Journal of the Czech Geological Society 49/3–4(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

Nejstarší 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 49/3–4(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 SW–NE 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, Šárka 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 15–20 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 49/3–4(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 of West Bohemia, Plzeò layers and lenses repeatedly interrupted the more or less (PCZCU). The other material is kept in the author’s col- continuous sedimentation of the volcanogenic material in lections in the Department of Biology, University of West the area. There is distinct increase in the development of Bohemia, Plzeò. the ironstone beds upwards in the succession of the Kla- bava Formation. Taphonomy and origin of the fossil assemblage A rapid change of sedimentation took place at the Kla- bava/Šárka formations boundary. The base of the Šárka The taphonomy of the fossils indicates that the accumu- Formation is marked by massive oolitic ironstone beds that lation did not take place in situ. The valves of brachio- are the start of a continuous succession of oolitic ironstones pod shells are always loosed, and commonly incomplete. without volcanogenic intercalations. The thickness of the Bryozoan zoaria, both ramose and discoidal, are fragmen- Šárka Formation is about 20 m in the area. These iron- tal, broken into small pieces. Lingulate brachiopods are stones lack the pebbles, coarse siliciclastic material and any preserved as isolated valves, often with broken margins macrofossils. The deposition of shore siliciclastic materi- or worn edges. Hollow gastropod and hyolithid shells are al had ceased from the beginning of the Šárka Formation, collapsed, being commonly incomplete, with apertures probably because of the shift of the shore line far to the broken. Almost all of the larger fossils, apart the bryo- NW. It is probable that the cliffs of Precambrian cherts, zoans, are cracked without the detachment of fragments the important source of pebbles in the ironstones of the by weak compression of still unconsolidated sediment. Klabava Formation, were buried by the onlap at the be- The intercalation of the shelly fauna mixed with abun- ginning of the Šárka Formation deposition. dant spicules of siliceous sponges within the ironstone represents a thanatocoenosis. The shells and dead zoaria Material and preservation were trapped in soft and possibly sticky mat of decayed sponges and algae (?) elsewhere and this primary accu- A rich fossil association has been collected from a sin- mulation was subsequently washed basinwards together gle thicker lens of the oolitic ironstone, about 2 m be- with ooids, small pebbles and some muddy material low the Klabava/Šárka Formations boundary. Almost all (Fig. 3). However, the origin of the ironstone lens could of the fossils come from an internal intercalation at about be not have been instantaneous. The first stage of ooid mid-thickness of the lens (Fig. 3 C). The maximum thick- sedimentation was interrupted by deposition of the ness of the intercalation is 3 cm and it tapers laterally, sponge mat with trapped fossils. Subsequent rapid sup- following the lateral wedging of the ironstone lens. The ply of ooids buried the mat and preserved the fossilifer- ironstone lens is estimated to have originally occupied ous intercalation inside the ironstone lens. The absence some 5–10 m2, although its actual size had been reduced of fossils below and above the fossiliferous intercalation by about a half by weathering when the fauna was col- also supports the conclusion about a stepped deposition lected in 1982. The ironstone contains fewer pebbles than of ironstone lens (Fig. 3 C). other ironstone lenses in the area. The original living place of the species preserved in a This fossil-rich intercalation is formed by a mat of sil- thanatocoenosis should be traced shoreward from their iceous sponges, bryozoan fragments, calcareous and or- burial place. In this instance it is indicated by the pres- ganophosphatic brachiopod shells mixed with ooids. The ence of layered bryozoan-like encrusting fossils on peb- carbonate and siliceous substances of shells have been re- bles within the intercalation and the absence of fossils moved and the fossils are preserved as variably deformed in adjacent volcanogenic sediments below and above the internal and external moulds. The bryozoan zoaria are ironstone lens. preserved as fragile mineral infillings of the zooecia, with original walls etched. Siliceous spicules of sponges are Taxonomic composition of fossil assemblage absent, leaving only external moulds or hollow superfi- cial incrustations. Internal moulds of larger shells (gas- It is notable that the intercalation is highly fossiliferous. tropods, hyolithids, brachiopods) are mostly imperfect or The species composition and abundance of fossils has no missing. Unlike the calcareous shells, the phosphatic counterpart in the Klabava Formation in the area. Although shells remain intact, with the original laminated shell sub- some elements of the thanatocoenosis (bryozoans, orthids stance favourably preserved (Fig. 2). All fossils are only Nocturnellia nocturna and Ranorthis lipoldi) can be loosely attached to the rock and their surface and inter- found in adjacent volcanogenic sediments, their relative nal details are soft and fragile. In great part this is the abundance is quite different. Several fossils are known result of dissolution of the original carbonate matrix be- only from the thanatocoenosis. tween fossils and ooids by weathering but the possibili- Apart from the sponge spicules, the fossil assemblage ty of an originally large amount of unpreserved organic consists mainly of orthid brachiopods, lingulate brachi- 130 Journal of the Czech Geological Society 49/3–4(2004)

