In the land of the trilobites

"Consequently, if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures."

Charles Darwin in "On the Origin of Species" (1859), the Silurian of Darwin's time corresponds to the modern Cambrian.

Fig.1. The restaurant "Trilobit" in the old city of Prague. The Czech Republic is very proud of its fossils and role in the history of geology, the city of Skryje for example has in it Civic Heraldry even a trilobite, inspired by the species Skreiaspis spinosus (thanks to Dr. Astudillo-Pombo for the information).

Barrandium is the denomination of the stratigraphic succession of a northwest-southeast stretching synclinale, also referred as Basin of Prague, in the Czech Republic. The sediments preserved here were deposited after the Caledonian orogeny (490-390Ma), and survived the Variscian orogeny (ca. 350-320Ma) without being metamorphosed - thus they are a unique window in a distant geological past and provided insights of life and habitats during the Palaeozoic, from the Cambrian (542Ma) until the Devonian (359Ma), but especially from the Silurian period (443-416Ma).

Fig.2. Trilobites-fragments from the Cambrian assemblage of Skryje-Tyrovice area, mostly Paradoxides and Hydrocephalus, below non identified, well preserved specimen.

It was the French Joachim Barrande (1799-1883) to describe and study for the first time these sediments, and the name of the succession and many animal species bear his name to remember this extraordinary man.

Fig.3./4. Barrande´s monument in the garden of the school of Skryje was made by the Czech sculptor M.V. Dobrovolny in 1969.
Joachim Barrande discovered the rich palaeontological localities at Skryje and Tyrovice in 1833 when he carried out there explorations for a planned horse-railway. Fossils of the Cambrian age, designated by Barrande as the "Primordial fauna" were at the time the oldest known fossil remains.
Below, plate 5 from the supplement to Barrande´s work "System Silurien du Centre de la Boehme" (1872).

Barrande was an engineer and geologist, but between 1833 and 1846 he gradually changed to palaeontologists and stratigrapher, publishing his important "Preliminary Notes" about trilobites and sediments of Bohemia (the western part of modern Czech) in 1846; 1846-1852 he successfully tried out his new "profession", and already in contact with many scientists in Europe, published in 1852 the first two volumes of his later famous "System Silurien du Centre de la Boehme"; finally from 1852 to his death he published 19 more volumes of about 6950 pages and 1148 tables.

Generations of researchers have followed his tracks, and tried to reconstruct the biostratigraphy and the development of the Basin of Prague. The ghost of Barrande, the history of research and the value of these sediments for the development of geology and palaeontology are well recognized in the beautiful city of Prague. Many unusual signs in the street celebrate the animals of the past, most iconic of them the extinct arthropods known as trilobites.


The Silurian is one of the most enigmatic epochs in the history of earth; marked by a glaciation at the beginning, it was at the end of this period that the landmasses, until them barren deserts, became settled by plants and animals. But the shelf-regions of the continents were a rich habitat, populated by trilobites, brachiopods, crinoids and other invertebrates.

In the Silurian the Basin of Prague was located at the northern shores of Peri-Gondwana-Land (the Southern Continent of these times), then during the Silurian Peri-Gondwana begun to move from the poles to the equator. These profound changes influenced also the deposited sediments; there is a progressive change from predominately clastic sedimentation (sandstones, volcanic deposits) to black shales of an anoxic ocean to limestone and "reef"-deposits of a warm, shallow sea. This is not only a result of the change of paleolatitude of Peri-Gondwana, but also a sign of earth's climate warming after a pronounced cooling during the Ordovician and Silurian.

Fig.6. Simplified Geology and Stratigraphy of the Barrandium.

