Bryozoans from the lower Silurian (Wenlock) Steinsfjorden Formation of Ringerike, southern Norway ANDREJ ERNST & HANS ARNE NAKREM A stenolaemate bryozoan fauna from the lower Silurian (Wenlock) Steinsfjorden Formation of the Ringerike district, southern Norway contains 10 species (2 cystoporates, 6 trepostomes, and 2 cryptostomes). Six species are new: Heterotrypa ringerikensis sp. nov., Trematopora maculata sp. nov., Amplexopora crassiparietum sp. nov., A. evae sp. nov., Orthopora worsleyi sp. nov., and Mediaporina kiaeri sp. nov. One species is described in open nomenclature: Eridotrypella sp. Trepostome bryozoans dominate this assemblage, both by diversity and abundance. The bryozoans oc- cur in carbonates associated with evaporite minerals and sedimentary structures indicating very shallow depositional settings. • Key words: lower Silurian, Bryozoa, Norway, taxonomy. ERNST,A.&NAKREM, H.A. 2015. Bryozoans from the lower Silurian (Wenlock) Steinsfjorden Formation of Ringerike, southern Norway. Bulletin of Geosciences 90(1), 65–87 (13 figures, 10 tables). Czech Geological Survey, Prague. ISSN 1214-1119. Manuscript received June 5, 2014; accepted in revised form September 11, 2014; published online Decem- ber 3, 2014; issued January 26, 2015. Andrej Ernst, Institut für Geologie, Universität Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany; [email protected] • Hans Arne Nakrem, Natural History Museum (Geology), University of Oslo, P.O. Box 1172 Blindern, NO-0318 Oslo, Norway; [email protected] The purpose of this study is to describe the bryozoan fauna beds of the Ringerike Group (late Wenlock to Ludlow age). of the lower Wenlock Brattstad Member, Steinsfjorden The age of the bryozoan beds are Sheinwoodian to Formation of the Ringerike district, Oslo Region, Norway Homerian as based on occurrences of ostracodes and (Fig. 1). The bryozoans present in these beds have been brachiopods as well as agnathans and thelodonts (on di- known since the work by Kiær (1908), but they have not rectly overlying beds) (Worsley et al. 1983, 2011). Geo- until now been given a systematic treatment. The number chemical analyses of interbedded bentonites (Hetherington of previously known species is low, and their distribution et al. 2011) support these datings. in Ringerike can add very little to the already established biostratigraphy of the area. The Brattstad Member (upper “Etage” 9e and 9f of Depositional environment Kiær 1908) is 30 m thick in the studied areas of Nes bus stop, Ødegårdsviken and Åsa (Fig. 2). The lithology con- Alternations of marls, red argillaceous dolomites, dolomi- sists of medium to thickly bedded limestones with tic limestones, and limestones, with occurrence of oospa- interbedded marls and dolomitic limestones. The lime- rite, oncolites, algal laminated dolomitic limestones with stones contain a relatively rich and varied fauna with thick mud cracks, stromatolites as well as evaporitic minerals massive favositid and oncolitic biostromes (the “Favosites (Fig. 3B) indicate developments of shallow subtidal to sup- Limestone” of Kiær 1908) near the top of the member from ratidal environments (Olaussen 1981). The fossil compo- which the bryozoan material derives. Kiær (1908, p. 113) nents are characterized by many individuals belonging to lists fossils from these beds (“Etage 9f”) in various locali- few taxa, notably gastropods, ostracods, brachiopods and ties in the Ringerike district, but only one bryozoan taxon, rare trilobites (Kiær 1908, Olaussen 1981). Tabulate corals “Monticuliporen [Amplexopora(?) sp.]”, is mentioned. (Fig. 3C) and echinoderms are locally present. Whitaker (1977, pp. 24–26) also refers to the bryozoan- The common microfacies are represented by float- rich beds as “polyzoan (Amplexopora)” beds. Spjeldnæs stones with abundant bryozoans (mainly trepostomes (1982) describes the bryozoan occurrences in these beds as (Fig. 3A, F), locally poorly washed rudstones (Fig. 3E), “colonies ranging from only a few cm in diameter to small and bryozoan and algal bindstones (Fig. 3B–D). The ma- bioherms with three-dimensional frameworks”. Bryozoans trix is usually micrite-rich, containing various skeletal grains are even found in the lowermost levels of the overlying red (Fig. 3G). Calcimicrobes are common on some localities, DOI 10.3140/bull.geosci.1521 65 Bulletin of Geosciences Vol. 90, 1, 2015 Figure 1. Map of the Ringerike District show- ing sample localities and major structural fea- tures. – Åsa locality, – Ødegårdsviken lo- cality, – Nes bus stop locality. Figure 2. Wenlock stratigraphy of the Ringerike District (based on Melchin et al. 2012; Worsley et al. 1983, 2011). represented by various Girvanella and Rothpletzella spe- Anstey & Perry (1970) for trepostomes, and Hageman