Taxonomy and Biostratigraphical Significance of the Toarcian Bivalves of the Genus Meleagrinella Whitfield, 1885

Abstract

The morphology of the ligament block and outer parts of the shells of Toarcian bivalves of the genus Meleagrinella (family Oxytomidae Ichikawa, 1958) were studied. The phylogeny of Meleagrinella and Arctotis in the Toarcian was reconstructed on the basis of the conclusions on the evolution of the ontogeny of the ligament pit, made as a result of studying extensive material from Eastern Siberia and Germany, and taking into account the data on the microsculpture of the ostracum. A revision of the species “Monotis” substriata (Münster, 1831), which is widespread in the Toarcian deposits of Europe, Asia, and North America, was carried out. By comparing specimens from different stratigraphic levels of the Lower Toarcian of Eastern Siberia, North-East Russia, and Germany, three species were substantiated: Meleagrinella (Praemeleagrinella?) golberti sp. nov., M. (Clathrolima) substriata (Münster) and M. (Meleagrinella) prima sp. nov. Figures of the ammonite Tiltoniceras sp. ind., Upper Plinsbachian-Toarcian bivalves of the genus Meleagrinella and Upper Toarcian bivalves of the genus Arctotis are given. For the first time, a microsculpture of an ostracum of the Toarcian representatives of the genera Arctotis and Meleagrinella is depicted. In the Upper Pliensbachian–Lower Toarcian, a sequence of oxytomid taxa was established and, based on the Boreal ammonite scale, an assessment of their biostratigraphic significance was given. The sequence is represented by four species: (1) Meleagrinella (Praemeleagrinella) deleta (Upper Pliensbachian), (2) M. (Praemeleagrinella ?) golberti (Tiltoniceras antiquum, Harpoceras falcifer zones), (3) M. (Clathrolima) substriata (Dactylioceras commune Zone), (4) M. (Meleagrinella) prima (Zugodactylites braunianus, Pseudolioceras compactile zones).