Facies

, 54:613

The role of bryozoans in fossil reefs—an example from the Middle Devonian of the Western Sahara

Authors

    • Institut für Geowissenschaften der Christian-Albrechts-Universität zu Kiel
  • Peter Königshof
    • Forschungsinstitut und Naturmuseum Senckenberg
Original Article

DOI: 10.1007/s10347-008-0149-1

Cite this article as:
Ernst, A. & Königshof, P. Facies (2008) 54: 613. doi:10.1007/s10347-008-0149-1

Abstract

Stenolaemate bryozoans with their stable calcitic skeletons play a significant role in reef building. In the Middle Devonian Sabkhat Lafayrina reef complex (Western Sahara), bryozoans are abundant and diverse. Although they do not form part of the principal framework of reefs, bryozoans are involved significantly in reef growth, especially in the initial stage. In this way, bryozoans are important with respect to initiating reef growth. They contribute greatly to sediment stabilization, making it possible for principal reef builders to grow on hardened and stabilized substrates, and also play sediment-baffling and sediment-filling roles. The aim of this study is to document the diversity of bryozoans in a Middle Devonian reef complex and to estimate their potential for initiation and contribution to reef structures.

Keywords

ReefsBryozoaDevonianWestern SaharaEcology

Introduction

Bryozoans are aquatic colonial animals, usually with calcitic skeleton, occurring mainly in marine habitats. They are known since the Lower Ordovician and have a fossil record with more than 20,000 extinct species. In the Palaeozoic, bryozoans inhabited various marine environments, being often rock-forming. In several time intervals, bryozoans played a relatively important role as reef-building organisms. The first bryozoan reefs are recorded from the Middle Ordovician (Cuffey 1977; McKinney et al. 1987; Vermeij 1987), a time interval that is characterized by a significant radiation of marine skeletal biota with a near-threefold increase in the number of known families (Sepkoski 1993). Silurian reefs were abundant and diverse. They occupied different facies settings from nearshore, to mid-outer shelf, and to deeper water settings, marking a period of probably warm climate that was connected with high sea-level stands (e.g., Wood 1999; Copper 2002). The Devonian period, particularly the Mid-Late Devonian (Givetian-Frasnian) is considered to represent possibly the largest global expansion of reefs in the Phanerozoic (Copper 1989, 2002). During this “greenhouse” interval, the sea level was high. Extensive reef tracts occurred around the world, such as in Canada, South China, central Asia, and North Africa, and also throughout Europe. These reefs were diverse and widespread and were dominated by microbialites, calcified cyanobacteria, and large metazoans, especially stromatoporoids and tabulate corals, whereas Carboniferous and Permian reefs were dominated by algae and mud mound communities, with minor participation of poriferans, echinoderms, bryozoans, and brachiopods (Fagerstrom 1987, 1994). The faunal diversity in Devonian reefs is high, with metazoan reef builders showing a tremendous variety of complex morphologies. However, there are only a few Devonian cases in which bryozoans were substantially involved in reefs and bioherms (e.g., Hedinger 1989; McKinney and Kříž 1986; Wyse Jackson 2006). Frequently, they are associated with so-called “algal laminites” (Smith 1981; Hollingworth and Tucker 1987; Paul 1996) and thrombolithic microbial crusts (Flajs and Hüsssner 1993; Flajs et al. 1996). The stromatolites and thrombolites form a hard substrate, which is necessary for the development of an overgrowing bryozoan framework. Bryozoans often played an important role in the pioneer communities of reefs, even if they were not principal reef constructers. They stabilized and trapped sediment, paving the way for the main reef builders (Cuffey 1970, 1977, 2006; Hollingworth and Tucker 1897). Furthermore, bryozoans occupied cryptic habitats and interspaces in reefs, often showing a high diversity.

In the Devonian, the taxonomic diversity of bryozoans underwent significant changes. The Silurian faunas were dominated by trepostome bryozoans. Bifoliate cryptostomes were also abundant and diverse in Silurian communities. During the Devonian, fenestrate bryozoans became important, whereas trepostomes and bifoliate cryptostomes diminished markedly. These changes also marked a shift from mainly branched, massive, or encrusting colony forms to planar unilaminate ones, like those of meshed fenestrates. This colony form provided a significant success for fenestrates in the Late Palaeozoic (McKinney 1986). Advantages of these colony forms were improved feeding (due to occupying higher tiers and larger colony volumes per unit substrate area), and reduction in competition on the substrate (McKinney and Jackson 1989). Erect fenestrate colonies also enhanced sediment trapping with their feeding currents, essentially contributing to mud mound building (McKinney et al. 1987). Fenestrates were involved significantly in Carboniferous and Permian reefs and bioherms. Erect rhabdomesine bryozoans also became diverse during the Devonian. However, due to their small size, they were not as important as fenestrates in reefal structures.

