Bivalve-Based Stratigraphy of the Toarcian Deposits of Eastern Siberia and Northeastern Russia (Family Oxytomidae Ichikawa, 1958). Part 1. A Brief Outline of the Development of Views on the Toarcian Zonal Stratigraphy in Northeast Asia. History of the Study of Oxytomids. Materials and Methods of Fossil Study

Interpretations of the stratigraphic range of the Toarcian Stage in Northeast Asia are analyzed using the concept of historical dynamics of science. Although Lower-Middle Jurassic deposits have been studied for more than a hundred years, and the biostratigraphic framework has been developed, some problems of the Lower and Middle Jurassic stratigraphy of Eastern Siberia and Northeastern Russia remain unresolved. These include substantiation of the lower and upper boundaries of the Toarcian and analysis of the completeness of the Toarcian Stage represented in sections. It is shown that current bivalve zonal scales for Eastern Siberia and Northeastern Russia are based on different principles, constructed to successions of taxa belonging to different families, and are used independently in the two regions. By identifying the structural and logical components of the research process, three stages are recognized in the history of the study of the family Oxytomidae Ichikawa, 1958. The relevance of the study of the bivalve family Oxytomidae is substantiated. New methods are developed for studying bivalves with a pterineoid hinge: growth stages, typification of the hinge apparatus, zoning and morphometry. The method of growth periodization of mollusk shells makes it possible to identify oxytomid growth stages in shells of different sizes. Use of a shell zoning method allows the relative boundaries of shell parts to be determined, and development of a unified terminology for morphological traits used in describing taxa. The method of hinge typification facilitates the study of ontogenetic changes in bivalves with a pterineoid hinge and helps clarify the phylogeny of the group. The morphometric method allows for a quantitative assessment of the characters used in the description of taxa and statistical testing of hypotheses about evolutionary transformations.