Arnstadt Formation

The Trossingen Formation, formerly the Knollenmergel (also known as the Arnstadt Formation), is a geological formation in Germany and Switzerland. It dates back to the late Norian-Rhaetian. It belongs to the Keuper Group of the Germanic Triassic. The ''Trossingen Formation'' is underlain by the Löwenstein Formation and erosively overlain by the Exter Formation, in part also directly by the Black Jurassic.

Definition

The horizontal boundary of the ''Trossingen Formation'' is a diachronous facies boundary and is characterized by the exposure of the sandstones of the Löwenstein Formation. The hanging wall boundary is also sharply defined facially and characterized by the erosive overburden with terrestrial sediments of various locally widespread strata of the Exter Formation (Malschenberg Clay or Sandstone, Contorta Clay, Tübingen Sandstone) or, in the case of the more extensive Upper Keuper Hiatus, directly with the dark marine mudstones of the Hettangian onwards Black Jurassic.

The ''Trossingen Formation'' itself consists of fairly uniform, reddish-brown to red, sometimes purple claystones with occasional layers of calcareous nodules (hence the name “Knollenmergel”). In northern Franconia, continuous limestone crusts several meters thick can also occur. Carbonaceous mudstones and silicified limestones also occur locally at the top of the succession.

The Trossingen Formation is restricted to the edge area of the Vindelizian Threshold and thus to the South German stratigraphic zone. The average thickness is about 40 to 50 m, in northern Franconia values between 55 and 60 m are reached. In southern Württemberg (Wutach area), the thickness decreases to 10 m. The differences in thickness are due on the one hand to the interlocking with the Löwenstein Formation and on the other hand to the Rhaetian/Early Jurassic erosion.

Chronostratigraphically, the ''Trossingen Formation'' is placed in the uppermost Norian or lower Rhaetian (upper Upper Triassic). The type locality or type profile is the impact slope of the Trosselbach near Trossingen (Tuttlingen district). It was named after the town of Trossingen. The type region is southwestern Baden-Württemberg.

The outcrops of the ''Trossingen Formation'' are characterized by so-called “hummocky” meadows. It forms very heavy soils (minute soil), which can be recognized by typical tree growth (crooked and leaning trees), and is prone to frequent landslides after rainfall. Tuberous marl landscapes can often only be used for orchards.Due to its richness in claystone, the Knollenmergel, like many Keuper formations, is a difficult subsoil for buildings of any kind because the claystones contain three-layer clay minerals that are swellable due to water retention in the intermediate layers. Intensive arable farming is therefore also problematic due to the fact that it is highly dependent on weather conditions. Like many claystone-dominated rock sequences, nodular marl tends to creep on slopes. This manifests itself in the sabre growth of trees on slopes.

The lithology of the ''Trossingen Formation'', particularly at the Trossingen dinosaur site, is dominated by fine-grained sediments with minimal stratification, indicating post-depositional disturbance. Sandstones are rare, appearing only as thin intercalations in certain areas. Is dominated by mudstones with varying compositions and colors, divided into three main stratigraphic units: the lower beds, dark purple, marbled mudstones rich in carbonates. They contain calcitic replacements of small roots and show signs of pedogenic processes, including mud cracks and pseudobreccias. The presence of carbonate is highest in these beds, contributing to their distinctive texture and appearance. Next are homogeneous red-brown beds of light reddish-brown mudstones with lower carbonate content. They contain occasional small, round carbonate nodules, which increase in size and clustering towards the lower part of the unit. The last beds range from light purple to brown mudstones and have moderate carbonate content. Some layers contain calcitic root replacements, scattered carbonate nodules, and mud cracks that transition into pseudobreccias. This unit lacks the marbled appearance of the Lower Beds but shares similarities in texture and composition.

Historical Work

The ''Trossingen Formation''’s excavation history dates back to the early 20th century when a schoolboy stumbled upon dinosaur bone fragments at a site known as "the Rutschete". His discovery caught the attention of local scientists, leading to initial excavations by Eberhard Fraas in 1911. By 1912, his team had uncovered scattered bones and two articulated skeletons, one of which remains one of the best-preserved plateosaur specimens.