In the Middle Devonian Sabkhat Lafayrina reef complex, Western Sahara (Scholz et al. 2005; Jansen et al. 2006; Königshof and Kershaw 2006; Fig. 1), abundant and diverse bryozoans were found in different parts of the described reef complex. The bryozoans play a significant role in the development of initial parts of reef as well as in certain facies inside and around the reef complex.
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Fig. 1

Location of the Sabkhat Lafayrina reef complex in the Tindouf Basin (star) near Samara, Western Sahara

Materials and methods

The morphology of the Sabkhat Lafayrina reef was recently mapped and described (Königshof and Kershaw 2006). The bryozoan palaeontology and sedimentology were investigated using thin sections studied with a transmitted light binocular microscope. More than 200 thin sections of different sizes (28×48 mm, 50×50 mm, and 50×75 mm) were used for bryozoan taxonomy and microfacies study. The material is housed at the Senckenberg Museum (Frankfurt am Main, Germany).

Geological setting

The Sabkhat Lafayrina reef complex is located on the southern flank of the Tindouf Basin (Western Sahara, Fig. 1), which is bounded on the south by the Precambrian Reguibat Massif and on the north and west by the African fold belt. Devonian reef structures which were developed at the northern fringes of Gondwana in a high palaeolatitude of about 45°S (Copper 2002; Golonka 2002; Scotese 2003) are mainly composed of stromatoporoids and corals accompanied by other organisms such as crinoids, brachiopods, and bryozoans. The Sabkhat Lafayrina reef complex is well preserved. Due to a lack of current vegetation cover, the reef structure in outcrop is revealed in three dimensions, and it is not tectonically deformed. Based on the first investigations in the late 1960s by Dumestre and Illing (1967) and recent studies by Wendt and Kaufmann (2006), six reef cycles can be distinguished in the Givetian of the Uein Terguet area. The reef basement is characterized by dark grey, thick-bedded oolites overlaying siltstones and sandstones. The reef cycles are separated by marly sedimentation. The Sabkhat Lafayrina reef complex, which was described recently (Jansen et al. 2006; Königshof and Kershaw 2006), represents the fourth reef cycle in the Uein Terguet area. The reef structure mainly consists of bulbous and tabular stromatoporoids up to 2.5 m across with a variety of tabulate and rugose corals. Corals of the genus Mesophyllum, which occur in three characteristic horizons, are very abundant. The occurrence of Heliolites porosus indicates a Late Givetian age.

Bryozoans are mostly associated with crinoidal limestones and stromatoporoid bindstones in the basal parts of the reef structure (Fig. 2). These limestones overlay sandstones and contain brachiopods as well as bryozoans. Sediments with abundant bryozoans are mostly represented by grainstones. The uppermost part of the reef structure is built up by large stromatoporoids which represent the last stage in reef development preserved.
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Fig. 2

Generalized lithological profile of the Sabkhat Lafayrina reef complex

Bryozoan growth forms and faunal diversity in the Sabkhat Lafayrina reef complex

Skeletal bryozoans develop various growth forms as an adaptation to different environmental parameters. The growth form of a colony is constructed by the skeletons of individuals (autozooecia), by different polymorphs and by extrazooecial skeleton. Some groups, such as cystoporid bryozoans, can produce large amounts of extrazooecial skeleton. Growth forms of bryozoans serve different aims, but generally two of them are crucial: habitat space occupation (McKinney and Jackson 1989), and regulation of feeding currents (Cowen and Rider 1972; Taylor 1979; McKinney 1981). A very comprehensive classification of growth forms has been suggested by Hageman et al. (1998). The analytical bryozoan growth habit classification is based on 11 characters, which describe the orientation of the colony and its occupation of, and placement in, space, which is very important in understanding the ecological claims of bryozoans.

