Name: The Hunsrück Slate
Location: Western Germany
Age: Devonian
 


TAPHONOMY


TAPHONOMY

Burial of sessile feeders such as crinoids in situ and the occurrence of arthropod trace fossils proves that the overlying water was oxygenated. Deposition of fine grained mudstone is not sufficient to preserve soft tissue, nor is rapid burial on its own. Rapid burial and pyritisation are needed.

Conditions for intact burial and pyritisation of fossils developed locally and were of short duration. This explains why the majority of the Hunsrück fauna are disarticulated and not exceptionally preserved. The best preserved fossils occur in localised areas, within certain units.

Although many fossils occur in situ, the more vagile forms were transported over very short distances to low lying areas of the sea floor. Turbidite currents resulted in rapid burial in a variety of orientations to bedding surfaces.

   

The trilobite Chotecops in a variety of orientations.

Taken from Bartels, Briggs and Brassel, The Fossils of the Hunsruck Slate, Marine Life in the Devonian, Cambridge University Press, figure 33, p. 48, collection Bartels in German Mining Museum Bochum, Coll Bartels, by kind permission of the Bergbau Museum, Germany.

Various organisms including the ophiuroid Furcaster palaeozoicus, a small crinoid and fragments of the bizarre arthropod Mimetaster hexagonalis.

Taken from Bartels, Briggs and Brassel, The Fossils of the Hunsruck Slate, Marine Life in the Devonian, Cambridge University Press, figure 23, p. 38, collection Bartels in German Mining Museum Bochum, Coll Bartels, by kind permission of the Bergbau Museum, Germany.

Rapid burial protects the fossil from scavengers that thrive in oxygen-rich environments. The various orientations seen in the diagrams of Chotecops and of the assemblage below indicate the flora and faunas were killed by a sediment-laden density current.

PYRITISATION

Early pyritisation not only preserves relief but it also allows the fossils to be analysed using x-radiography. Pyrite has the ability to absorb x-rays whilst they penetrate the mudstone matrix.

Extensive pyritisation is confined to the Bundenbach and Gemünden areas. Skeletons can be either covered with a thin film of pyrite or replaced by fine crystals. Appendages of arthropods occur because after muscle has decayed pyrite precipitates from within the exoskeleton to infill appendages before the collapse of the limbs.

The trilobite Chotecops and the remarkable preservation of its appendages.

Taken from Bartels, Briggs and Brassel, The Fossils of the Hunsruck Slate, Marine Life in the Devonian, Cambridge University Press, figure 113, P136, collection Bartels in German Mining Museum Bochum, Coll Bartels, by kind permission of the Bergbau Museum, Germany.

Pyritisation of soft tissue is rare in the fossil record, other examples are: Beecher's Trilobite Bed in Upper Ordovician of New York State and La Voulte Sur Rhone in the Middle Jurassic of SE France.

Several factors are critical for preservation of soft tissue - rapid burial and the chemistry of the sediment. Surrounding sediments must be low in organic matter so that dissolved iron and sulphur can diffuse to the carcass instead of precipitating in the sediment. High iron concentrations are also needed to promote rapid pyrite formation in association with decaying organic soft parts. Different fossils within a small region and different parts of the same fossil can show varying degrees of pyritisation.

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