Cretaceous sponges from the Campanian of Misburg and Höver
Tremabolites megastoma is a lumpy to cushion- or mushroom-shaped sponge with very characteristic, large (10 mm) oscules scattered over the upper surface. The oscules have a slightly protruding rim. The upper part of the sponge is covered by a dense, smooth, siliceous membrane, ending abruptly in an edge along the perimeter, which probably represents the former water-sediment-interface. The underside is very rough and usually obscured by adherent sediment.
Three views of the same specimen (top, side, bottom).
Notice the root-like process on the left side. It appeaers that these roots intruded into the sediment at a shallow angle, and that the sponge was actually resting on its rough underside. Such lateral roots have been observed only with specimens from the Upper Campanian, not from the Lower Campanian. Tremabolites megastoma is thus essentially an encrusting species.
Tremabolites megastoma is closely related, if not identical with Cystispongia monostoma. The small specimen of Tremabolites megastoma shown here has 4 oscules and is hardly larger than typical individuals of Cystispongia monostoma. Also all other features, such as the structure of the underside or the spiculation seem to be equal.
Cushion-shaped specimen of Tremabolites megastoma from Höver.
The enlarged image below of the central part ot the sponge shows two anomalous oscula: The apertures of both oscula have been reduced at some stage, and the right one has subsequently been completely closed. In other cases it has also been observed that single oscula were shut by sieve plates. It thus appears that Tremabolites megastoma was capable to modify or adjust its canalization over its lifetime, or in response to environmental changes.