Ecology of sponge reefs from the Upper Bathonian (Middle Jurassic) 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 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. The fauna and habitat of the reefs are compared with those of Upper Jurassic lithistid sponge reefs in southern France and Germany, and with those of Middle and Upper Jurassic sponge accumulations in Great Britain.The Platychonia magna fronds underwent early diagenetic loss of their original hyaline silica skeleton, together with early lithification of the fine sediment which permeated their canals after death. This appears to be a common occurrence in fossil lithistids.