AC

B

DE

Fig. 2. Taphonomy of fossils in the thanatocoenosis. A – Orbithle rimosa Mergl with original organophosphatic shell, in left part pre-diageneti- cally broken; B – Lithobolus plebeius Mergl, with three post-diagenetical fractures of original shell; C, D – Branches of zoaria of bryozoans, with carbonatic material completely removed and zooecia preserved as mineral infillings; E – Nereidella pribyli (Havlíèek) preserved as external moulds with calcareous shells completely removed; A, B – ×5.8; C, D, E – ×2.8. Klabava Formation (Arenigian); Ejpovice. opods, bryozoans, sponges, one gastropod, one hyolith, mound shaped with large zooecia. The third species has and one trilobite species. a low, single-layered irregularly discoidal zoarium of Orthid brachiopods are represented by Nereidella large polygonal zooecia. However, the bryozoans cannot pribyli (Havlíèek), Ranorthis lipoldi Havlíèek, Noctur- be studied by sectioning and their taxonomic position is nellia nocturna (Barrande), Protohesperonomia ? sp., open. The encrusting bryozoan-like species Berenicea ? Nicolella ? sp. and Poramborthis sp. Only Nereidella, vetera Prantl covers the surface of small pebbles scattered Ranorhis and Nocturnellia are common. All the brachi- in the thanatocoenosis. opods were epibenthic, with the largest being Nereidella Gastropod and hyolithid shells are less common. The and Poramborthis, while shell width of the other genera first are represented by a hyperstrophic clisospirid Mi- is always less than 5 mm. mospira helmhackeri (Perner). Hyoliths are known from Lingulate brachiopods are represented by Lithobolus one newly discovered species Quadrotheca sp. that is plebeius Mergl, Orbithele rimosa Mergl, Numericoma described below. The trilobite Pseudopetigurus hoffmani vulcanogena Mergl, and Elkania lineola (Havlíèek). Perner is very rare. There are no remains of pelmatozo- Apart from Lithobolus, which probably lived as a shallow ans. Sponges are preserved as clusters of loose spicules burrower in a soft bottom, other genera were epibenthic which can be referred to Hyalospongia. The globular spi- or attached to elevated substrates. Absence of siphonotre- nose bodies associated with spicules are similar to those toid brachiopods is noteworthy, since these brachiopods described by Poèta (1898) and referred by him to the are abundant in more basinward sediments of the Klabava sponge Pyritonema feistmanteli Poèta. They are probably Formation of the same age. All the lingulate brachiopods skeletons of a large radiolarian, probably the genus Ana- are fairly thick-walled relative to their shell size. Their ma- crusa Nazarov (cf. Nazarov – Popov, 1980). ture size is about 10 mm except for the only 2 mm high The approximate relative abundance of specimens conical shell of the ephippelasmine Numericoma. (apart from sponges) is presented in Fig. 4. However, Bryozoans are represented by three or more poorly their quantitative presence is strongly affected by their known species. The first has a ramose zoarium of long fragmentary state and different modes of shelly material slender zooecia. The second species is massive, low- preservation. Journal of the Czech Geological Society 49/3–4(2004) 131

