Upper Cretaceous

The morphological diversity of diosculumic sponges, known as Paracraticularia cylindrical, results from the fact that many fossils assigned to this species prove to be fragments of the apical segments of internodes (branches) from major colonies that are rarely encountered as totally preserved fossils. The structures of the branch apical segments and the oscula outlines reflect various manifestations of hexactinellida synchronous division conductive to morphological diversity of the descendants, the succeeding complicatedly structured colonial sponges.

Tremabolites sponges known among the Middle-Late Cretaceous beds in Europe are reliably identifiable due to the numerous openings against the background of the skeleton smooth upper surface. The ideas of the level of organization in these sponges and of the species characteristics are far from being univocal. The paper deals with the species diversity of Tremabolites common in the Coniacian – Santonian rocks from the Volga Region. Examples of regeneration and budding in the representatives of the group are presented.

The features of areal distribution and stratigraphic confinement of calcareous spicular sponge forms in the southeast of East European Platform are considered. The skeletons of calcareous sponges essentially representatives of the genus Porosphaera are found in the carbonates of Turonian, Coniacian, Campanian and Maastrichtian, as well as at the base of siliceous formations of the Paleocene. The Maastrichtian calcareous sponges are characterized by the highest species diversity and a wide lateral extension.

Terminal layers of carbonate rocks of Maastrichtian and lower intervals of Paleocene aleurite-pelitic formations are accessible for investigation in Krasny Oktyabr and Bolshevik quarries (Volsk Depression). In the bottom of the Paleocene accumulation of skeletons of calcareous and siliceous sponges has been confirmed, mollusk and elasmobranch assemblage has been determined. Biozonal stratification of the location section and oryctocomplex analysis have contributed to specification of the processes in succession of benthic forms at the Upper Cretaceous – Paleogene boundary.