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The Earliest Brachiopod-Bryozoan Dominated Community in The

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The Earliest Brachiopod-Bryozoan Dominated Community in The Journal of the Czech Geological Society 49/34(2004) 127 The earliest brachiopod-bryozoan dominated community in the Ordovician of peri-Gondwana and its ancestors: a case study from the Klabava Formation (Arenigian) of the Barrandian, Bohemia Nejstarí spoleèenstvo s dominantními mechovkami a brachiopody v ordoviku peri-Gondwany a jeho pøedchùdci: pøíklad z klabavského souvrství (arenig) Barrandienu, Èechy (Czech summary) (6 figs) MICHAL MERGL Department of Biology, University of West Bohemia, Klatovská 51, 30619, Plzeò, Czech Republic; [email protected] A remarkably rich fossil assemblage with bryozoans, orthid and lingulate brachiopods, siliceous sponges and other fauna is described from the uppermost layers of the Klabava Formation (Arenigian) in the western part of the Prague Basin, Bohemia. Its taxonomic composition is evaluated and its depth and local position toward the ancient shore line is discussed. The fossil assemblage probably represents transported material from a fissure in the steep rock walls of the shore cliff. The life assemblage was probably analogous to Recent cryptic communities of the rocky shore. The species Quadrotheca sp. is described, representing the earliest known orthothecid hyolith in the Ordovician of the Prague Basin. Key words: Bryozoans; brachiopods; hyolithids; Arenigian; Ordovician; palaeoecology; palaeoclimate; palaeogeography Introduction nificant. The associated brachiopod fauna in the later communities has a similar composition in the Mediter- In European sector of peri-Gondwana, the bryozoan-bra- ranean Realm, with the dominance of orthid brachiopods chiopod-pelmatozoan (BBP) communities diversified (Svododaina, Drabovia, Onniella, Onnizetina, Hirnantia, from the Middle Ordovician and reached their maximum Jezercia, Gelidorthis, Saukrodictya and others). Stro- in the mid-Ashgillian time. There are only rare occurrenc- phomenids Rafinesquina, Iberomena and Blyskavomena, es of the BBP communities in older Ordovician stages a plectambonitid Aegiromena, a triplesiid Bicuspina, a in contrast to the tropical and temperate palaeocontinents rhynchonellid Rostricellula and a large discinid Trema- (Laurentia, Baltica). In European peri-Gondwana, the tis are locally common. Apart of the bryozoans and bra- BBP communities rapidly spread from the late Berounian chiopods, there are other remarkable and typical elements (Caradocian) to the mid Ashgillian. The shallow of the BBP faunas of Upper Ordovician in the Mediter- shelves of the Iberian Peninsula (NW and NE Spain), ranean Realm: cystoids (Aristocystites, Echinosphaerites, Montagne Noire, SW Sardinia, Libya (North Tripolita- Codiacystis, Heliocrinites, Hippocystis, Fungocystites, nia), the Carnic Alps, and submarine volcanogenic ele- Macrocystella), crinoids (Caleidocrinus), trilobites with vations of the Armorican Massif showed a significant dominant illaenids (Stenopareia) and homalonotids (Ca- increase in carbonate deposition and thick carbonate se- lymenella), gastropods of the large Oriostoma-type, large quence appeared there (Spjeldnaes, 1961, Havlíèek, 1981, benthic Tentaculites and other but generally rare fauna. 1982, Havlíèek et al., 1987, Villas, 1985, 1995, Villas et al., The most famous BBP communities in the Prague Ba- 1987, Mélou, 1990, Babin et al., 1982, Bergström Mas- sin are of the late Berounian age. The Polyteichus fa- sa, 1991, Boucot et al., 2003, etc.). cies (= Polyteichus Horizon; Hirnantia plateana Com- Carbonate deposition in mid-Ashigillian times was munity of Havlíèek, 1982) is known from the shallow, rapidly interrupted at the beginning of the Hirnantian as tectonic rising zone in the otherwise deep Prague Basin a response to cooling, regression and growth of the polar in the late Berounian (Zahoøany and Bohdalec forma- ice cap. Thinner carbonate-rich beds are reported from tions) (Havlíèek Vanìk, 1966, Havlíèek, 1982). Slightly mid-Ashgill of Thuringia, Belgium and the Prague Basin. older examples of the BBP communities in the Prague All occurrences are isochronnous and indicate a brief Basin are greatly restricted but are indicated by local ac- warm interval preceding the Late Ordovician glaciation cumulations of the characteristic taxa. There are exam- (Boucot et al., 2003). ples from the lower Berounian (scattered data from the The older stratigraphical records of the BBP commu- Libeò Formation; top of the Letná Formation and the lower nities in peri-Gondwana are less distinct and their warm Vinice Formation, Bicuspina Community of Havlíèek, climate origin is more speculative. In general, the com- 1982). Their presence often preceded or followed the moner occurrences of the bryozoans, crinoids and cys- oolitic