Evolution of Permian Non-Marine Bivalve Fauna from Continental Deposits of the Severnaya Dvina River Basin

Abstract

Four stages (Late Urzhumian-Early Severodvinian, Late Severodvinian, Terminal Severodvinian-Early Vyatkian, Late Vyatkian) have been established in the evolution of Permian non-marine bivalve fauna from continental deposits of the Severnaya Dvina River Basin. These stages have been singled out based on estimation of the number of autochthonous cosmopolitan, endemic, and allochthonous generic taxa in the assemblages of bivalves. The Late Urzhumian-Early Severodvinian stage is characterized by the dominance of the Angarian genus Prilukiella. The cosmopolitan subgenus Palaeomutela (Palaeomutela) has a subordinate significance. The Late Severodvinian stage is distinguished by the dominance of the cosmopolitan genus Palaeomutela s.1. Representatives of the endemic genus Opokiella appear in the beginning of this stage. The Terminal Severodvinian-Early Vyatkian stage is characterized by a sharp increase in the number of endemic genera and the appearance of rare representatives of the Angarian genus Concinella. The cosmopolitan genus Palaeomutela s.1. has a predominant significance during the Late Vyatkian stage. The number of endemic genera decreases in this stage. It has been assumed that the disappearance of Angarian non-marine bivalves is caused by the relative warming as evidenced by the general direction of the positive excursions of carbon and oxygen isotopes on the isotope curves.

Keywords

• stratigraphy
• non-marine bivalves
• Permian system
• Severnaya Dvina River basin