Bivalve-Based Stratigraphy of the Toarcian of Eastern Siberia and Northeastern Russia (Family Oxytomidae Ichikawa, 1958). Part 3. Toarcian–Lower Aalenian Zonal Scale Based on Oxytomids. Bivalve-Based Stratigraphy and Correlation

As a methodological basis for spatio-temporal modeling of the studied sedimentary series, the relational-genetic space-time concept of Steno–Vernadsky and V.I. Vernadsky’s paradigm of biological time. The biochronological approach to the development of a zonal scale as an independent methodological direction is defined. Based on a revision of the phylogenetic system of the family Oxytomidae, a spatiotemporal framework of the zonal scale was constructed. The direction of the time scale is determined by the sequence of index species interconnected by a single chronocline of changes in the states of the characters of the ligament block. The direction of evolution in the phylogenetic lineage Meleagrinella–Arctotis, combined with periods of stable state of some characters, has its own (relational) time, therefore the scale is considered as biochronological and is a tool for dating sediments. According to paleontological and stratigraphic criteria, the elementary divisions of the scale—Oxyto-zones are phylozones, deposits containing the species representing segments of the phylogenetic lineage of the genera Meleagrinella and Arctotis. The updated Toarcian and Lower Aalenian zonal scales consist of six oxyto-zones. In the boundary deposits of the Upper Pliensbachian, Beds with Meleagrinella deleta are recognized. In the Upper Aalenian–Lower Bajocian boundary deposits, Beds with Arctotis sublaevis are recognized. In the Upper Toarcian–Lower Aalenian, an auxiliary biostraton is recognized—parallel Beds with Oxytoma jacksoni. The correlation potential of the scale was assessed based on tracing oxyto-zones in sections of Eastern Siberia, Northeast Russia, Germany, France, Western and Arctic Canada. In the terminal part of the Pliensbachian, the Beds with Meleagrinella deleta are recognized in the Siberian, Far Eastern and Western European paleobiogeographic provinces within the Arctic and Boreal-Atlantic regions. In the Lower Toarcian, the Meleagrinella golberti and Meleagrinella substriata oxyto-zones of were identified in the Arctic, Boreal-Atlantic and Boreal-Pacific paleobiogeographic regions. In the upper part of the Toarcian and in the Aalenian–Lower Bajocian, the Meleagrinella prima, Arctotis marchaensis, Arctotis similis oxyto-zones and Beds with Arctotis sublaevis were recognized in the Siberian and Far Eastern provinces within the Arctic paleobiogeographic region, parallel Beds with Oxytoma jacksoni were traced in the Arctic and Boreal-Pacific paleobiogeographic areas. Tracing oxyto-zones and Beds with oxytomids allows the intra- and interregional correlation at the zonal and substage levels.