Геология

The description of the representatives of the genus Labyrintholites is presented. The type species of the genus was published I. F. Sinzov in 1879. The considerable amount of the fossil material from the Santonian rocks found in the Saratov Volga Region has made it possible to substantiate the diagnosis of this genus within the order Hexactinosa and to present its taxonomic diversity within eight species. Among the Hexactinellids, the Labyrintholites are distinguished for their attachment to the substrate solid immobile elements.

The offering paper covers the history of the Aptian stage stratigraphy and ammonites studies of the central part of the Northern Caucasus, which was subdivided by the author into five stages. The first (mid-to-late 19th century), second (1901–1951) and third (1952–1969) stages are reviewed. The first is associated with the beginning of research in the region, at this time the attempts of the stage and substage dividing were made.

The article discusses the content and distribution of natural radioactive elements (uranium, thorium, and potassium) in the Upper Jurassic deposits of the Kogalym-Pokachevsky region, based on data from nine wells. Through comprehensive studies, eight stages of deposition have been identified, including deposits from shelf silts to the prefrontal zone of the beach. Certain patterns of migration and accumulation of these radioactive elements have been observed for each stage, and their limiting values have been determined.

Ammonite finds at the Miroshniki site, within the Albian-Cenomanian boundary interval, make it possible to identify the presence of the analogues of the Upper Albian zones Mortoniceras fallax (autochthonous burial within aleurolites) and Mortoniceras rostratum (allochthonous occurrence among phosphorite nodules). In this very section, an elasmobranchian assemblage has been isolated from the phosphorite interlayers; this is represented by 9 series and 13 genera. Small sand sharks Eostriatolamia subulata (Ag.) are dominant in its content.

The Albian-Cenomanian rocks compose a single complex of terrigenous deposits over the Volga Region. Delineation of the boundaries of the Cretaceous system units generally tends to cause difficulties, accounted for by rare finds of the fossils. In the northern part of the DonMedveditsa dislocations, natural outcrops of the Albian and the Cenomanian beds in the Burluk river valley have been examined since the 1950s. The authors have earlier made some case studies on the Gordienki, Krasny Yar, Melovatka and Moiseyevo sections.

The record of a mandibular tooth plate of the chimaeroid fish Ischyodus sp. from the Lower Cretaceous deposits of the north of the Saratov Volga River Right Bank is described. The presumed age of the find is the Lower Aptian. This is the first recorded occurrence of chimaeroid remains in the Lower Cretaceous (pre-Albian) deposits of the Middle and Lower Volga regions, and the fourth record in deposits of this age in Eastern Europe.

The Late Cretaceous and the Paleocene representatives of the Aphrocallistes from the East European Province are known from the scanty skeletal fragments. The fragmentary preservation state of the fossils has brought about some contradictory characteristics of the genus and the idea of those sponges diversity. Colonial Aphrocallistes are regarded as the typical representatives of the Maastrichtian spongiofauna from the west of the European region.

The final, late Campanian – early Maastrichtian stage of development of echinoids Micraster and Isomicraster, was studied based on extensive material from the Upper Cretaceous sediments of the North Caucasus, Mangyshlak, Kopetdag, Volga and Transvolga regions. The changes in the complex of morphological characters in the youngest species, which are predominantly connected with variability in the structure of the oral surface of the test, considered in detail.

The Polyptycha sponges have been registered for the first time in the Upper Cretaceous beds from the Volga Region. The skeletons of those true colonies are peculiar for bushy morphotype complicated by anastomoses of the parent modules at their bending. The systematic position of the genus is disputable. This results from the incomplete understanding of the diverse skeleton morphologies in the Late Cretaceous hexactinellids and, correspondingly, from the lack of any standard criteria as regards the taxa of the “family” and the “genus” levels.