada and their equivalents: the upper subzone of the Tropites welleri zone in British Columbia and the upper part of the Tropites subbullatus zone in the Alps.