- Lower Devonian Hunsrück Slate of Germany
in the region of the Rhine and Moselle Valleys cannot help but notice the charming
slate roofs of buildings in the many picturesque towns encountered. The Hunsrückschiefer,
along with the Lower Jurassic Posidonia shale and Upper Jurassic
Solnhofen Limestone are deposits that have been exploited since ancient times
for building purposes. During quarrying operations, fossils are occasionally discovered.
The quarrymen have learned that such specimens are often of great value to collectors.
While the specimens from the dark gray Hunsrück Slate are mostly small, their
diversity goes far towards affording a glimpse of life in the dim past.
best localities for well-preserved fossils are in the communities of Bundenbach
and Gemünden. The slates were widely quarried in the past, primarily from
small pits, of which over 600 are known. Today, only a single quarry remains open
in the main fossiliferous region of Bundenbach. Unlike other famous Lagerstätten,
the Hunsrück Slate has never undergone detailed sedimentological examination.
slates are of Early Devonian
age, deposited during the Late Pragian to Early Esmian Ages, making them 396-390
million years old. The sediments were deposited to thicknesses estimated at 4,000
meters (4,400 feet) in a number of offshore basins that were separated by swells.
The main belt of deposits runs for about 150 km (100 miles) from northwest to
southeast. To the south the thickness drops to only 200 meters (220 feet). Thus
deposition appears to have occurred in subsiding basins that were separated by
swells which had reduced and partly sandy sedimentation. Water depth in the offshore
environment of the slates has been estimated at 40-60m (44-66 feet), an estimate
borne out by the well-developed eyes of arthropods and vertebrates found here.
remains are very often covered by a pyritic surface layer. When
burial occured rapidly, fossils were commonly completely replaced
with pyrite, in which case preservation was excellent. Such pyritized
fossils are often examined by the X-ray technique. This considerably
facilitates the preparation process, as the fossils are not otherwise
easily separated from the fine-grained sediment matrix. Having
an image of the as-yet-unexposed fossil
makes the task of the preparator less of a hunt-and-pick operation.
deposits are not a perfect representation of the ecology, however, for several
reasons: 1. Due to the dissolution of thin calcareous shells of brachiopods, gastropods,
and cephalopods, they are rarely represented 2. Because the slates became famous
for their arthropods and echinoderms, the quarrymen concentrated on their collection.
It must be realized that the excellently-preserved fossils are confined to localized
areas, mainly around Bundenbach and in some other areas around the Rhine valley,
where they are restricted to only a few horizons. Soft-body preservation is quite
date, over 260 species have been described from the Hunsrück Slate. In the
main depositional basins of Kaub, Bundenbach, and Gemünden, echinoderms are
concentrated in the southwestern area around Bundenbach, with brachiopods predominating
in the northeast. The presence of corals and trilobites with well-developed eyes
from the central basin areas implies a shallow-water environment. Breaks in sedimentation
in these shallow-water areas indicate episodic emergence. Remains of land plants
are found only rarely.
fossils include jellyfish, sponges, corals, brachiopods, cephalopods,
conularians, gastropods, and traces of worms. Certain horizons
to the south are filled with tiny tentaculoids indicative of the
more pelagic environment to be found there. The main attractions
are the trilobites and echinoderms, as well as the rare vertebrates.
Crinoids and starfish are the predominant representatives of the
echinoderms, although holothurians (sea cucumbers) are also represented.
The vertebrates include the flattened jawless fish that lived
on or near the sea floor. There are over 60 species of crinoids
found in the Hunsrückschiefer.
commonly attached themselves with radicular cirri (roots) to hard objects on the
muddy bottom such as the shells of the ammonite Orthoceras. In many instances,
crinoids attached themselves to the stems of other crinoids as well. Juvenile
individuals were particularly fond of attaching themselves to the stems of adults.
Evidently crinoids that were attached directly to the bottom were uncommon. In
a few instances, crinoids have been found attached to sponges.
fossils represent mainly benthic (bottom-dwelling) organisms and
include many trilobite tracks, known as Cruziana. The shallower
parts of the area were fully oxygenated and as such specimens
they are not pyritized. The classic fossil sites of Bundenbach
and Gemünden are preserved in interchannel areas with oxygenated
bottom water that allowed for the establishment of benthic communities.
Occasionally, density currents would bury the animals, often in
situ, and the thick layers of sediment protected them from scavengers.
This thick iron-rich sediment eventually became anoxic, providing
the conditions needed to promote pyritization. It is this rich
pyritised fauna which is so highly prized by collectors.
Bartels, C, Briggs, D, AND Brassel, G., The fossils of the Hunsrück
Slate. Marine Life in the Devonian. Cambridge Univeristy Press.