Preparation of Brown-Shelled Pleuroceras spinatum (Upper Pliensbachian, “Holzbachacker” Clay Pit, Buttenheim)

 

These brown-shelled specimens of Pleuroceras sp. originate from the so-called “Hard Nodule Bed” of the Upper Pliensbachian in the “Holzbachacker” clay pit near Buttenheim. This layer, approximately one meter thick, consists of several superimposed nodule horizons containing hard, pyrite-rich geodes. It lies about two meters below the boundary to the Toarcian. The best and most complete ammonite specimens are typically found in the lowest nodule horizon, where the nodules often display an ammonite-like shape even externally. The bed is underlain by small, golf ball–sized geodes containing glendonite crystals. The average diameter of the ammonites is around 4–5 cm, while larger, well-preserved specimens are extremely rare. Due to the generally poor separation between fossil and matrix, ammonites are often discovered only by breaking open the geodes—frequently resulting in fragmentation.

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Other fossils, such as Amaltheus sp. or gastropods like Angulomphalus expansus, occur in the Hard Nodule Bed but are far less common than ammonites.

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The preparation of these fossils requires great patience and precision. After coarse exposure, detailed work is usually carried out under a stereomicroscope. Very fine pneumatic chisels, needles, and scalpels are used to gradually uncover the ribs, spines, and whorls. Because the thin ammonite shell is extremely brittle and fragile, it must be regularly stabilized with thin cyanoacrylate (superglue). Toward the end of preparation, the specimens are carefully cleaned at low pressure using an air-abrasive unit with iron powder. If necessary, missing areas are reconstructed with Apoxie® Sculpt and color-matched. Finally, a stone sealant is applied to enhance the natural coloration and surface texture.

Depending on size, shell quality, degree of separation, and pyrite encrustation, the complete preparation of a Pleuroceras specimen can require anywhere from 20 to 40 hours of work.