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