Fig. 3. Suggested origin of the described ironstone lens with an intercalation of the rich fossil assemblage. A – Configuration of the shore: model of shore with cliffs and small bays with gravels and ooidal sand, and crevices bearing sheltered rich fauna of brachiopods, bryozoans and sponges; B – Detail of a crevice with layered encrusting biota on exposed rocky walls (bryozoan-like Marcusodictyon and Berenicea ? vetera) and ramose and discoidal bryozoans, orthid brachiopods, burrowing lingulate brachiopods and sponges on the walls of sheltered areas. The death assemblage accumulated at the bottom of the crevice; C – Origin of ironstone lens with the intercalation of fossils (2): currents brought together ooids and pebbles from beaches in two short episodes (1, 3) with an intercalated episode of fossil deposition by currents that brought the death assemblage from a crevice (2).

Affinity of the assemblage to other benthic Havlíèek. This orthid-dominated community is locally communities in the area enriched by a diverse fauna including the bellerophontid Modestopira mergli Frýda, ostracods, conodonts and rarely The benthic communities of the upper part of the Klaba- the clisospirid Mimospira helmhackeri (Perner). Lingu- va Formation are well known, despite the limited outcrop late brachiopods Numericoma vulcanogena Mergl and of the formation and lack of precise stratigraphical data Fagusella indelibata Mergl are locally common, indicat- in some lithofacies (Havlíèek, 1982, 1989, Mergl, 1992). ing, together with other rare lingulate brachiopods, a There are two main littoral communities: The Nocturnel- slightly deeper environment and less firm bottom. Other lia nocturna Community and the Marcusodictyon Com- benthic fossils are highly localised, and remains of bry- munity. ozoans, trilobites and tunicates are exceptional. Sponges The Nocturnellia nocturna Community was wide- are somewhat local and commonly absent. The Noctur- spread along the NW margin of the basin, and flourished nellia nocturna Community is a typical-level bottom in a shallow subtidal environment. It is a typical low-di- community. The bioclasts of this community were sort- versity community, with prominent dominance of the ed by currents and waves into shelly pavements and also small enteletoid Nocturnellia nocturna (Barrande), rarely transported with volcanogenic material over long distanc- associated with orthoids Prantlina bohemica (Barrande), es along the ancient SW-NE coast of the basin. and Prantlina desiderata (Barrande). Mergl (1992) sub- The Marcusodictyon Community is a shallow, hard- divided this community to two sub-communities, main- bottom benthic community known from rocky shore ly on presence of the small plectorthoid Ranothis lipoldi cliffs. The biota are represented by weakly diversified 132 Journal of the Czech Geological Society 49/3–4(2004)

