Pleurope lacunosa

Figure 1 - Pleurope lacunosa. Alemannia, Höver.
Most complete specimen of its kind known to date.

Figure 2 - Pleurope lacunosa. Alemannia, Höver.
Transition from stem to plicated funnel.

Figure 3 - Pleurope lacunosa. Oberg, near Peine.
Two views of a typical stem section.

Figure 4 - Pleurope lacunosa. Alemannia, Höver.
Typical stem sections with holdfasts.

Synonyms:

Pleurostoma lacunosum Roemer 1841

Occurence:

Teutonia, Misburg, Upper Campanian (spiniger zone). Rare.

Alemannia, Höver, Lower Campanian (senonensis zone). Rare.

Previously, only the stems of Pleurope lacunosa were known and were actually mistaken for the sponge itself. The specimen shown here in Figure 1, although incomplete, reveals the true nature of Pleurope lacunosa.

The height of the specimen shown in Figure 1 is 210 mm, the original diameter of the funnel was approximately 120 mm; the ribs are approximately 6 to 7 mm thick; the wall thickness is 1 mm at the outer rim.

Pleurope lacunosa has a flat, scabbard-like stem with large oval openings (parietal oscula) along its narrow margins. However, approximately at half-height of the sponge, the stem widens to form a longitudinally folded funnel with a trumpet-shaped top (Figures 1 through 3). The inward-facing longitudinal folds of the lower funnel section turn into a set of pronounced radiating arched ribs. The outward-facing longitudinal folds of the lower funnel section grade into the flat sectors between the ribs.

Pleurope lacunosa is not known to form branching roots. The basal parts shown here apparently penetrated other objects to obtain some anchorage. However, the specimen shown in Figure 1 reveals tufts of siliceous fibres radiating laterally from the stem base and erecting at some distance (10 to 20 mm) from it.

Figure 5 - Pleurope lacunosa. Alemannia, Höver.
Cross section through stem. Etched specimen.

Figure 6 - Pleurope lacunosa. Alemannia, Höver.
Dictyonal skeleton. Etched specimen.

The skeleton of Pleurope lacunosa is composed of several layers of dictyonal lychnisks with pronounced cortex formations on the gastral and dermal surfaces (Figure 5). Notice smaller hexactines attached to the lychnisk skeleton. Such hexactines are abundant near the narrow edges of the stem, i.e.around the parietal oscula, but absent otherwise.

The dictyonal skeleton looks very regular in side view (Figure 6). However, as seen in Figure 5 (cross section), the lychnisk layers form an acute angle with the dermal and gastral surface, introducing some mismatch between subsequent layers. This feature is probably restricted to the stem section. No particular epirhyses or aporhyses are developed. Instead, water circulation passes through the meshes of the skeleton.

Figure 7 - Pleurope lacunosa. Alemannia, Höver.
Dermal cortex. Etched specimen.

Figure 8 - Pleurope lacunosa. Alemannia, Höver.
Dermal cortex viewed from inside. Etched specimen.

Figure 9 - Pleurope lacunosa. Alemannia, Höver.
Gastral cortex. Etched specimen.

The dermal surface is covered by a rather massive cortex (Figure 7), which leaves round pores (ostia) on every other mesh of the dictynal skeleton (chess board pattern), while the other half of the dictynal meshes are shut by a porous netting (Figure 8). The surface of the dermal cortex is smooth.

The gastral surface is covered by a less massive cortex (Figure 9). However, the general structure is similar to that of the dermal surface, leaving round pores (postica) on every other mesh of the dictyonal skeleton (chess board pattern). The gastral cortex has a velvet-like surface (Figure 5 may give a vague idea).

Figure 10 - Pleurope lacunosa. Alemannia, Höver.
Filamentous overgrowth. Etched specimen.

The lower stem sections and particularly the narrow margins and peripheries of the parietal oscula are covered with a peculiar filamentous network of anaxial silica strings. The latter are interconnected by synapticula, forming ladder-like structures, and are connected to the underlying cortex by similar synapticula. Figure 10 is a transect from an oscule rim (left) to the middle of the broad side of the stem, showing the filamentous network in the left and the normal dermal cortex in the right.