Taxonomic analysis reveals 21 bryozoan genera in the Sabkhat Lafayrina reef complex so far (Table 1). The most abundant and important genera are fenestrates (eight genera), followed by bifoliate and rhabdomesine cryptostomes (six genera), trepostomes (three genera), and fistuliporids (two genera). Phylloporines, which are regarded as closely related to fenestrates are represented by two genera.
Table 1

Preliminary taxonomic composition, relative abundance (a abundant, c common, r rare) and growth forms of bryozoan fauna from the Sabkhat Lafayrina reef complex

Taxa

Relative abundance

Growth forms

Fistulipora sp.

a

Encrusting

Fistuliphragma sp.

c

Ramose (tubulose)

Leioclema sp.

a

Encrusting

Atactotoechus sp.

c

Ramose

? Cyphotrypa sp.

r

Encrusting

Acanthoclema sp.

a

Ramose

Rhabdomesid sp. indet

r

Ramose

Intrapora sp.

r

Ramose

Euspilopora sp.

c

Ramose

Ptilodictyonid sp. indet 1

r

Ramose

Ptilodictyonid sp. indet 2

r

Ramose

Penniretepora sp.

c

Pinnate ramose

Filites sp.

c

Pinnate ramose

Fenestella sp.

a

Reticulate

Alternifenestella sp.

a

Reticulate

? Speotrypa sp.

r

Reticulate

Reteporidra sp.

a

Reticulate

Rectifenestella sp.

a

Reticulate

Semicoscinium sp.

r

Reticulate

Prolixicella sp. (Ernst and Schroeder 2007)

c

Branched ramose

Phylloporinine sp. indet.

r

Branched ramose

Bryozoans in the Sabkhat Lafayrina reef complex developed different growth forms (Table 1): ramose (ten), reticulate (six), encrusting (three), and pinnate ramose (two). Ramose Atactotoechus and tubulose ramose Fistuliphragma develop robust stems, with diameters of about 1 cm and 5–15 cm in length. Others are rather delicate branched colonies, with diameters about 1–3 mm. Reticulate bryozoans, Reteporidra, ?Speotrypa and Semicoscinium have large and robust colonies, with colonies about 10 cm high and branches more than 1 mm wide. Other fenestellids have smaller colonies (5–7 cm high), with branches about 0.3–0.4 mm wide.

Bryozoans and their sedimentological role in the Sabkhat Lafayrina reef complex

Bryozoans appeared in pioneer communities preceding the reefs, and acted as sediment stabilizers (encrusting cystoporates and trepostomes), sediment baffler (fenestrates) and sediment fillers (all taxonomic groups).

Sediment stabilizing

Encrusting bryozoans e.g., Fistulipora, Leioclema covered large areas and stabilized the substrate by binding the fine-grained sediment. Large-branched trepostomes (Atactotoechus) and cystoporates (Fistuliphragma) stabilized the sediment additionally by their colonies, sharing also a sediment filling role (Figs. 3b–f, 4c, d), and providing a suitable substrate for other encrusting organisms, including also bryozoans.
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Fig. 3

Microfacies of the reef basal parts in Sabkhat Lafayrina reef complex. a Carbonate sediment, stabilized by encrusting trepostome bryozoan Leioclema sp. (Lei) (SMF-HF-23). b–f Grainstones containing different bryozoans: trepostome Atactotoechus sp. (At), diverse fenestellids (Fen), cystoporate Fistuliphragma sp. (Fis) and rhabdomesine cryptostome Acanthoclema sp. (Ac) (b SMF-HF-28, c SMF-HF-25, d SMF-HF-29, e SMF-HF-22, f SMF-HF-26). Scale bars are 3 mm

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Fig. 4

a, b Rock samples with large fragments of fenestellid bryozoans. Scale bars 10 mm. c, dFistuliphragma sp. c Transverse section of a tubular colony (SMF-HF-24), scale bar 3 mm. d Longitudinal section (SMF-HF-27), scale bar 0.5 mm. e, fLeioclema sp., encrusting colony: e tangential section, f longitudinal section; scale bars 0.5 mm (SMF-HF-21)

Sediment baffling

Large fenestrate colonies like Reteporidra and Rectifenestella (Fig. 4a, b) are known to be effective in trapping sediment (e.g., Troell 1962; McKinney et al. 1987; Christopher 1990). This is due to their net-like shape and also because of the activity of their polypides. Polypides create feeding currents that accelerate the sediment precipitation in the vicinity of the colony (McKinney et al. 1987; Fagerstrom 1987).

Sediment filling

All calcified bryozoan skeletons can contribute carbonate to reef sediment. In contrast with many other groups (e.g., gastropods, bivalves; Bathurst 1964), skeletons of bryozoans can persist a long time without being dissolved. Most Palaeozoic bryozoans had skeletons consisting of the low Mg calcite (Lowenstam 1963; Sandberg 1983; Boardman and Cheetham 1987; Taylor 1999), which is very stable to diagenetic alteration. Different genera contribute varying amounts of carbonate to the sediment. Large colonies of Atactotoechus, Reteporidra,Rectifenestella and Fistuliphragma contribute more carbonate to the reefs, whereas the rarer Reteporidra, ? Speotrypa and Semicoscinium contribute less. Small-sized bryozoan colonies can contribute significantly to the sediment volume if they occur in large numbers (Fig. 3e).

