Not all aragonitic molluscs are missing: taphonomy and significance of a unique shelly lagerstätte from the Jurassic of SW Britain

摘要: The Blue Lias Formation at Lyme Regis (Dorset, UK) includes an exceptional pavement of abundant large ammonites that accumulated during a period profound sedimentary condensation. Ammonites were originally composed aragonite, unstable polymorph calcium carbonate, and such fossils are typically prone to dissolution; the occurrence rich association aragonitic shells in condensed bed is highly unusual. Aragonite dissolution occurs when pore-water pH reduced by oxidization hydrogen sulphide close sediment-water interface. Evidence suggests that, this case, oxygen concentrations overlying water column low deposition. This inhibited oxidation sulphides associated lowering pH, allowing aragonite survive long enough for shell be neomorphosed calcite. loss impacts upon estimates past biodiversity carbonate accumulation rates. preservational model presented here implies diagenetic will greatest those areas where dysoxic-anoxic sediment lies beneath oxic waterbody but least depleted. Unfortunately, bias through biotopes which most diverse lowest due restriction.