EXCEPTIONAL PRESERVATION IN AN UNEXPECTED ENVIRONMENT: FE-MUMMIFICATION OF CRYPTIC BRYOZOANS FROM A HIGH-ENERGY, PLIO-PLEISTOCENE SANDWAVE FACIES

Most marine Konservat-Lagerstätten consist of muddy sediments deposited in low-energy regimes, often in anaerobic conditions where early diagenetic phosphatization can occur. The Red Crag Formation of southeast England is a striking exception to this rule. This Plio-Pleistocene shallow marine deposit comprising medium- to coarse-grained, shelly, quartzose sands containing cross-bedded sets up to 5 m high. As the name suggests, the Red Crag is red in outcrop but this colour is due to oxidation: unweathered samples from the subsurface may be green. Convex-upwards, disarticulated bivalve shells (e.g. Glycymeris) are locally abundant, some encrusted on their concave inner surfaces by cyclostome and cheilostome bryozoan colonies. These bryozoans can be exquisitely preserved, with delicate articulated spines and calcified opercula in situ. While this exceptional preservation was facilitated by the bryozoans growing in cryptic microhabitats on the sheltered undersides of shells, diagenetic factors were also involved. Remarkably, the calcified skeletons of the bryozoans are normally lacking, and the colonies are preserved as high-fidelity mummies coated by rusty red goethite. Scanning electron microscopy of the Fe-coatings occasionally reveals tiny threads, suggesting a bacteriogenic origin. Coatings must have formed very early in diagenesis before complete decay of organic tissues that held articulated parts of skeletons together. The original mineralogy of the coatings is unknown but they may have been the same green mineral that gives unweathered Red Crag its colour and has been variously identified in the literature as glauconite or chlorite. Conversion of this mineral to goethite was accompanied by dissolution of the entirely calcitic skeletons of the cyclostome and most of the cheilostome bryozoans. Paradoxically, the aragonitic bivalve substrates, as well as aragonitic skeletons of the cheilostome Phylactella aff. labrosa, were not dissolved, contrary to the routine dissolution of aragonite but survival of calcite in the rock record. No unmineralized parts of the bryozoans have been observed but this unusual mode of exceptional preservation in an unexpected environment may yet furnish soft parts.