Bivalve-Based Stratigraphy of the Toarcian of Eastern Siberia and Northeastern Russia (Family Oxytomidae Ichikawa, 1958). Part 2. Ontogeny. Classification and Taxonomic Assessment of Characters. Phylogeny. System of the Family Oxytomidae. Taxonomic D

The system of the family Oxytomidae (subfamily Oxytominae) was revised based on Mesozoic material from different stratigraphic levels, using our own and museum collections. Variability of the hinge plate and byssal block of shells in the ontogeny of these mollusks was assessed using the age periodization method. Taxa were classified using a posteriori character weighting. Relationships between taxa were established by the constant presence of a character or assembly of characters in morphologically similar taxa. To establish the relationship, the similarity of homologous characters was used—the shape of the ligament pit at the postlarval stage, the type of the ligament pit at the juvenile stage, the subtype of the ligament pit and the subtype of the lower margin of the ligament pit at the subadult stage, the type of the byssal furrow, the type of the byssal notch, the presence of a protuberance, the presence of an inclination of the ligament area relative to the shell axis. The phenoclines of these characters were identified and the directions of morphological evolution within the subfamily Oxytominae were reconstructed based on the study of ontogenetic and evolutionary variability in the shape of the ligament pit, byssal ear, and the anterior and posterior wing of the left valve. The chronoclines were reconstructed based on the chronological succession of taxa with unidirectional changes in characteristics. When the phenocline and chronocline coincided, probable phylogenetic relationships between the taxa were suggested. The transformation of the ligament pit of the Jurassic-Cretaceous Meleagrinella and Arctotis is accepted as the main cluster of the phylogeny of the family Oxytomidae in the Jurassic. Based on the phylogenetic reconstruction, the composition of the subfamily Oxytominae was emended. It contains three genera—Oxytoma, Meleagrinella, and Arctotis. Using the method of morphometric assessment of morphological characteristics, the index species of the Pliensbachian-Aalenian oxyto-zones and of the Beds with oxytomids were systematically described and compared with other species in the taxonomic clade.