Name: The Soom Shale
Location: Cape Province, South Africa
Age: Late Ordovician (Ashgill)


Taphonomy


The Soom Shale is a black shale deposit, characteristic of anoxic bottom waters where little or no bioturbation occurs during deposition. This allows increased probability of fossil preservation.

The fossils of the Soom Shale show varying degrees of preservation. At its best, the detail is exceptional, and includes three-dimensional features of soft tissues right down to a sub-cellular level. All of the fossils found in the Soom Shale show no biomineralised hard-part preservation, perhaps a result of acidic bottom water conditions.

Muscle fibres preserved in conodont from Sandfontein site (Gabbott 1998b). Image used with permission of the Palaeontological Association. Muscle fibres showing longitudinal lineation preserved in conodont from Sandfontein site (Gabbott 1998b). Image used with permission of the Palaeontological Association.

The majority of the animals found in the Soom Shale were nektonic or nektobenthic, which indicates that they must have settled out rapidly from the water column upon death. There are no sedimentary structures, apart from fine laminations, which suggests no transportation took place.

Preservation of soft tissues

The preservation of soft tissues in the Soom Shale fossils was a complex process. The Soom Shale is unique in that the fossils can show sub cellular preservation by clay minerals. This fidelity is only seen in other deposits by the replication of organics by phosphate mineralisation.

The transformation of soft tissues to more resistant minerals in the Soom Shale, such as clays, was controlled by many factors including the decay of the individuals and the sedimentary geochemistry. However, it is most likely that clays were directly involved in the replication of animal tissues at the time of preservation.

Chitinozoan chain showing preservation of organics (Gabbott 1998). Image used with permission of the Palaeontological Association.

In normal circumstances, organic material and clay particles both have a negative charge and are mutually repulsive. Perhaps positive ions such as sodium or calcium acted as a bridge in the replication process, and allowed the clays to 'plate' the organic tissue.

The absence of biomineralised tissues in the Soom Shale was most likely due to acidic bottom water conditions. This is suggested also by the occurrence of pyrite in the substrate. However clay precipitation, of the chemistry seen in the Soom Shale, is inhibited by high acidity. A possible theory to explain this uses the decomposition of organic material to reduce the pH of the water surrounding the animal, which would then create a more alkaline area in the immediate vacinity of the fossil, allowing clay precipitaion.

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