The unmetamorphic stratigraphy of the Barrandium, the many distinctive horizons of volcanic ash and the rich fossil fauna made it a preferred place to define here various GSSP (Global Boundary Stratotype Section and Point), outcrops used as reference sites where the single limits of the chronostratigraphic periods, used to subdivide geological time, are defined.
The Pridoli-epoch (418,7-416,0Ma) is the last epoch of the Silurian and it is named after the Pridoli-Formation (or Pozary-Formation), and the official GSSP is situated since 1985 in the Pozary-quarry, located at the periphery of Prague. Here in various quarries of the 19th century the complete sediment- succession of the upper Silurian until the lower Devonian is accessible, first clays with tuff-layers (Kopanina-Formation), followed by layers of biodetrital limestone, rich in fragments of cephalopods, brachiopods and trilobites, and finally a succession of thin bedded marls and limestone (Pridoli-fm). This succession is also visible in the nearby quarry of Muslovka.

Fig.7. The quarry of Muslovka, where the transition from the Kopanina-fm (Ludlow, 422-418Ma) to the Pozary-fm (Prídolí, 418-416Ma) is well visible- Legend: a) mudstones and limestone with ash-layers of the Encrinuraspis beaumonti zone, b) limestone of the Metacalymene baylei zone; c) limestone of the Ananapsis fecunda zone ; d) bedded limestone of the Prinopeltis archai zone.

Fig.8. Fragments of crinoid-stems and brachiopod Septatrypa sp. of the Pozary-fm found in the Muslovka quarry.

To observe the lower Silurian stratigraphic succession and find more fossils the quarry of Kosov, located near the city of Beroun, is a perfect place. Here black shales of the Motol-fm with abundant planktonic graptolites can be found.

Fig.9. Black shales of the Motol-fm with Neocullograptus kozlowskii (?).

The dominance of the graptolites, the lack of benthic organism and the lamination of the rocks was caused by a reduction of water currents and circulation of the Silurian sea, resulting in a hostile, anoxic environment where the mud accumulated undisturbed by digging organisms. One possible explanation for the reduced circulation observed in many similar deposits could be the global cooling at these times.
In the upper part of the Motol-fm single limestone layers appear, until a gradual transition to a succession of bioclastic limestone and clays of the Kopanina-fm occurs. The limestone layers and the abundance of fossils are evidence of a restored circulation and oxygenation of the water. Single volcanic events, recognizable by the ash and clay-layers, killed from time to time the organism living in this sea, buried and conserved them until today.

The graptolites, and especially the vast group of the trilobites dominated the Palaeozoic, but finally all of them disappeared. The graptolites in the Carboniferous, the trilobites in the mass extinction at the end of the Permian about 250 million years ago - only the fossils remain to tell their extraordinary story.

Fig.10./11. Kopanina Formation as seen in the Kosov quarry, with typical alternation of dm-thick layers of limestone with clay-layers and tuffites - the small image shows also the typical lens of limestone interbedded in the layers. Below image of thorax/pygidium of Acantholomina minuta found in an ash-layer.

Bibliography:

BARRANDE, J. (1872): Systeme Silurien du centre de la Boehme. 1st Partie: recherches Paleontologique. Supplement au Vol. I - Trilobites, Crustaces divers et Poissons. Prague - Paris.
CHLUPAC, I. (1993): Geology of the Barrandian - A field trip guide. Senckenberg- Buch 69. Verlag Waldemar Kramer, Frankfurt am Main.
KRÍZ, J., (1989): The Prídoli Series in the Prague Basin (Barrandium area, Bohemia). In: Holland, C. H. and Bassett, M. G. (eds.). A global standard for the Silurian System. National Museum of Wales, Geological Series 9, Cardiff: 90-100
LEHNERT, O.; FRYDA, J.; BUGGISCH, W.; MUNNECKE, A.; NÜTZEL, A.; KRIZ, J. & MANDA, S. (2007): 13C records across the late Silurian Lau event: New data from middle palaeo-latitudes of northern peri-Gondwana (Prague Basin, Czech Republic). Palaeogeography, Palaeoclimatology, Palaeoecology. Vol. 245 (1-2): 227-244
RICKARDS, R.B. & WRIGHT, A.J. (1999): Systematics, Biostratigraphy and Evolution of the Late Ludlow and Prídolí (Late Silurian) Graptolites of the Yass District, New South Wales, Australia. Records of the Australian Museum Vol. 51: 187-214