ironstones deposition confined to local rising zones toids should indicate shallower environment. Bryozoans (Havlíèek, 1982). are not fully described, but ramose, discoidal and encrust- The older units in the Ordovician succession of the ing, mostly trepostomate and cyclostomate taxa are sig- Prague Basin lack any typical components of the BBP 128 Journal of the Czech Geological Society 49/34(2004) communities. The only exception has been found in the of the Klabava Formation comprises conglomeratic beds, western part of the Prague Basin near the village Ejpov- but these are not exposed at present time in the open ice, close to the top of the Klabava Formations (late Areni- mine. In the NE part of the open mine, the volcanogenic gian). The remarkable mixture of stratigraphically old sequence is in a direct contact with the pre-Ordovician and new faunal elements, and the taxonomic composi- basement, represented by shore cliffs of Precambrian tion of the fossil association with abundant bryozoans and cherts. The cliffs are a part of the Upper Arenigian rocky orthid brachiopods is suggestive of the late Ordovician shore line that trends in a SWNE direction. The cliffs BBP communities. It represents the earliest known report and their adjacent cover rocks have yielded the oldest of the BBP-like community in the Mediterranean Realm known fossils of the rocky shore yet reported (Mergl, of peri-Gondwana. Its taxonomic composition, taphono- 1983). They are represented by stromatolites, encrusting my and affinity are discussed herein. bryozoans and other bryozoan-like biota. The volcanogenic sequence of the top of the Klabava Geological setting Formation commonly contains heterofacial intercalations and lenses of oolitic ironstones. These layers and lenses The fauna has been found in the northeast part of the are 2 to 15 cm thick, with erosional bases and contain abandoned open iron ore mine near the village of Ejpov- large amount of rounded to subangular, 5 to 20 mm sized ice, some 10 km east of Plzeò (Fig. 1). The open mine chert pebbles. Siliciclastic material is a less significant exhibits one of the best exposures of the Klabava, árka component of the ironstones. The matrix of ironstones is and Dobrotivá formations (Middle Ordovician) in the mostly muddy, with some carbonate. The ooids of iron- Barrandian. The facies succession indicates the gradual stones are 1 to 2 mm in size, usually flattened and with upward deepening of the area, from the near shore, shal- nuclei of quartz grain or fragments of phosphatic brachi- low littoral lithofacies in the Klabava Formation (Areni- opod shells. The ironstones rapidly and unevenly taper gian) to the deep, open-shelf environment with the depo- laterally. Deformations of the unconsolidated underlying sition of the black shale in the lower Dobrotivá Formation sediments are sometimes present. Fossils in the ironstones (upper Darriwillian). are rare and differ from those of the adjacent volcano- The upper 1520 m of the Klabava Formation is a suc- genic rocks. In the ironstones, there are mostly organo- cession of various volcanogenic rocks, mainly of reworked phosphatic lingulate brachiopods, bryozoan-like biota laminated tuffs and tuffaceous shale with a varied con- encrusting the pebbles, and broken pieces of stromatolitic tent of haematite and calcareous cement, commonly with mats. In the volcanogenic rocks, the shells of orthid bra- calcareous and phosphatic bioclasts. The older succession chiopods and hexactine spicules of siliceous sponges pre- Praha Praha Plzeò 25 km Berouneroun field roa Proterozoic chert ds cliffs Komárov FT Mýto FT OP EN M INE Rokycany LAKE Ejpovice fiel d ro ads rai lroa Ejpovice d 250 m Fig. 1. A The Bohemian Massif showing the Czech Republic and the Ordovician of the Prague Basin; B The Ordovician of the Prague Basin showing the described locality in Ejpovice (FT), and localities of the Nocturnellia nocturna Community (asterisks) and the Marcusodiction Com- munity (rectangles); C Map of the abandoned and flooded ironstone open mine near Ejpovice showing the outcrop of Proterozoic chert cliffs (black), localities of the Nocturnellia nocturna Community (asterisks) and the Marcusodiction Community (rectangles), and the site of described locality with its rich bryozoan-brachiopod association (FT). Journal of the Czech Geological Society 49/34(2004) 129 vail. Unlike the ironstones, diverse trace fossils are com- material (e.g. algal mat) between the fossils cannot be mon in the volcanogenic rocks. excluded. In total, some 1100 specimens were determined The ironstone beds may be interpreted as a replaced in addition to uncounted sponge spicules. material of shallow littoral origin. The ooids and pebbles Figured specimens are deposited in the palaeontolog- were washed basinwards and their deposition in discrete ical collections of the Museum
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