Document Type : European UNESCO Geoparks: Review Article

Author

Werkforum and Fossil Museum, Holcim (Süddeutschland) GmbH, Dotternhausen, Germany

Abstract

The Lower Jurassic Posidonia Shale of Southern Germany is famous for its excellently preserved fossils. First of all, the large and spectacular ichthyosaurs, pterosaurs and crocodiles impress. Fish, crinoids, ammonites and belemnites are witnesses of a very special living world in the former Posidonia Shale Sea. The rather small and inconspicuous bivalves, brachiopods and serpulids provide important clues about conditions in the ecosystem. The Posidonia Shale is one of the best-known fossil deposits in the world, and for centuries has provided scientists with an enormous wealth of information. One of the most important fossil collections is displayed in the Fossil Museum of the Werkforum in Dotternhausen. Together with the quarries (active and recultivated), the museum offers unique opportunities for scientific and geoeducational purposes.

Keywords

Main Subjects

Introduction

 The Posidonia Shale of Southern Germany has been known for several centuries for its excellently preserved fossils (e.g., Quenstedt 1885; Hauff & Hauff 1981; Riegraf et al. 1984; Urlichs et al. 1994). Faunal content, sedimentary structures and the high amount of organic matter indicate oxygen deficiency and very calm depositional conditions (e.g., Seilacher 1982, 1990; Röhl et al. 2001; Schmid-Röhl et al. 2002). Generally, benthic life was rare and, if present, limited to a few adaptable species. The bituminous marls were deposited over about 3 million years in a shallow basin, a marginal sea of the Tethys. Due to the multitude of research methods used in the past, a thoroughly dynamic overall picture of the ecosystem with recurring, relatively short phases allowing benthic colonization is emerging (Röhl et al. 2001).

 

The dark gray rock sequence of the 180 Ma Posidonia Shale Formation is in the foothills of the Swabian Alb (Fig. 2). There, the cement factory and Fossil Museum owned by Holcim (Süddeutschland) GmbH, is located in the small village of Dotternhausen. Active rock mining of the Posidonia Shale currently occurs close to the neighboring village of Dormettingen. Dotternhausen and Dormettingen are situated between Balingen and Rottweil, about 80 km southwest of Stuttgart. There the total thickness of the Posidonia Shale Formation is about 12 m.

 

Fossils and Paleoenvironment

Fossils

Completely preserved vertebrate finds are rare in the fossil record. Therefore, a Fossillagerstätte like the Posidonia Shale is of outstanding importance for paleontology. Numerous excellently preserved skeletons of fish, ichthyosaurs and rarer finds of crocodiles and pterosaurs could be recovered in Southern Germany, especially in the Holzmaden area but also in Dotternhausen and Dormettingen. Extraordinary ichthyosaur skeletons – some of which even show soft tissue preservation – were found in the dark bituminous clay- and marlstones of the Posidonia Shale (Fig. 1). It is all the more astonishing that, after such a long time of intensive collecting and research, new discoveries are being made again and again, providing new pieces of the puzzle for understanding the former ecosystem. In recent years, scientists described two new vertebrate species from the Posidonia Shale of Dotternhausen: the shark species Crassodus reifi (Maisch & Matzke 2016) and recently the ichthyosaur species Hauffiopteryx altera (Maxwell & Cortés 2020).

 

Figure 1. Skull of the ichthyosaur Temnodontosaurus trigonodon found in the Posidonia Shale of Dotternhausen (length: 1,12m). Werkforum and Fossil Museum Dotternhausen, FWD_0130.

  

Among the most common macro-invertebrates of the Posidonia Shale are bivalves, ammonites and belemnites. They often occur in high abundance but low diversity indicating an environment with poor living conditions. At first sight, the often badly preserved bivalve shells and imprints appear unspectacular. The individuals are mostly inconspicuously small, yet they are of crucial importance for the interpretation of living conditions (Röhl et al. 2001; Schmid-Röhl & Röhl 2003). The bivalve Posidonia (today Bositra) gave its name to the dark clayey marlstones, but there are still many uncertainties about its way of life. Bositra occurs repeatedly in masses in certain strata. The small shells with up to 20,000 individuals/m2 lie close together. For a long time, the way of life of Bositra was controversial. Interdisciplinary studies show that Bositra had a bottom-related way of life and was able to colonize the seafloor quickly and at high density.

 

In general, ammonites are very common in the Posidonia Shale. The most abundant genera are Dactylioceras, Harpoceras, Hildoceras, Lytoceras and Phylloceras. After the death of the ammonite the shells were dissolved and compressed during compaction of the clayey sediment. Only the thin pyritized periostracum is preserved and gives the ammonites their pretty golden-brownish color. Belemnites are abundant in certain strata of the sequence but are absent in the lower part of the black shale facies. They seem to have had a more bottom-related way of life, as they are mostly absent in layers with very high organic carbon content. On layers with accumulations of numerous belemnite rostra they can serve for the reconstruction of paleocurrents.

 

Paleoenvironments

The Posidonia Shale has been the subject of intense scientific investigation for 400 years. Since 1900, questions concerning the depositional conditions and ecology during the Early Jurassic became the focus of interest. To explain the formation of the rocks, comparisons were made with modern ecosystems, such as the Black Sea or the Baltic Sea. Various studies could prove that oxygen availability in the Posidonia Shale Sea was subject to strong fluctuations. Phases of benthic colonization alternated with anoxic or even euxinic conditions. Both colonization on elevated benthic islands (Kauffman 1978, 1981) and a pseudoplanktonic way of life (Seilacher 1980, Matze & Maisch 2019) were "survival strategies" in the ecosystem of the Posidonia Shale Sea.

 

Changes in oxygen content in the benthic environment of the Posidonia Shale can be explained by climatic changes. During the Early Jurassic, the subtropical location of Central Europe, a high sea level, a warm balanced greenhouse climate with mild water temperatures and a seasonally changing mega-monsoon influenced the environmental conditions. When the bituminous sediments were deposited, southwest Germany was located at about 30° northern latitude, which corresponds roughly to the current latitude of cities like Cairo or New Orleans. The position of the continents and the existence of the huge continental mass Pangaea led to a special paleoclimatic situation. The resulting wind systems, which were strongly seasonal, were mega-monsoons. They influenced the circulation in the sea basin by changing sedimentation rate, nutrient input and evaporation rates.