384 ders and small shaded caves. They have a quite different fau- na compared to exposed places of the adjacent open bottom (Asgaard – Bromley, 1991, Rug- giero, 2001). The material (bro- ken zoaria, shells) detached from the walls of some crevice or cave was probably trapped in a 114 litter of sponges and (?) algae at about the same place. This clus- 8 22 1 73 tered accumulation has subse- quently been transported basin- wards by occasional, probable orthid brachiopods (384 specimens) bryozoans (73 specimens) storm currents, together with trilobite (1 specimen) gastropods (22 specimens) ooids and pebbles from the nearby sea bottom. The accumu- hyolith (8 specimens) lingulate brachiopods (114 specimens) lation was deposited as a dis- Fig. 4. Relative abundance of fossils in the thanatocoenosis based on 602 specimens; siliceous sponges crete layer, now inside the iron- and radiolarians are omitted. stone lens. The known local shore configuration suggests encrusting and commonly colonial organisms (Marcusod- that transport was only several tens of meters, with some iction, bryozoan-like Berenicea ? vetera, trepostomate 100–200 m as the maximum. bryozoans and microbial and algal mats). The communi- ty is known from N slope of open mine in Ejpovice, some Ancestors and offsprings of the assemblage 100–200 m from the locality with the described fossil as- semblage. Thin stromatolitic mats attached to abraded The ancestors of the taxa of the thanatocoenosis may be rock walls of the cliffs, with encrusting Berenicea ? vet- found in the older units of the Prague Basin and in oth- era and a discoidal small trepostomate bryozoan have er, more distant areas. The earliest Ordovician unit in the been found there. Other encrusting mats on the cliffs are Prague Basin, the Tøenice Formation (Upper Tremado- formed by the bryozoan-like Berenicea vetera and Mar- cian) has several genera that may be considered as the cusodictyon sp. Both species are known on abraded and direct ancestors of some of the taxa present in the thana- worn surface of the cliffs, on boulders, and the former tocoenosis (Fig. 5). also on pebbles trapped in cliff fissures and in adjacent The plectorthoid Ranorthis lipoldi, one of the dominant sediments. Gravel bottoms with pebbles in small beach- species in Ejpovice, is also locally common in other sites es adjacent to cliffs were probably occupied by the en- of the upper Klabava Formation. The ancestor species Ra- crusting bryozoan-like Berenicea ? vetera. The same en- northis prima (Havlíèek) is known from a banded haema- crusting species are known in a few other sites along the tite lens from Holoubkov (locality Ouzký) where it is com- almost 60 km long NW limb of the basin. mon and represents the earliest known species of the genus. The thanatocoenosis in Ejpovice contains elements of A similar species is known from Bavaria (Sdzuy et al., both communities. There are species characteristic of the 2001). Although the genus is known in the upper Areni- Nocturnellia nocturna Community, especially the Noc- gian of France (Melou, 1982), its centre of distribution was turnellia/Ranorthis Sub-community (Mergl, 1992). Sev- the Baltic area with several successive species in the Bill- eral species in the thanatocoenosis are unknown outside ingen and Volkhov stages (Egerquist, 2004). it. The presence of bryozoans indicates a hard- or firm The genus Poramborthis is very rare in the thanato- marine bottom. The dominance of epibenthic, fixo-sessile coenosis and fragments of its shell with characteristic orthid, elkaniid and acrothelid brachiopods favours the microornament occur less frequently in the tuffaceous suggestion about the supposed hard-bottom origin for the sediments with the Nocturnellia nocturna Community. organisms preserved in the thanatocoenosis. The genus is represented by several stratigraphically suc- The original life assemblage preserved in the thana- cessive species in the Tøenice and Mílina formations in tocoenosis probably lived on the rocky bottom of supral- the Prague Basin (Havlíèek, 1977). It is also known in itoral and shallow sublittoral on the cliffs or in their close the Lower and Upper Tremadocian of Bavaria (Sdzuy, proximity. However, erect ramose zoaria of bryozoans 1955, Sdzuy et al., 2001) and Spain (Havlíèek – Joso- indicate some sheltered places in the shore rocks, proba- pait, 1972). It is a typical species of European sector of bly any crevices, small caves or similar places. Studies peri-Gondwana, being unknown outside the area. of Recent fauna show that the cryptic communities can The other orthid brachiopods are unknown in the old- be present at as much as 1–2 metres depth, below boul- er units of the basin, but they have relatives among fau- Journal of the Czech Geological Society 49/3–4(2004) 133 na of the Baltic area. The small enteletoid Nocturnellia ic. It is similar to Protohesperonomia Williams – Curry nocturna is more related to the Baltic Upper Ordovician (Family Hesperonomiidae Ulrich – Cooper), know only genus Oanduporella Hints than to Mediterranean Realm from Ireland in a fauna of Laurentian affinity. However, genera, such as Drabovia Havlíèek, Drabovinella Hav- it is also worth noting that some orthid genera (Apheoor- líèek or Hirnantnia Lamont. The commonest orthid in the thina Havlíèek, Robertorthis Havlíèek, Eoorthis Walcott) thanatocoenosis, the dalmanelloid Nereidella pribyli, is in the diversified orthid brachiopod assemblage from the related to Baltic species of the genus Paurorthis Schuchert base of the Tøenice Formation are similarly puzzling in – Cooper, especially to P. resima (Rubel) from the Bill- a palaeogeographic sens, since they have no relatives in ingen Stage. the earliest Ordovician of the Mediterranean Realm. The affinity of other rare orthid species from Ejpovice The other shared feature with older units of the Pra- is less clear. Protohesperonomia sp. decribed by Mergl gue Basin is the presence of hyperstrophic gastropod (1991) is known only in external moulds with character- Mimospira. This clisospirid genus is known in the Tøen- istic parvicostellate ornament and its affinity is enigmat- ice and Mílina formations (Rùžièka, 1927, Mergl, 1984)

