sponges

In the structure of modular organization of the Late Cretaceous hexactinellid sponges, the terminal position is assigned to colonial forms, with skeletons peculiar for complex, intricate structures. Primitive (primary) and genuine (secondary) colonies have been revealed. Skeletons of the primary forms, generally with multiple numbers of equivalent modules and oscula, differ morphologically from the genuine colonies in undeveloped module connecting elements, such as junctions and wall closures, and in the lack of ostia

In morphogenesis of the Late Cretaceous skeletal sponges – hexactinellids – certain tendencies may be traced that allow to distinguish forms of diverse levels of modular organization (?polyoscula? sponges) among the unitary representatives of the group. Skeletons of the sponges from the primary level of modular forms, autonomies, represent an example of the initial module polymerization, with the principal polymerization mechanism involving diverse variants of gemmation and/or fission

Certain tendencies are supposed to show up in morphogenesis of the Late Cretaceous skeletal sponges, hexactinellids; those tendencies make it possible to distinguish some forms of diverse-level modular organization among the unitary representatives of the group. Generation of primitive ?modular? forms, defined as the perifrontal ones, is associated with incomplete gemmation and isolation of solitary cross canals, apo- or prosochetes

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

Tendencies of morphogenesis in the Late Cretaceous modular forms contributed to formations of complexly structured secondary (genuine) colonies. Skeletons of genuine colonial sponges differ not just in numerous and diversely arranged equivalent oscula-carrying modules, but in the presence of the module connecting elements – junctions, as well. Characteristic elements of genuine colony skeletons – ostia – have been recognized between the modules and the junctions.

The regularly arranged openings acting as additional oscula – subos­cula – in the skeletal structures of numerous hexactinellids made the basis for recognizing a transient level in those sponge organization: a transition between the unitary and the colonial forms. Unitary, autono­mous and colonial sublevels have been recognized in the transition structures. The morphological diversity of the Late Cretaceous transi­tions was called forth by manifestations of mosaic morphogenesis in the representatives of the Lychniscosa and Hexactinosa orders.

Hexactinellids comprise diverse types of sponges viewed as solitary ones. They used to constitute the basis of sponge communities in the Middle-Late Jurassic and Middle Cretaceous of the European region. The morphological diversity of unitary forms is determined by the skeletal structure with combination of a goblet, a shaft, a false shaft and a system of rhizoids. The goblet wall is peculiar for the highest morphological flexibility, which contributes to substantial habitus changes of the skeleton original morphotypes.

Partial and complete regeneration phenomena, cases of skeleton habitus restoration have been recorded in representatives of the Late Cretaceous hexactinnelids. Regeneration incidences are in many ways associated with the ability of silicon-producing cells responsible for constructing skeleton-generating elements, spicules, to transform, to rearrange their functions in response to the changes of the organism position on the substrate surface and/or upon the changes of the aquatic environment parameters.

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.

Basic structure types have been specified in hexacinellids with soldered spicule lattice. The fossils of those occur as skeletal forms. Certain tendencies in Lychniscosa and Hexactinosa morphogenesis were identified as conductive to shaping the spectrum of skeleton archetypes. The increase of sponge morphological diversity was in many ways predetermined by thickness reduction of the skeleton-forming walls, associated with reduction of the irrigation system elements. That has brought about forms of complicated tubular labyrinth habits.