The Middle Oxfordian to lowermost Kimmeridgian ammonite succession at Mikhalenino (Kostroma District) of the Russian Platform, and its stratigraphical and palaeobiogeographicai importance

The Mikhalenino section on the Russian Platform has yielded numerous ammonites from the Middle and Upper Oxfordian and lowermost Kimmeridgian, collected bed by bed. The ammonites belong mostly to the Boreal family Cardioceratidae, but also to the Subboreal family Aulacostephanidae; additionally at some levels there were collected various Submediterranean ammonites (Perisphinctidae, Oppeliidae and Aspidoceratidae). The co-occurrence of ammonites representative of different faunal provinces makes possible recognition of the Boreal, Subboreal, and partly also Submediterranean standard zonations. In consequence, it is possible to make a close correlation between these zonal schemes. The Oxfordian/Kimmeridgian boundary at the Pseudocordata/Baylei zonal boundary of the Subboreal zonal scheme corresponds precisely to the Rosenkrantzi/Bauhini zonal boundary. This boundary of the stages defined well faunistically in the Flodigarry section (Isle of Skye, Scotland) and proposed as a candidate for the uniform Oxfordian/Kimmeridgian boundary, can be also recognized in the Russian section studied. The boundary can be traced in the Mikhalenino section using the same criteria as used at Staffin: the appearance of the first representatives of Pictonia [M]–Prorasenia [m] (Subboreal), and the first appearance of Amoeboceras (Plasmatites) (Boreal). This indicates the large correlation potential of the boundary defined in this way. The research on the Mikhalenino section has provided the new palaeontological findings described in this study. These include the first discovery in the territory of the Russian Platform of ammonites of the genera/subgenera Decipia, Vineta and Pomerania (Pachypictonia). Two new species: Decipia (?) kostromensis sp. nov., and Pictonia mesezhnikovi n. sp. are described.