The Sachrang Formation or "Posidonienschiefer" Formation (common name the "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 marine biota, listed below. Composed mostly of 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 the fossil hydrocarbon jet which, especially jet derived from wood remains, is used for jewelry. The exceptional preservation seen in the Posidonia Shale has been studied since the late 1800s, finding that a cocktail of chemical and environmental factors led to such an impressive preservation of the marine fauna. The most common theory is that changes in the oxygen level, where the different anoxic events of the Toarcian left oxygen-depleted bottom waters, stopped scavengers from consuming the dead bodies.
Biological interactions
The "Monotis"-Dactylioceras bed 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.
Several empty ammonite shells from Holzmaden have been found with associated decapods inside. This includes a possible member of the genus Paleastacus inside a chamber of a Harpoceras. This decapod is related to the family Erymidae, which are considered to be possible bottom-dweller carnivores 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 within the shell. More recent studies had recovered new data about the inquilinism of decapods inside ammonites, this time, however, recovering three eryonoids together within a body chamber. The eryonoids most likely used the ammonoid as some kind of shelter, possibly due to the shell being an ideal location to molt, protection against predators, a source of food or that the shell 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 inhabited a different shelter due to being unable to make their own. Other biota are found related to the decayed ammonite shells, such as serpulid annelids and bivalves, creating what was denominated as "benthic islands". 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.). The genus Clarkeiteuthis and its predatory behaviour, found associated with fishes of the genus Leptolepis. Based on the position of prey and predator, it was suggested that the cephalopods caught and killed the fishes while the schools were still in well-oxygenated waters and then descended into oxygen-depleted water layers where the cephalopod suffocated and died attached to its prey. The cephalopods' arms were contracted over the fish, likely killing it quickly by cutting its spine. Several Geotheutis have been reported with eumelanin preserved along with their ink sacs. A specimen of Jeletzkyteuthis found in Ohmden has appeared predating a Parabelopeltis. The association of these 2 genera shows the predatory behaviour of this group when its members lived in epicontinental seas, which is rather different than extant Vampyromorphs. A pabulite (fossilized meal which never entered the digestive tract) was recovered from Holzmaden, being composed of an associated Passaloteuthis laevigata with its arms embracing an exuvia of a crustacean. The belemnnite itself can be seen as the remnant of a failed predation attempt from a Hybodus, corroborating a possible tropic chain.
One of the most complex organism interactions on the Posidonia Shale were the crinoid megarafts that grouped a wide variety of animals, creating large floating ecosystems. The largest megaraft found measured 18 metres (59 ft), and is based on an Agathoxylon trunk, where different animals were attached. The first attached animals would have been the growing community of oysters, bivalves and crinoids, that would add a small weight to the raft (about 800 kilograms (1,800 lb)). The presence of these megarafts were in part possible due to the absence of marine wood worms which destroy tree logs in less than 3 years along with the absence of modern raft wood predators (that appeared on the Bathonian). These rafts could last up to 5 years, which is the main reason the crinoids attached were able to reach huge sizes. These rafts were likely also essential to distribute animals along sea basins. Seirocrinus & Pentacrinites were some of the main crinoid colonizers of the floating rafts. Seirocrinus is the main representative of the pelagic crinoids, being among the longest animals known, reaching 26 m long in the largest documented specimen. The ecology of the genus is widely known, with it being known that the smallest stems were among the first animals to colonize the rafts, with at least 2 generations of crinoids found per raft, where the hydrodynamic changes of the log influenced the settlement of the crinoids. It is believed that Seirocrinus underwent seasonal reproduction linked to monsoonal conditions that sent new logs to the sea. The large crinoids would have feed on pelagic micronutrients, and after the log fell to the bottom, all of the colony would have died. Thoracic barnacles of the genus Toarcolepas became the oldest epiplanktonic barnacle known in the fossil record, probably motivated by the appearance of the giant crinoid rafts. It has been found in situ associated with fossil wood.
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