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Travels & Trilobites in Geology

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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 -epoch of Darwin’s time corresponds to today’s Cambrian.

Barrandium” is the denomination of the stratigraphic succession found in the Basin of Prague (Czech Republic). The sediments preserved here were deposited after the Caledonian orogeny (490-390Ma years ago) and survived the Variscian orogeny (ca. 350-320Ma) without significant deformation or metamorphosis. The Barrandium is named after the French engineer and fossil collector  Joachim Barrande (1799-1883). In the year 1833, during fieldwork for a planned horse-railway  near the villages of Skryje and Tyrovice, Barrande discovered various fossiliferous sites. These fossils of the Cambrian  were at the time the oldest known petrified remains of large organisms.

Fig.1. Barrande´s monument in the garden of the school of Skryje, made by the Czech sculptor M.V. Dobrovolny in 1969.

Fig.2. Trilobites-fragments from the Cambrian assemblage of Skryje-Tyrovice area, mostly Paradoxides and Hydrocephalus.

Barrande became fascinated by the fossils of trilobites, extinct arthropods characteristic of the Paleozoic Era. In 1846 he published his “Preliminary Notes” about trilobites and the sediments found in Bohemia (the western part of modern Czech republic), followed in 1852 by the first volumes of his “System Silurien du Centre de la Boehme

Fig.3. The “System Silurien du Centre de la Boehme” comprises 21 volumes with 6.950 pages and 1.148 tables, describing and depicting fossils attributed by Barrande to the “Primordial Fauna”, as it was believed that these animals were the most ancient, maybe even the first organisms, to exist on earth.

Still today the Silurian (some 443,7-416 milion years ago) is one of the most enigmatic epochs in the history of earth; marked by glaciations at the beginning, it was at the end of this period that the landmasses, until then barren deserts, became colonized by plants and animals. The sea was already populated by a rich assemblage of trilobites, brachiopods, crinoids and other strange invertebrates.

At the beginning of the Silurian the Basin of Prague was located at the northern shores of the continent of Peri-Gondwana-Land. During the Silurian Peri-Gondwana moved slowly from the South Pole to the equator. This change of location is marked also by the change of the deposited sediments; from predominately clastic sedimentation (sandstones, volcanic deposits), formed by the erosion of the landmass, to black shale of an anoxic ocean to limestone of a warm, shallow sea. Earth also recovered from a phase of pronounced cooling during the Ordovician and Silurian and the thriving life forms formed the first larger reef-formations.

Fig.4. Simplified Geology and Stratigraphy of the Barrandium.

The black shale of the Motol-Formation is characterized by the presence of beautiful fossils of graptolites. The dominance of the graptolites – colonies of organisms floating in the open sea – missing ground-dwelling organism and the lamination of the rocks show that these sediments were deposited on the bottom of the sea, where mud accumulated undisturbed by currents or burying animals.

Fig.5. Black shales of the Motol-fm with Neocullograptus kozlowskii (?).

Maybe cool global temperatures reduced significantly the marine currents at the time. In the upper part of the Motol-fm single layers of limestone appear, until the gradual transition to a succession of fossiliferous limestone and shale of the Kopanina-Formation. The limestone layers and the abundance of fossils are evidence of a restored circulation and oxygenation of the ancient sea. Single volcanic events, recognizable by the ash-layers, killed from time to time the organism living in this sea, burying and preserving them until today.

The unmetamorphic sediments of the Barrandium, the many distinctive horizons of volcanic ash and the rich fossil fauna made it a preferred area to define here various GSSP of the Paleozoic. A GSSP – Global Boundary Stratotype Section and Point -is an outcrop or site where the boundaries of a chronostratigraphic period, used to subdivide geological time, are defined.

The Pridoli-epoch (418,7-416,0 Ma) is the last epoch of the Silurian and is named after the Pridoli-Formation (or Pozary-Formation). The GSSP of this epoch is found in the abandoned Pozary-quarry located at the periphery of Prague. Here the stratigraphic succession of the upper Silurian and the lower Devonian was exposed during quarrying activities in the 19th century. The succession comprises shale with tuff-layers (the Kopanina-fm), followed by layers of limestone, rich in fragments of cephalopods, brachiopods and trilobites, and finally a succession of thin bedded marls and limestone (Pridoli-fm).

Fig.6. The quarry of Muslovka, near the Pozary-quarry, 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.7. Kopanina Formation as seen in the Kosov quarry, with typical alternation of layers of limestone with shale and ash-layers- the small image shows also lens of limestone interbedded in these layers.

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

Fig.8. The restaurant “Trilobit” in the 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 a trilobite, inspired by the species Skreiaspis spinosus (thanks to Dr. Astudillo-Pombo for the information).

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

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.



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