New Data on Ammonites from Basal Beds of the Valanginian Stage (Lower Cretaceous) in the Lower Reach of the Anabar River, Northern Yakutia

A collection of ammonites from the basal beds of the Valanginian stage in the Klimovskii Utes key section and in the Uryung‒Khaya outcrop in the lower reaches of the Anabar River (northern Yakutia) has been studied. For a number of taxa (Bojarkia cf. mesezhnikowi, Neotollia cf. venusta, and N. maimetschensis), descriptions and images from these localities are first provided. It has been established that the basal beds of the Valanginian stage can include fauna of different ages. Here, both late Ryazanian Bojarkia ammonites and early Valanginian Costamenjaites and Subtemnoptychites have been found together. At the same time, it has not been reliably determined whether the ammonite complex is mixed (the condensation horizon) or stratification is preserved in the ammonite distribution (the condensed section). In the first case, the subface of these beds will also be the Valanginian base and, in the second case, the Valanginian base should be located inside these beds. The authors are inclined to the first interpretation, as suggested by previous researchers.