The bigger part of the rock containing the ammonite in cross-section

The counterpart on a smaller slab of stone

The living chamber is worn off on this side

The fossil emerges step by step

The ammonite was flipped and inserted into the large stone

The final result after 15 hours of meticulous preparation work

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This ammonite is a Neosilesites sp. from the lower Cretaceous of Japan. Its special feature: Except for the living chamber, the whole ammonite is preserved in calcite. Normally these ammonites are found as steinkern, showing relatively poor sculpture. However, the calcite preservation might have preserved special features, but at the same time presents a challenge for the preparation. The coarse-grained calcite appears to have fused with the hard matrix and is extremely fragile. 

The ammonite was found torn in the median, with the already roughly exposed living chamber on the smaller piece of rock heavily weathered on one side. Since consequently the other side is better preserved, this side should be prepared. However, the larger stone half is clearly better suited as a matrix piece for the presentation of the ammonite. For this reason the ammonite was sawed out of the rock, exposed and then reinserted upside down into the rock piece. The preparation was easily accomplished by careful blasting under the binocular. Only the calcite had formed small cavities in places, some of which collapsed during exposure and had to be filled or restored. 

After completion of the preparation a specimen with astonishingly well preserved ribbing, which could not be observed so far in the stone core preservation of these ammonites is the final result.

Fossil: Neosilesites sp. 

Collection: Ryohei Hashimoto

Diameter: 5cm (2 inches)

Work required: 15 hours