This is a very complete and nicely preserved specimen of the fascinating species Pravitoceras sigmoidale. Its a very rare ammonite from the upper cretaceous of Japan with a stunning heteromorph shape. The whorls are loosely coiled and the final coil of Pravitoceras forms a distinctive “S” shape. The living chamber is folded back on itself to form a hook. 

The preservation of ammonites from that locality is quite unique and extremely challenging for preparation. Althought the matrix is very much weathered and often broken into several pieces with many cracks and crevices, it is partially extremely hard and very sticky. It seems like the process of building a geode around the ammonites was interrupted or heavily disturbed. The core is often very soft, just like the whorls of the ammonites, which are sometimes even hollow. It requires a lot of glue to actually recover a specimen in the field and it took more than two bottles of superglue to secure this very specimen during preparation. Some small missing pieces have been filled up and restored. But in the end all the effort was well worth it, being able to admire natures bizarre forms of cretaceous marine life.

Fossil: Pravitoceras sigmoidale YABE 1902

Locality: Awaji Island, Japan

Formation: Upper Cretaceous, Campanian, Seidan Formation, Izumi Group

Length: 25,5 cm (10 inches)

Work required: 30 hours

Collection: Hashimoto Ryohei