Hexactinellida

The fouling of skeletons and shells of benthic organisms and the formation of multi-tiered skeletal formations are characteristic of carbonate-producing Porifera, which participated in the structure of reef communities during the Phanerozoic. The accretion of flint sponges, Hexactinellidas, to the solid elements of the substrate is a little-known phenomenon, since many representatives of this group were fixed on the sediment surface by rhizoid outgrowths.

Arterial directions in formation of module organization of skeleton hexactinellids are caused by vegetative forms of reproduction. It is supposed that some tendency of selection of protozoa module forms can be traced in morphogenesis of separate generic types of late Cretaceous sponges. The concept “polyosculumic„ sponges is considered in vast context personally in reference to skeletons with some osculumics formed in the result of partial regeneration of organism

Guettardiscyphia skeletons are peculiar for thin walls and highly dense apo- and prosopores. Wall mobility was conductive to formation of conical lobate skeletons of various, occasionally geometrically regular outlines. Substantial number of fossil samples from variously composed rocks from all the Upper Cretaceous intervals has made it possible to trace skeleton transformations in sponge ontogenesis and depending on their environmental conditions.

The earliest assemblage of Cretaceous sponges has been found in the Cenomanian terrigenous rocks (the Melovatka suite) from the south of the Volga right bank region. A noticeable element of the assemblage is composed of small-sized Camerospongia, newly recognized in the region and peculiar for forming monotype colonies as well.

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. 

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.

The first representatives of Cameroptychium on the territory of Russia, which are established in the rocks of the Upper Cretaceous of the Volga region, are described. The labyrinthine structure of the skeleton and the modular, colonial level of organization of these sponges are considered, which is typical for many Late Cretaceous Hexactinellids. The widespread manifestation of isomorphism indicates the finiteness of the formation of representatives of Hexactinellida. The origin of Cameroptychium is correlated with Santonian Plocoscyphia.