Subdivision of the Ladinian Stolleyites tenuis zone in northern Okhotsk region

Abstract

The views on the zonal subdivision of the uppermost part of the Ladinian Stage in Northeast Asia, which is characterized in this region and the Svalbard Archipelago by ammonoids from the genus Stolleyites, and their evolution are discussed with revision and specification of the taxonomic composition and stratigraphic distribution of ammonoids through the Stolleyites tenuis Zone in the section at the Pravaya Vtoraya Sentyabr’skaya River (Yana Okhotskaya River basin) in northern Okhotsk region. It is established that the ammonoid species Stolleyites tenuis (Stolley) and S. terminalis sp. nov. are confined to the lower and upper parts of the tenuis Zone (in its original range), respectively. In this connection, the position of the upper boundary of the tenuis Zone in this section is displaced, with the Stolleyites terminalis Beds emerging as a new biostratigraphic unit overlying the tenuis Zone (in its new range). No presice correlatives of the Stolleyites terminalis Beds of northern Okhotsk region are established so far in other boreal regions, which indicates the stratigraphic completeness of the examined section and the appropriateness of its use for reconstructing the evolutionary features of the boreal ammonoids at the Middle–Late Triassic boundary. The species Stolleyites terminalis sp. nov., which differs from S. tenuis (Stolley) in its shorter early ontogenetic stage with evolute shell, relatively narrower whorls, absence of regular umbilical folds, and dentate patterns of all lobes in the outer part of the whorl, is described.