SiberiaSib P A N T H A L L A AEGIRAEGIR S Baltica IC

a asu i um t Sib n e r u IAPETUS South a TORNQUIST L China Annamia Perunica rica Armo

South ia Pole n lo va A

Gondwana

PA NT HA LL AS IC

Fig. 5. Palaeogeographical reconstructions for the early Ordovician (after Cocks – Torsvik, 2002) with the distribution of some key Tremadocian – Arenigian genera indicated: Ranorthis (triangles), Poramborthis (dots), Mimospira (rectangles). 134 Journal of the Czech Geological Society 49/3–4(2004) in allied and hitherto undescribed species related to Mi- Systematic palaeontology mospira helmhackeri. The clisospirids are typical warm water gastropods, known mainly in the Baltic area in the Class H yolitha Marek, 1963 Ordovician (Frýda, 1989), but in the Prague Basin they Order Orthothecida Marek, 1967 are known only in the Lower and early Middle Ordovi- Family O r t h o t hecidae Syssoiev, 1957 cian and from the mid-Ashgillian (the top of the Králùv Dvùr Formation; Frýda, 1989). The later occurrence is Genus Quadrotheca Syssoiev, 1958 clearly related to the pre-Hirnantian climate warming episode (Boucot et al., 2004). Type species: Hyolithes (Orthotheca) quadrangularis Holm, 1893; Upper Ordovician, Sweden. Bryozoans are unknown in the earlier Ordovician in the Prague Basin, but the rapid radiation of the phylum Quadrotheca sp. begun only in the Arenigian. The cystoids, both rhomb- Fig. 6 iferans and diploporites are known from the Tøenice and Mílina formations where they are also associated with the Material: Six incomplete shells, (specimens PCZCU 10657-1062) brachiopods Ranorthis and Poramborthis. The cystoids preserved as external moulds in oolitic ironstone. are unknown in the Klabava Formation, but with a few exceptions, echinoderms are totally in this unit; their ab- Description: The conch is straight and long with an sence is probably related to some physical limits of the estimated maximum length of 30 mm. The shell is thin environment. The cystoids, however, are common ele- and fragile. The angle of divergence is less than 10°, ments in the Tremadocian of Bavaria where they occur probably between 7–8° (apical part of the shell is always with the orthid genera Poramborthis and Ranorthis. broken off). The cross section is rectangular, almost There are also common genera among the lingulate square. All four sides are flat to weakly concave in the brachiopods. Orbithele is known in the lowest part of the adult part of the conch. Tøenice Formation. It is common in the Mílina Forma- The ornament consists of thin and high costellae of two tion and Orbithele rimosa from the intercalation in Ejpov- sizes. The coarser costellae are some 0.4 mm apart. The ice is the youngest known species of the genus yet re- intercalated costellae are about their half height; number- ported. Elkania is known also from the Tøenice Formation ing four or five between the coarser costellae. This orna- and the rare species from Ejpovice is also the youngest ment is present on the ventral, dorsal and lateral sides of known occurrence of the genus in the Prague Basin. the conch. Growth lines are low and weak. Lines are rarely Sponges having large hexactine and pentactine spicules extended into coarser very short lamellae. The lateral sur- are almost unknown in the younger Ordovician units of face of the coarser costellae bears growth lines inclined the Prague Basin, in contrast to their abundant presence anteriorly, indicating that along the shell aperture these from the Tøenice Formation to the top of the Klabava coarser costellae formed a margin of short spines. Shell Formation. In general, the species-rich communities near interior and the operculum are unknown. the base of the Tøenice Formation near Holoubkov, and Remarks: Despite the lack of data on the shell