cies. Microbial crusts are common, often forming oncoids (1993) for cryptostomes. The following morphologic char- (Fig. 3C). acters were measured for statistical use (Fig. 4): Branch The faunal composition and microfacies characteristics Width, Branch Thickness, Exo- (Endo-) zone Width, Auto- are reduced due to stressed conditions in an environment zooecial Aperture Width, Autozooecial Aperture Spacing with fluctuating salinities. (Along Branch/diagonally for cryptostomes), Acantho- style Diameter, Lunarium Width/Length/Thickness, Vesi- cle Diameter/Spacing/Number per Aperture, Wall Thick- Material and methods ness in Exozone, and Macular Diameter (Spacing), Autozooecial Diaphragm Spacing, Meso- (Exila-, Hetero-) Bryozoan samples were collected from the localities Nes zooecia Width, Mesozooecial Diaphragm Spacing. bus stop, Ødegårsviken and Åsa (see Fig. 1). The investiga- The spacing of structures was measured as the distance ted bryozoans were studied from thin sections using between centres. Additional quantitative characters in- a transmitted light binocular microscope. 107 oriented clude the Number of Mesozooecia, Exilazooecia and and non-oriented thin sections were used. The material is Acanthostyles surrounding each autozooecial aperture. housed at the Natural History Museum (Geology), Oslo, Statistics were summarized using arithmetic mean, sample under numbers PMO 221.750–221.841. standard deviation, coefficient of variation, and minimum Morphological character terminology is adopted from and maximum values. Figure 3. Carbonate microfacies of the Steinsfjorden Formation of Ringerike, southern Norway. • A – bryozoan floatstone, Nes bus stop, PMO 221.766. • B – bryozoan bindstone (Trematopora maculata, Fistulipora nummulina). Arrow – relicts of gypsum crystals, Ødegårdsviken, PMO 221.820. • C – coral-oncolitic floatstone, Åsa, PMO 221.813.•D–bryozoan bindstone (Trematopora maculata, Fistulipora nummulina), Ødegårdsviken, PMO 221.824.•E–poorly washed rudstone, Ødegårdsviken, PMO 221.779.•F–bryozoan floatstone, Ødegårdsviken, PMO 221.792.•G–matrix, Nes bus stop, PMO 221.766. 66 Andrej Ernst & Hans Arne Nakrem Bryozoans from the lower Silurian (Wenlock) Steinsfjorden Formation of Ringerike A BC D E F G 67 Bulletin of Geosciences Vol. 90, 1, 2015 Results and discussion Table 1. Measurements of Fistulipora nummulina Nicholson & Foord, 1885. Abbreviations: N – number of measurements; X – mean; SD – stan- dard deviation; CV – coefficient of variation; MIN – minimal value; The bryozoan colonies are generally well preserved and MAX – maximal value. show little signs of abrasion. Large, thin branches up to 5 mm in diameter are often not broken, the whole fauna has N X SD CV MIN MAX only been transported a short distance, and they may repre- Aperture width, mm 33 0.16 0.027 16.46 0.12 0.22 sent time intervals of more normal marine conditions. How- Aperture spacing, mm 31 0.24 0.035 14.41 0.17 0.32 ever, they are not considered preserved in situ. The bryo- Vesicle width, mm 23 0.10 0.031 32.55 0.03 0.15 zoans are therefore interpreted as typical for the Vesicle spacing, mm 25 0.10 0.028 28.98 0.06 0.15 depositional environment in which the sediments were for- Vesicles per aperture 8 6.0 1.414 23.57 4.0 8.0 med. The three bryozoans species previously known have a Wenlock or Ludlow stratigraphic distribution elsewhere. The new species described herein have many similarities Material. – PMO 221.756, PMO 221.758, PMO 221.790, with species known from time equivalent strata in Europe PMO 221.764, PMO 221.817, PMO 221.820, PMO and the USA, but only further research may reveal their 221.822, PMO 221.824, PMO 221.827, PMO 221.829. biostratigraphic as well as biogeographic value. Description. – Encrusting colonies, 0.25–0.85 mm in thick- ness. Autozooecia growing from 0.002–0.008 m thick Systematic palaeontology epitheca, bending sharply at their bases towards colony surface. Autozooecial apertures circular to oval. Basal dia- Phylum Bryozoa Ehrenberg, 1831 phragms rare to absent. Lunaria well developed, horseshoe Class Stenolaemata Borg, 1926 shaped to slightly triangular, 0.06–0.08 mm wide, Order Cystoporata Astrova, 1964 0.03–0.04 mm long and 0.03–0.04 mm thick. Vesicles Family Fistuliporidae Ulrich, 1882 abundant, 4–8 surrounding each autozooecial aperture, irregularly shaped, medium in size, often large at the Genus Fistulipora M’Coy, 1849 base, polygonal in tangential section, box-like to hemi- spheric, with plane or concave roofs in exozone. Autozo- Type species.–Fistulipora minor M’Coy, 1849. Carboni- oecial walls granular, 0.003–0.005 mm thick. Extensive ferous; England. extrazooecial material in exozone developed. Maculae not observed. Diagnosis. – Massive, encrusting or ramose colonies. Cy- lindrical autozooecia with