No abstract

The Maurynya section exposed on the northwestern margin of West Siberia is one of few continuous sections of the Jurassic–Cretaceous boundary deposits in Boreal regions. In the meantime, it can be considered a reference section for the transitional Volgian–Ryazanian beds formed in shallow water environments of the West Siberian sea basin. This study is a synthesis of the new data with previous results of lithologic, paleontological, biostratigraphic, and (O, C, Sr) isotope studies of the Upper Volgian–lowermost Ryazanian of the Maurynya section. It has been established that the beginning of the Cretaceous (corresponding to the middle Late Volgian) is associated with a sharp increase in species number and diversity of cephalopods and bivalve mollusks on the northwestern margin of the West Siberian sea basin. This can be explained by an increase in its depths and their stabilization at a level which appeared optimal for the habitat of nekton, nektobenthos, and benthic fauna communities, superimposed on the general trend of warming. At the same time, the percentage of phytoplankton significantly increased, indicating the onset of transgression, which affected largely coastal landscapes and type of vegetation: The forests dominated by conifers were gradually succeeded by forests mostly composed of Ginkgoaceae.

No abstract

The discovery of Tethyan and Boreal fossils in Bathonian deposits of the Sokur section in the vicinity of Saratov (Central Russia) reopened the possibility of direct correlation of the Bathonian sections in different paleobiogeographic provinces, which require further integrated studies involving both paleontological (based on different groups of fauna) and physicochemical (paleomagnetic and isotope-geochemical) methods. Stable carbon isotope data for belemnite (Cylindroteuthididae) rostra from the Sokur section show that the Subboreal Oraniceras besnosovi Zone and Boreal Arcticoceras harlandi Subzone are correlated with the Zigzag Zone of the standard scale, whereas the Boreal A. ishmae Subzone can be correlated completely or, even partly, with the Tenuiplicatus Zone. We identified equivalents of chrons of the geomagnetic polarity time scale (GPTS): e-Bath N, m-Bath R, and lt-Bath N, corresponding to the Lower–Middle Bathonian and, possibly, lowermost Upper Bathonian.

Berriasian (Early Cretaceous) oyster Pycnogryphaea weberae from Crimea is one of the earliest known pycnodonteine oysters. Examination of shell morphology and microstructure of this species shows that it shares characters of the subfamilies Gryphaeinae and Pycnodonteinae. Shell microstructure of Pycnogryphaea weberae lacks vesicular structure and is similar to microstructure of the genus Gryphaea. At the same time the shell of this species possesses morphological characters typical of pycnodonteins: development of chomata and well-defined circumferential curb. Pycnogryphaea weberae is regarded as a transitional taxon between the subfamilies Gryphaeinae and Pycnodonteinae and referred herein to a new genus Pycnogryphaea within the Pycnodonteinae. This suggests that the subfamily Pycnodonteinae likely originated from the Late Jurassic representatives of the genus Gryphaea (Gryphaeinae) in Berriassian. First pycnodonteins preserved shell microstructure of the genus Gryphaea but developed chomata along all the perimeter of both valves.

No abstract

No abstract

No abstract