Friedrich von Huene, aware of the site since 1904 but initially lacking funds, launched his own major excavation in 1921, supported by the American Museum of Natural History and local entrepreneurs. His team unearthed 12 ''Plateosaurus'' skeletons, employing university students under spartan conditions. His findings were split between Tübingen, the AMNH, and later Yale University. Huene originally theorized that the dinosaurs perished in an arid desert setting but later modified his view to suggest that the animals died of exhaustion near a muddy water source.

A large-scale excavation followed in 1932, led by Reinhold Seemann from the Stuttgart Natural History Museum. With a team of 25, he unearthed 39 individual skeletons, noting that the Lower Dinosaur Bed contained better-preserved material. Seemann refuted Huene’s desert hypothesis, instead proposing that the region once held a lake, and supported the idea that many Plateosaurus specimens became trapped in mud. His meticulous field notes and maps provided a detailed record of the site, though much of the material was later destroyed in World War II.

Interest in the site waned after 1932, with subsequent research relying on past records. Later studies, including those by Weishampel in the 1980s and Sander in 1992, refined theories on the formation’s depositional history, emphasizing miring as a key factor in preserving the dinosaurs.

Excavations resumed in 2007, conducted by the Staatliches Museum für Naturkunde Stuttgart. Over multiple campaigns, new fossil-bearing layers were identified, revealing additional ''Plateosaurus'' skeletons and providing fresh insights into the site’s sedimentology and paleoenvironment.

Paleoenvironment

The ''Trossingen Formation'' facies formed as a result of episodic sheet floods in a well-drained playa basin. This region was heavily influenced by the Triassic Pangaean monsoon system, often referred to as a " megamonsoon". While many Keuper clay beds contain evaporitic sulphate minerals, the deposits are unique in their lack of such minerals, despite showing desiccation features like mudcracks. This suggests they were deposited on a dry mudflat where groundwater was too deep for evaporite formation, allowing pedogenic processes to dominate. The playa stretched from northern Switzerland through parts of Germany, including Baden-Württemberg, Bavaria, Thuringia, and Saxony-Anhalt, transitioning into central playa lake and lacustrine facies further north.

The paleoenvironment, particularly the Knollenmergel claystones, suggests a landscape shaped by alternating wet and dry conditions. The presence of features like pedogenic slickensides, pseudoanticlines, and deep desiccation cracks indicates that the sediments underwent significant soil formation processes. These characteristics align with vertisols-soils that form in climates with strong seasonal moisture fluctuations.

The Knollenmergel paleosols likely developed through repeated deposition of clay-rich layers, each becoming integrated into the soil through swelling and shrinking processes. The widespread evidence of root structures suggests that vegetation was present at times, though oxidation may have destroyed other plant remains. Carbonate nodules, sometimes forming vertical "pillars," point to periods of increased aridity, leading to the formation of calcretes. The Red-Brown Beds, distinct in color and composition, likely represent a more arid phase, contrasting with the more humid conditions inferred for the purple vertisol layers.

The Trossingen site also preserves numerous dinosaur fossils, particularly ''Plateosaurus'', with many skeletons found in life-like postures. This suggests rapid burial, likely in soft, clay-rich sediments that acted as natural mud traps. Such conditions are consistent with the presence of gilgai—depressions in vertisol landscapes that can retain water and become hazards for large animals. The overall environmental picture is one of a floodplain subject to periodic drying and wetting, supporting intermittent vegetation and experiencing long-term soil development processes.

Paleobiota

Molluscs

Fish

Amphibia

Therapsida

Sauropterygia

Archosauromorpha

Lepidosauria

Testudinata

Pseudosuchia

Sauropodomorphs

Theropods

Flora

See also

* List of dinosaur-bearing rock formations
* List of fossiliferous stratigraphic units in Germany
* List of fossiliferous stratigraphic units in Switzerland

References

{{Reflist, 3

Triassic System of Europe
Triassic Germany
Triassic Switzerland
Norian Stage
Shale formations
Mudstone formations
Sandstone formations
Conglomerate formations
Marl formations
Deltaic deposits
Fluvial deposits
Lacustrine deposits
Shallow marine deposits
Fossiliferous stratigraphic units of Europe
Paleontology in Germany
Paleontology in Switzerland