Aphrocallistes alveolites
Roemer 1841
Aphrocallistes alveolites is a moderately common hexactinnelid sponge at Misburg and Höver.
It forms branching tubes, 10 to 20 mm in diameter, with walls approximately 2 mm thick. The tube walls are perforated by radial channels, giving rise to the very regular, almost honeycomb-like pore structure which makes the species easily recognizable.
The images show larger fragments consisting of subhorizontal, branching tubes with closed, finger-like terminations. On top of this basal tube system, which forms a foundation for the sponge on soft substrate, numerous chimney-like, vertical branches are observed. These chimneys have wide oscules covered with characteristic sieve plates, whose structure resembles that of the tube walls, but the pore sizes are distinctly larger.
The second and third image are two different views of the same specimen of Aphrocallistes alveolites. It shows a chandelier-like growth. While the symmetrical structure may be accidental, the specimen shows how the upright cylindrical sections with terminal sieve plates emerge from and are connected by subhorizontal tube sections. The rounded, closed, finger-like parts point downward and may grade into root-like supports.
The chimneys of Aphrocallistes alveolites show a variation of forms. Slender cylindrical forms with straight, parallel walls are characteristic for the Upper Campanian specimens, while those of the Lower Campanian tend to be barrel shaped, somewhat reminiscent of the heads of tobacco pipes.
The sieve plates are separated from the chimney tubes by a clear suture. In some cases, the sieve plates are framed by a strong rim. Sieve plates may be concave, plane, or convex.
Photomicrograph of Aphrocallistes cylindrodactylus (etched specimen), showing the typical honeycomb structure.
The tubular canals, which are open to both sides, are more or less hexagonal in cross-section. The canal walls consist of delicate meshworks made up from fused hexactines (see below), while the canal endings are fortified by heavily silica-mantled scleres. The
Near the outer (dermal) surface, the pore channels sometimes show very delicate, web-like membranes. Presumably all pore openings had such webs, but they are generally not preserved.
Two photomicrographs of Aphrocallistes alveolites showing the walls of pore canals.
These walls are formed by fused hexactines, which are slightly flattened along the "octahedral direction of the cubic coordinate system", and the hexactine arms are connected in a manner such that mostly triangular mesh outlines are produced. The pore walls are very delicate and consist of only a single layer of interconnected hexactines.
In some parts, the axial canals of the hexactines are clearly visible as black lines caused by (bacterially precipitated ?) pyrite fillings.
Photomicrograph of an etched specimen of Aphrocallistes alveolites showing detail of a sieve plate.
Sieve plates differ considerably from the walls of Aphrocallistes alveolites. They are lacking the reinforced pore rims typical for the sponge walls, and the canal walls of the sieve plates are much thicker, consisting of a hexactine meshwork several meshes thick.
Aphrocallistes cylindrodactylus
Schrammen 1912
Aphrocallistes cylindrodactylus is composed of a more or less globular central part with numerous finger-like protrusions. The "fingers" resemble very much the description for A.alveolites.
Aphrocallistes lobatus
Schrammen 1912
Aphrocallistes lobatus is less common. Its characteristical feature is the formation of pouch-shaped structures as the one shown in the picture. The inside of the pouch consists of sieve plates, similar to the terminal sieve plates of A.alveolites (see above).