The Posidonia shale found in the Holzmaden region is renowned for its fossils. Duke Carl Eugen of Württemberg, documented in 1749, stands as one of the earliest fossil collectors. It was in 1892 that the first detection of soft tissue fossil remnants in an ichthyosaur occurred during preparation. From that point on, the influx of Holzmaden fossils into museums worldwide began.
The fossils found in the Posidonia Shale offer a glimpse into an ancient Central European shelf sea, approximately 180 million years old. These fossils, including vertebrate skeletons, echinoderms, crustaceans, mollusk shells, and even preserved soft tissues with impressions, were compressed to thin membranes due to the sludge's compression during deposition. Discoveries notably included coal-black skin remnants of an Ichthyosaur. The fascination with Holzmaden fossils began centuries ago, with documented reports dating back to the 16th century. Duke Carl Eugen von Württemberg initiated fossil trading in 1749, introducing remarkable specimens, like an Ichthyosaur fragment carrying an embryo—the oldest dinosaur found in Central Europe. The Holzmaden site continues to surprise due to enhanced preparation methods, holding unparalleled quality and frequency of vertebrate fossils worldwide.
The "Posidonia Slate" derives from the shell Posidonia Bronni Quenstedt, honoring the Greek sea god Poseidon, and is pivotal in this deposit. These marlstones, found on continental margins, owe their dark, blue-gray hue to abundant bitumen and finely pyrite pebbles. Oxygen levels played a crucial role in the deposition and fossilization process. The Holzmaden area's fossil richness has various explanations. Recent studies reveal a climate with summer monsoons and winter droughts, causing significant seasonal shifts in seawater oxygen content, slowing tissue decomposition. Dead animals were swiftly covered by clays and limestone, eventually solidifying over millions of years, encapsulating the Jurassic Sea's seasonal changes in layers of petrified mud.
The Holzmaden slate quarries, active since Medieval times, focused on extracting the thin, exquisite Fleins layer, only 18 centimeters thick and often splitting twice. The striking rough surface, adorned with Posidonia shells, was prized for interior design. Initially, quarry pits measured around 5 by 8 meters, employing a technique where the mined section was refilled with overburden. As the accessible areas, roughly 2 to 4 meters deep, got exhausted, operations shifted to deeper areas—up to 10 meters. Nowadays, the remaining quarries reveal Fleins at a depth of approximately 12 meters, hosting most vertebrate and sea lily fossils, except crocodiles found in the "Untere Stein" layer.
An adept eye is necessary to unearth fossils. Typically, the surface of the rock reveals very little about the fossil, perhaps just a flat protrusion at most. Only in cross-section fractures can bones be identified due to their brown hue against the gray slate. The skilled individual's job then becomes deciphering the fossil's type, size, and position within these fractures, piecing together fragments of often broken slate to reveal the fossilization.
Within the laboratory, the preparator assesses the entirety, condition, and integration of the fossil. The rock and fossil are fused, not easily separable. Tools like chisels, knives, diggers, and eventually delicate blasting tools aid in matrix removal, while meticulous work is conducted under a binocular microscope. Each bone is meticulously uncovered, occasionally relying solely on the fossil's color contrast.