Gabriele Kuhl

Coffee-table book on the Devonian fossils of the Hunsrūck Slate. Slate’s an unusual rock to find fossils in; there are probably plenty of fossils in slate, it’s just unusual to find them. The reason is slate is metamorphic; the original bedding planes have been destroyed. Instead heat and pressure align clay minerals to form “slaty cleavage”. The direction of slaty cleavage depends on the way metamorphic deformation forces were applied to the original shale, and it’s more or less randomly oriented compared to the original bedding planes. Since fossils in shale are normally found on the bedding planes, the only ways you can find them in slate are (1) if the deformation didn’t smear them out of recognition, and (2) they have been mineralized with a radio-opaque mineral, like pyrite and you X-ray the rock, or (3) the slaty cleavage just happens to line up with the original bedding. As it happens, the Hunsrūck slate has all three conditions; although distorted a little the fossils are still recognizable, they are pyrite mineralized, and the cleavage planes are more or less aligned with the original sedimentary bedding planes.

That would be interesting enough, but the Hunsrūck slate is also a Konservat Lagerstätten; one of those rare sites, like the Burgess Shale or the Mazon Creek beds, which preserve soft parts of organisms. Thus it’s full of “weird wonders”: polychaetes, a couple of marrellomorph arthropods, lots of trilobites with the delicate legs preserved, and an anomalocarid (which means that group is known from the Cambrian, Ordovician, and Devonian).

The Hunsrūck fossils were known all the way back to the 1860s; they were subject to the vagaries of the roofing slate industry. The original specimens were discovered, prepared, and sold as curiosities by slate quarry workers, who prepared them by scrubbing with bronze bristle brushes. When the slate industry declined (due to replacement by asphalt shingles), paleontologists were limited to working through spoil heaps; at about the same time X-ray equipment became available and a lot of specimens were discovered and described that way. In the 1970s the roofing slate industry picked up again as the price of petroleum products went up; old quarries began producing again and the quarry owners cooperated with paleontologists. Fossils continued to show up on X-rays and the new technique of microbead blasting allowed better preparation.

Alas for me, the book isn’t quite technical enough. It’s large format, with beautiful pictures; since the fossils are pyritized, many look like exotic jewelry. But although the basics of the geological setting and the fossil systematics are covered I’d like to see a lot more; there are some more technical works available but they are mostly in German, which I only read slowly and with a technical dictionary at hand.… (lisätietoja)