Urasterella sp. - pyritized fossil starfish from the Devonian of Germany. (OSU 50430, Orton Geology Museum, Ohio State Universtity, Columbus, Ohio, USA)

The Devonian-aged Hunsrück Slate Lagerstätte is one of the most famous soft-bodied fossil deposits. Hunsrück fossils have been pyritized (replaced with pyrite/“fool’s gold” - FeS2). They occur in a black slate matrix - the result is beautiful brassy gold colored fossils on a dark background. A diverse biota has been described, including trilobites with preserved legs & gills, crinoids, starfish & brittle stars, “worms”, molluscs, corals, conulariids, sponges, cnidarians, land plants, “algae”, fish, plus various unusual arthropods & echinoderms.

The Hunsrück Slate Lagerstätte occurs in the Kaub Formation (Hunsrück Slate Group, lower Emsian Stage, upper Lower Devonian) in the Budenbach area of western Germany.

Preparation of Hunsrück fossils is notoriously difficult, and many of the pyritized fossils crumble to dust from pyrite disease over many years. Relatively recently, a “secret” air abrasion method has been successfully used to prepare Hunsrück fossils, using an abrasive grit composed of metallic iron powder mixed with potato starch.

Shown here is a complete, pyritized Urasterella fossil starfish from the Hunsrück. Two species of Hunsrück Urasterella have been described: U. asperula is the common form and U. verruculosa is the rarer form. This specimen appears consistent with U. asperula.

From exhibit signage:

Hunsrück Slate

Slate from this area, located in western Germany along the Rhine [River] near the villages of Budenbach and Gemünden, was used by the Romans and for medieval castles and churches. The Lower Devonian (400 million years old) fossils, although rare, are wonderfully preserved, in some cases with soft parts replaced by pyrite. Over 270 species have been found, the most common being crinoids, starfish, brittle stars, and arthropods, including trilobites. Also found are glass sponges, corals, bryozoans, brachiopods, clams, cephalopods, sea cucumbers, and fish.

Classification: Animalia, Echinodermata, Asterozoa, Asteroidea, Platyasterida, Palasteriscidae

Replacement is a fossil preservation style involving the crystal structure and the mineral of an organism's hard parts being changed.

The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification). Many silicified fossils have rounded to pustulose structures covering their surfaces. These are called beekite rings, but they're composed of ordinary quartz.

Other common replacment materials include the mineral pyrite (FeS2 - iron sulfide) and calcium phosphate. These replacement styles are called pyritization and phosphatization.

Numerous other minerals have been found replacing minerals - many of them are quite rare. Reported fossil replacement minerals include: anglesite, apatite, barite, calamine, calcite, cassiterite, celestite, cerargyrite, cerussite, chalcocite, cinnabar, copper, dolomite, fluorite, galena, garnet, glauconite, gumbelite, gypsum, hematite, kaolinite, limonite, magnesite, malachite, marcasite, margarite, opal, pyrite, romanechite/psilomelane, siderite, silica/quartz, silver, smithsonite, specular hematite, sphalerite, sulfur, uranium minerals, and vivianite.

(List mostly from info. in Hartzell, 1906 and Klein & Hurlbut, 1985)

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