The Sachrang Formation or "Posidonienschiefer" Formation (Whose Vulgar name is "Posidonia Shale") is a geological formation of southwestern Germany, northern Switzerland, northwestern Austria, southeast Luxembourg and the Netherlands, that spans about 3 million years during the Early Jurassic period (early Toarcian stage). It is known for its detailed fossils, especially sea fauna, listed below. Composed mostly by black shale, the formation is a Lagerstätte, where fossils show exceptional preservation (Including exquisite soft tissues), with a thickness that varies from about 1 m to about 40 m on the Rhine level, being on the main quarry at Holzmaden between 5 and 14 m. Some of the preserved material has been transformed into fossil hydrocarbon Jet, specially wood remains, used for jewelry. The exceptional preservation seen on the Posidonia Shale has been studied since the late 1800s, finding that a cocktail of chemical and environmental factors let to such an impressive conservation of the marine fauna. The most common theory is the changes in the oxygen level, where the different anoxic events of the Toarcian left oxygen-depleted bottom waters, with the biota dying and falling to the bottom without any predator able to eat the dead bodies.
Biological interaction frozen in time
Several animal behaviours have been recovered on the Posidonia Shale. The Monotis-Dactylioceras bed is one of them, as it shows an accumulation of the bivalves Meleagrinella substriata and the ammonite Dactylioceras, that were the most abundant representatives of its group on the Altdorf region, and were probably washed to near epicontinental waters by a rapid event, or as result of a large succession of events. This assemblage has been compared with modern Brazilian coastal Mangroves and also linked to Tsunami events. Related to the Ammonite fauna, on Holzmaden there have been found several empty shells of this cephalopods, with associated crustaceans inside. The original specimen was reported in 1995 and consisted on a possible member of the genus Paleastacus inside a chamber of a Harpoceras. Other epizoans are found related to the decayed Ammonite shells, such as Serpulid Annelids and Bivalves, creating what was denominated as "benthic islands" as reference at be isolated benthic units that attracted fauna. The Decapod is related to the family Erymidae, that are considered as possible bottom-dweller carnivorous or carrion feeders. The associated fossil has several spherical structures that had been interpreted as decapod coprolites, implying that the animal lived for a long period on the shell, and maybe changes on the bottom oxygen stopped the process. More recent studies had recovered new data about the inquilinism of decapods inside ammonites, this time, however, recovering three Eryonoidea lobsters together within a body chamber. The lobsters most likely used the ammonoid as some kind of shelter, where was excluded the possibility of transportation into the body chamber by bottom currents. There are several theories about the gregarious inquilinism showed by this specimens, such as that the shell was and ideal location to molt, as has not been proved to be corpses or molts; that the shell provided protection against predators; the decomposing soft body of the ammonoid was a source of food that attracted the decapods or that was used as a long-term shelter. One key aspect found was that the muddy bottom was not suitable for burrowing, implying that the decapods look for a different shelter due to being unable to make their own. Beyond trace fossils, several vertebrate specimens show associations with crustacean exoskeletal remains such as GPIT-PV-31586 and SMNS 58389 (Pachycormus macropterus) with necrophagous interaction as well SMNS 55934 (Stenopterygius quadriscissus) or SMNS 95401 (Metopacanthus sp.).