PYRITE TEXTURES OF SKIN-LIKE MEMBRANES IN BRITTLESTARS (OPHIUROIDEA, ECHINODERMATA) FROM THE HUNSRÜCK SLATE (LOWER DEVONIAN, EMSIAN) OF GERMANY: PRELIMINARY RESULTS

The Hunsrück Slate is known for the exceptional preservation of pyritized invertebrates and vertebrates. Aside from fully articulated skeletons, pyritized soft-tissue has also been reported. Among the echinoderms, soft-tissue has been described in holothurians, crinoids, and stelleroids. However, the validity of these reports has been challenged, with some arguing that they are misinterpretations of conventional skeletal structures or artifacts. Nevertheless, unequivocal preservation of tube feet was recently documented in several specimens of the protasterid ophiuroid Bundenbachia beneckei.

Traditionally, authors have pointed to the skin-like membranes preserved in Hunsrück ophiuroids (e.g. Loriolaster, Cheiropteraster) as examples of soft-tissue preservation. Alternatively, Otto (1994) proposed that these structures are the product of the fusion and overgrowth of tightly packed skeletal sclerites that pyritized only after dermal tissues decayed.

To characterize the pyrite textures of the skin-like membranes, two specimens of Loriolaster were studied using Scanning Electron Microscopy. The dermal sclerites of Loriolaster are between 10 and 100 µm in diameter and interstitial spaces between 10 and 200 µm. Sclerites are covered in subhedral to euhedral pyrite crystals. Elemental analysis detected both original high-magnesium calcite as well as quartz. The latter forms an amorphous mass between the pyrite crystals. In contrast, the interstitial spaces between sclerites are almost exclusively filled with tightly packed pyrite framboids between 10 and 15 µm in diameter. Occasional (<1%) subhedral pyrite crystals float within this groundmass. Original histological structures are not preserved.

Pyrite framboids can be formed through biogenic and inorganic processes. However, they are associated with soft-tissue preservation and are known to form relatively early in the decay process. In contrast, late stage pyrite, as infill or overgrowths, is typically in the form of crystal aggregates. Our preliminary results suggest that pyritization of the interstitial spaces likely occurred during initial decay when dermal tissue was still present. Hence, origin of the skin-like membranes remains indeterminate but could represent a pyritic cast or permineralization of the original tissue.