inte- moderately diversified fauna of the Mílina Formation in rior and the operculum, the shell is very similar to Q. (?) the Komárov area, can be taken as forerunners of the fau- rediviva Marek 1967, especially in its rectangular cross- na preserved in the thanatocoenosis in Ejpovice. section and its ornament. Quadrotheca sp. differs in its The immediately overlaying Šárka Formation has more elongate shell with smaller apical angle and the or- nothing common with the fauna from the intercalation, nament more clearly divided into coarser and finer cos- but this difference is related to rapid subsidence and deep- tellae which are also on the lateral sides of the conch. ening of the basin and lack of data about shallow littoral The species is much older in its stratigraphical occur- environment. Some similar faunistic elements are known rence, since the Q. (?) rediviva is known from the Zahoøa- from the Libeò and Letná formations (Berounian), but the ny and Bohdalec formations (Berounian Stage). The spe- bryozoans did not become abundant in the Prague Basin cies thus represents the earliest representative of the until the deposition of the Zahoøany and Bohdalec for- genus yet known. It is also the earliest representative of mations. The remarkable hyolith Quadrotheca known in the order Orthothecida Marek, 1967 in the Ordovician of the intercalation in Ejpovice is known from the Zahoøa- the Prague Basin. Another orthothecid genus, Bacrothe- ny Formation (Berounian). ca Novák, 1891, is common in the overlaying Šárka For- The fauna from Ejpovice is a remarkable mixture of mation. However, this genus, represented by the common the earlier Ordovician groups and genera, (Ranorthis, species B. teres (Barrande, 1867) differs from Quadroth- Poramborthis, Orbithele, Elkania, Mimospira and eca sp. in its distinctly rounded cross section of the shell hexactinellid sponges) and of new Middle and Upper and more uniformly sized costellae. Ordovician elements represented by bryozoans, orthids The presence of Quadrotheca sp. in association with Nocturnellia and Nereidella, the ephippelasmatine Nu- bryozoans and abundant orthid brachiopods is reminis- mericoma and the orthothecid Quadrotheca. Many ele- cent of younger fossil associations of the Zahoøany and ments have clear Baltic affinity. Bohdalec formations. The genus probably represents a Journal of the Czech Geological Society 49/3–4(2004) 135 common and characteristic element of the BBP commu- Acknowledgements. This research was supported by Grant nities not only in the Prague Basin and its presence may Agency of Czech Republic grant 205/03/0670 “Fauna and be suggested in other parts of the peri-Gondwana. Stratigraphy of the Tremadocian in Bohemia”. It is a con- Occurrence: Upper Arenigian, Klabava Formation, tribution to IGCP Project 503 “Early Palaeozoic Palaeo- uppermost part; Barrandian, Ejpovice open iron mine. geography and Palaeoclimate”. Submitted November 21, 2004 Conclusions

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Nejstarší spoèenstvo s dominantními mechovkami a brachiopody v ordoviku peri-Gondwany a jeho pøedchùdci: pøíklad z klabavského souvrství (arenig) Barrandienu, Èechy

Z nejvyšší èásti klabavského souvrství (arenig) ze západní èásti pražské pánve je popsána druhovì bohatá asociace fosilií s mechovkami, orthidními a lingulátními brachiopody, køemitými houbami a další faunou. Její taxonomické složení a pozice vùèi bývalé bøežní linii je vyhodnocena a komentována. Fosilie pravdìpodobnì reprezentují materiál se stìn nebo ze stìrbin pobøežních skalisek. Spoleèenstvo bylo analogické recentním kryptickým spoleèenstvùm skalnatých pobøeží. V systematické èásti je popsán druh Quadrotheca sp., který je nejstarším ordovickým zástupcem orthothecidních hyolitù v pražské pánvi.