Exceptional Preservation in the Jurassic of Osteno

The Sinemurian deposit of Osteno, discovered in 1964, is remarkable for the exceptional preservation of its fossils. They are almost exclusively non-calcareous organisms such as fishes, crustaceans, polychaetes and nematodes. Their exceptional fossilization is due to a metasomatic process implying a molecule-for-molecule replacement of the organic material by colloidal calcium phosphate, a process that has permitted the preservation of the soft tissues of the organisms in some cases even to cellular level. The Osteno deposit formed on a poorly oxygenated sea bottom inhabited by fauna with a low taxonomic diversity, in some cases monotypic. Infaunal organisms are not found in the deposit since the H_2S-O_2 boundary was probably situated slightly below the water-sediment interface.