Discussion of the role of bryozoans in reefs

Bryozoans contribute to reef construction in different ways: as principal frame builders, accessory frame encrusters, sediment formers and sediment-movement inhibitors (Cuffey 1977), but generally bryozoan-dominated reef structures are small “patch reefs” or “reef knolls” (Cuffey 2006). Bryozoans rarely appear as principal frame builders, either alone or in conjunction with other organisms; an exception is a fossil reef from Vermont, USA, of Ordovician age (Pitcher 1964). These reefs are constructed by densely packed colonies of trepostomid bryozoans in upright growth position (Cuffey 1977). In Silurian reefs of Gotland, bryozoans accompanied by corals, produced small reef bodies on the flanks of the main reef complex (Fanterna type reefs, Manten 1971, p. 473). In Upper Permian Zechstein reefs, which reach about 100 m in thickness fenestrate bryozoans appeared in large quantities. However, they were not principal frame builders. This role was filled by algae and microbial mats which triggered a fast synsedimentary lithification and produced wave-resistant reefs (Kerkmann 1967). Similar accumulation patterns have been described by Antoshkina (1996) in Ordovician reefs of the Urals. However, accessory frame encrusting roles involve bryozoans growing upon or within reefal frameworks constructed earlier by other frame-building organisms. In this way, bryozoans play a role of “veneerers of dead reefs” (Cuffey 1977). After the principal frame builder such as corals are killed by severe conditions, bryozoans can overgrow dead reef building organism as a “veneer”. Examples of such reefs veneered by cheilostome bryozoans are known from the Miocene of the Black Sea (Martin 1943). More commonly, bryozoans appear in living and fossil reefs as cryptofauna, producing considerable biomass. Their roles in this concept are reduced to hidden encrusters, cavity dwellers, and cavity fillers (Cuffey 1977; Fagerstrom 1994). Due to their relative independence from daylight to small sizes, bryozoans inhabit the free space between framebuilders and other reef dwellers. Based on their variety of growth forms bryozoans can stabilize loose or movable sediments. Sometimes they appear as active sediment binders consolidating sediment grains to larger components by encrusting. Such examples are known from bryozoan patch reefs in the Middle Ordovician of North America (Ross 1981). Other examples of compact crust mounts have been reported by Cuffey and Taylor (1989) and Cuffey et al. (1991). In some Palaeozoic reefs, bryozoans acted as sediment stabilizers (e.g., Cuffey 1970, 1977; McKinney et al. 2001) similar to those described from the Sabkhat Lafayrina reef complex.

Fossil reef structures have been variously reconstructed (Copper 1989, 2002, cum lit.), but have not been fully considered in the light of interactions between skeletal organisms, microbialites, and community successions of reef builders (Adachi et al. 2006). These complex biotic interrelationships are still poorly understood even in Devonian reefs; however, this excellent reef complex provides the opportunity for more detailed studies.

Conclusions

The Middle Devonian Sabkhat Lafayrina reef complex in Western Sahara represents one of the most prominent Middle Devonian reef structures in northern Africa. The complex belongs to the fourth reef cycle in the Uein Terguet area. The occurrence of bryozoans is mainly associated with crinoidal limestones, and to a lesser extent with stromatoporoid bindstones in different units in the basal part of the reef complex. Based on the diverse fauna, bryozoans grew in optimum conditions either at the sediments surfaces or within the reefal framework.

Although bryozoans are not primary reef builders in Devonian reefs, they sometimes played other important roles within the reefs. In the Middle Devonian reefs in Sabkhat Lafayrina, Western Sahara, bryozoans appeared in pioneer communities, playing roles as sediment stabilizers, sediment bafflers, and sediment fillers. Bryozoans stimulated the initial phase of reef growth, providing suitable growth substrates and/or stabilizing unconsolidated substrates for the main reef-building organisms. These organisms may be useful for palaeoecological and palaeoenvironmental reconstructions.

Acknowledgments

We are very thankful to Caroline Buttler, Cardiff, for language correction and helpful comments on the manuscript and to Wolfgang Reimers, Kiel and Marion Demmel, and Jana Anger, Frankfurt, for their help in preparation of thin sections. We also thank to Patrick Wyse Jackson, Dublin, and an anonymous reviewer for helpful comments of the manuscript. The present study was accomplished during project ER 278/4-1 and 2 supported by the Deutsche Forschungsgemeinschaft (DFG). This paper is a contribution to IGCP 499 (Devonian land-sea interaction: evolution of ecosystems and climate).

Copyright information

© Springer-Verlag 2008