Preservation and taphonomy of the fossils of the Herefordshire (Silurian) Konservat-Lagerstätte

The Herefordshire Lagerstätte represents a fully marine Silurian ecosystem, three dimensionally soft-bodied fossils, preserved by sparry calcite, which are recovered from carbonate concretions. Although, a taphonomic model exists it does not determine the pathways that led to their preservation, nor has it been tested on the other taxa recovered from the Lagerstätte. X-ray fluorescence (XRF) analysis indicates the sediment originated from an andesitic volcano from a destructive plate margin. Field relationships suggest that the bed was deposited from either a turbidity current or debris flow. Given the highly reactive nature of the volcanic ash, the cation exchanges that occur provide a mechanism for supersaturating the pore-fluid with carbonates. Examination of the four common taxa; Offacolus kingi (arthropod), Tanazios dokeron (arthropod), Kenostrychus clementsi (polychaete worm) and Acaenoplax hayae (mollusc), indicates a similar pathway of preservation. Energy-dispersive X-ray spectroscopy (EDX) identifies the only impurity within the fossil calcite is manganese; in comparison the concretion carbonates contain a variety of different cations. Therefore, the fossils and the concretions are the result of different processes, which were not coeval. Comparing the preservation Kenostrychus clementsi (polychaete) against experimental work indicates that preservation occurred within 6 days and rules out the occurrence of a void stage as previously thought. In addition, it also suggests that preservation was either instantaneous or there must have been an intermediate “medusa” stage that preserved the tissue prior to templating. Electron microprobe analysis (EMPA) data indicate that the clay minerals that precipitated around the decaying animal were templating the organic matter. To precipitate the sparry calcite without the void stage, an hypothesis is proposed in which the sparry calcite nucleates and grows through the soft tissues. Isotopic data indicates that the concretions are not produced by organic matter decay. This is supported by the lack of correlation between the concretion and the fossil and the occurrence of barren concretions. Radial variation in the mineralogy, chemistry and isotopic ratios support the hypothesis that these concretions grew concentrically around a non-organic nucleus. These exceptional conditions may account for a preservation style that is so far unique to this single exposure.