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

Abstract

The morphology of the hinge plate and outer parts of the Toarcian bivalves of the genus Meleagrinella (family Oxytomidae Ichikawa, 1958) was studied. The Meleagrinella and Arctotis phylogeny in the Toarcian was reconstructed on the basis of the conclusions on the ligament pit ontogenesis evolution by study of the extensive material from Eastern Siberia and Germany and taking into account the ostracum microsculpure data. The species “Monotis” substriata (Münster, 1831), widespread in the Toarcian deposits of Europe, Asia, and North America, was revised. By comparing specimens from different stratigraphic levels of the lower Toarcian of Eastern Siberia, Northeast 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. The ostracum microsculpture of the Toarcian Arctotis and Meleagrinella representatives is depicted for the first time. In the upper Pliensbachian–lower Toarcian, a sequence of oxytomid taxa was identified, and on the basis of the Boreal ammonite scale, their biostratigraphic significance was assessed. The sequence is represented by four species: (1) Meleagrinella (Praemeleagrinella) deleta (upper Pliensbachian), (2) M. (Praemeleagrinella (?)) golberti (Tiltoniceras antiquum and Harpoceras falcifer zones), (3) M. (Clathrolima) substriata (Dactylioceras commune Zone), and (4) M. (Meleagrinella) prima (Zugodactylites braunianus and Pseudolioceras compactile zones).