Posidonia Shale, Jurassic geological formation in Holzmaden, Germany

The Posidonia Shale is an Early Jurassic (Toarcian) formation of fine-grained, bituminous oil shales and bituminous limestones in thin alternating layers. It is not a slate: the apparent cleavage is a very fine lamination of the sediment. It crops out in southwestern Germany, most notably near Holzmaden and Dotternhausen, and is known in Germany as the Sachrang Formation.

The first fossils from these layers were recorded in 1598 by Johannes Bauhin, marking the beginning of centuries of paleontological study. Since then, the formation has become one of the most important sources of information about ancient marine reptiles and their world.

The name comes from the bivalve Posidonia bronni, now reclassified as Bositra buchii, whose shells are common in the rock. The term is technically wrong, since the stone is not a slate but a finely laminated bituminous claystone. Near Holzmaden the Urwelt-Museum Hauff displays finds on 1000 m², and the Petrefaktensammlung at Kloster Banz holds another collection.

Fossils from the formation are best seen in museums like the Urweltmuseum Hauff rather than in active quarries, which may have restricted access. Visiting during warmer months allows for better conditions if you want to explore quarry areas on foot.

The preparator Bernhard Hauff showed that Stenopterygius had a dorsal fin and an upper-lobed tail fin, body parts without skeletal support that normally do not fossilise. Hauff recorded over 350 ichthyosaurs at Holzmaden, among them many females fossilised during birth or with embryos inside, evidence that the animals gave birth in this sea.