Ecology of Sponge Reefs From the Upper Bathonian of Normandy

Platychonia magna (d'Orbigny), a lithistid sponge previously regarded as a calcisponge (Cupulospongia magna ), forms small bioherms in rocks of Upper Bathonian age (sub-zone of Clydoniceras hollandi ), at Saint-Aubin-sur-Mer on the Normandy coast. In addition to the main frame-builder, four subsidiary faunal groups are preserved. These are: (i) encrusting organisms which attached to the sponge fronds and gave the reef additional strength; (ii) byssate and pedically attached filter-feeding nestlers; (iii) vagile strollers which predated, scavenged, or grazed; (iv) borers. The encrusters may be further subdivided into a low diversity assemblage on the upper sides of the Platychonia fronds, and a high diversity assemblage on their undersides. The reefs are envisaged as having grown in the lower photic zone, below normal wave-base but above that reached during storms.