sponges

Knowledge on the regularities of vegetative propagation and comprehension of the osculum (oscula) and subosculum structural-functional positions and interrelations in the skeleton structure have made it possible to specify the organization levels in the Late Cretaceous hexactinellids: the unitary (solitary, perifrontal and autonomous forms), transitory (solitary, autonomous and colonial forms) and colonial (primary and true colonies) ones.

In the ?Ozerki? sections, in the terrigenous and carbonate rocks, a faunal complex has been determined – unusually diversified and relatively abundant for the right-bank part of the Saratov Region. Examination of the specified groups of fossil invertebrates has allowed to propose variants for biostratigraphic division and correlation of the sections under consideration. The carbonate rocks are supposed to belong to the Turonian-Coniacian and the underlying terrigenouscarbonate rocks are regarded as the Lower Turonian ones. Formation of the ?sponge?

The Balantionella genus has been recognized by A. Schrammen (1902) from a single specimen and two skeleton fragments from a locality in the «quadrate» Cretaceous rocks in Germany. In the southeast of the Russian Plate, representatives of the genus have been determined within units from all the stages of the Upper Cretaceous. This has made it possible to provide characteristics and to trace the morphogenetic tendencies in the representatives of the group. Three subgenera and 12 species have been recognized.

The examination of the modular organization in fossil skeletal sponges (Hexactinellida) is mainly related to a study of their vegetative reproduction. The concepts of the reproductive forms for both Recent and Mesozoic hexactinellids can be ambiguous. Examples of the skeletons of fossil sponges from the Late Cretaceous of the Volga River basin, with morphologically detached and naturally distributed sprouts interpreted as buds, are shown. 

The Ventriculitidae representatives used to hold noticeable and occasionally dominant position within the Coniacian – Maastrichtian sponge assemblages. Relatively simple skeleton construction in the representatives of the family among the Late Cretaceous hexactinellids has made it possible to trace variations in skeleton morphology, changes in the goblet habitus and in the structure of irrigation system depending on the width variations of the skeleton-building wall.

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.

Among the Late Cretaceous Ventriculitidae, with the most simply constructed pariform skeleton, representatives characterized by unusual neoplasms were identified. On the paragastric surface after the lapel of these sponges (Contubernium), cone-shaped submodules are regularly based, the appearance of which is associated with incomplete blastogenesis. Paragastra of submodules are not associated with the atrium, therefore, Contubernium is considered as a perifrontal organism, occupying an intermediate position between single and modular forms of spongia.

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 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.

Study of the skeletal morphology of a new representative of the Leptophragmidae family, with no suboscula in its structure, provides support for the earlier specified regularities in generation of Hexactinellida modular forms. Good preservation state of a major colony has made it possible to specify the aspects of paleoecology and taphonomy in the later representatives of the Middle Coniacian – Early Santonian succession of siliceous sponges.