May 27. Hunsrück slate

I think it’s time to visit another lagerstatten – one of those amazing collections of fossils that are remarkable in their preservation and completeness. This time, it’s the Hunsrück slate, in southwestern Germany near the border with France and Luxembourg.

Brittle star from Hunsruck slate. Photo by James St. John (creative commons license)

Slate is a hard, dense rock with fine partings that make it come apart easily into flat slabs. Huge flat slabs of black slate used to make chalkboards in schools, and smaller pieces have been used for centuries to make roofing shingles. Slate is a metamorphic rock, formed by subjecting fine-grained shale to heat and pressure. The main difference between shale and slate is often just how indurated it is, how hard and tightly bound together the tiny grains are. The heating and pressure might drive off any water in the rock, and might mobilize the chemicals just enough to make the rock hard, rather than crumbly.

So how do you get fossils in slate, a metamorphic rock? Well, there are all levels of metamorphism, and even the process of going from loose sediment to solid rock might be called really low-grade metamorphism, but we usually reserve the word to mean a later cooking or pressurizing that changes the minerals at least a little. But not necessarily a lot.

The Hunsrück slate was laid down as mud and clay that became shale in a sea along the southern margin of the Early Devonian continent of Europe. As we’ve learned, fine-grained sediments are great for preserving details in fossils, and dark shales often indicate anoxic conditions, where even fragile organic structures might not oxidize and decay.

The Hunsrück fossils include more than 260 animal species, and even soft parts are preserved. Most of the kinds of marine life of the Devonian are present, including spectacularly preserved crinoids and brittle stars with long delicate arms intact, as well as fish, trilobites, and sea cucumbers. The strange and extinct carpoids, a kind of echinoderm, are also found with outstanding preservation.

The process of turning fossil-rich shale into slate must have been quite gentle, as metamorphism goes. The shale dates to the Emsian Stage of the Lower Devonian, about 400 million years ago. Sometime during the Carboniferous Period, perhaps 80 to 100 million years later, the rocks were metamorphosed to slate without significant destruction of the fossils.

—Richard I. Gibson

Photo by James St. John under Creative Commons license.  

Carpoids