Relict Ordovician brachiopod faunas in the Lower Silurian of Asker, Oslo Region, Norway

B. GUDVEIG BAARLI & DAVID A. T. HARPER

Baarli, B. G. & Harper, D. A. T.: Relict Ordovician brachiopod faunas in the Lower Silurian of Asker, Oslo Region, Norway. Norsk Geologisk Tidsskrift, Vol. 66, pp. 87-98. Oslo 1986. ISSN 0029- 196X.

The diverse brachiopod fauna of the lowest part of the Solvik Forrnation (Lower Llandovery) in the Asker area of the Oslo Region is a well- organised association overwhelmingly dominated by relict genera more typical of the Ordovician. These taxa appear to have survived the extinction events of the late Ordovician in deep water facies in or adjacent to the central Oslo Region. The gradual disap­ pearance of Ordovician elements through the sequence is considered to have resulted from un­ successful competition with immigrant stocks of Silurian aspect that may have originated around the shelves of archipelagos created during the late Ordovician regression. Thus competition is suggested to account for the final pulse of fauna) extinction above the Ordovician-Silurian boundary.

B. G. Baarli, Dept. of Geology, Williams College, Williamstown, Massachusetts 01267, USA. D. A. T. Harper, Dept. of Geo/ogy, University College, Galway, Ire/and

An extended extinction event just prior to the plete at and beyond the platform edges or in in­ Ordovician-Silurian boundary has been recog­ tracratonic basins where sections may be continu­ nised as one of a number of significant extinction ous across the system boundary. These sequences events during the Phanerozoic (Raup & Sepkoski are, however, the most likely to be destroyed 1984). Information regarding the earliest Llan­ during subsequent orogeny and continental col­ dovery faunas, however, is sparse compared with lision. Nevertheless, such continuous sequences that for immediately younger and older faunas. have been reported from Anticosti Island (1\ven­ The widely recognised glaciation at the end of the hofel 1928, Barnes & McCracken 1981, Bolton Ordovician caused a considerable fall in sea leve! 1981) and Quebec (Lesperance & Sheehan 1976, in the order of 50-100 m (Brenchley & Newall Lesperance et al. 1981), both of which are situ­ 1980). This led to the drainage of the broad conti­ ated at platform edges. The present paper docu­ nental shelves and was followed by a rapid rise in ments a comparable section from an intracratonic sea-leve! at the beginning of the Llandovery setting in the central Oslo Region. Within the last (Sheehan 1973, 1975). The resulting disconfor­ twenty years there have been many records of the mities seen at most Ordovician-Silurian junctions latest Ordovician Hirnantiafauna (see Rong 1979 are accompanied by a marked change in fauna and 1984a for reviews). This distinctive and wide­ from late Ordovician assemblages, which still re­ spread fauna probably inhabited intermediate tain a degree of provincialism compared to the shelf depths and comprised a variety of ecological more cosmopolitan faunas of the Silurian (Berry associations (Rong 1984b). It was decimated dur­ & Boucot 1973, Sheehan 1973). ing the late Hirnantian (Sheehan 1982). Thus The change in fauna is significant and in some whilst shelf faunas were clearly most susceptible cases such as that of the North American epicon­ to external stress, those inhabiting deeper water tinental platform there is little connection be­ environments may have been shielded from such tween the highly endemic late Ordovician fauna pressures by virtue of the evenness of their habi­ and that of the earliest Silurian (Berry & Boucot tat. We discuss herein a deep-water, lowest Llan­ 1973). Recognition and analysis of strata and fau­ dovery, fauna dominated by typically Ordovician nas equivalent to those apparently missing at the forms whose gradual extinction was probably the boundary in man y parts of the world would give a result of unsuccessful competition with immig­ more accurate picture of the transition of Ordov­ rant Silurian stocks during the period of the in­ ician to Silurian faunas as a whole. Evidence of itial Llandovery transgression. such a fauna! transition potentially is most com-

7 Geologisk Tidsskr. 2186 88 B. G. Baarli & D. A. T. Harper NORSK GEOLOGISK TIDSSKR!Ff 66 (1986)

Geological setting The marine Lower Palaeozoic succession of the Oslo Region was deposited in an intracratonic depression on a foreland area to the south-east of the developing Scandinavian Caledonide oro­ genic belt (Nicholson 1979, Bruton & Owen 1982, Worsley et al. 1983). The basin had a SSE­ NNW trend with sediments of the Asker area ori­ ginating at intermediate depths, whilst deposition was shallower to the west and deeper towards the east. The Asker sequences (Fig. l) consist of mixed siliciclastic and carbonate rocks and are lo­ cally well exposed and abundantly fossiliferous. Although the exact palaeogeographic position of the Oslo Region during the late Ordovician-early Silurian is not certain (e.g. Webby 1984), the Fig. l. Locality map for the Asker area of the Oslo-Asker dis­ presence of sediments of the bahamitic type, in­ trict of the Oslo Region; localities from whence collections cluding ooliths, in the highest Ordovician strata made are indicated by pointer. loset shows position of Asker of the region, indicates a tropical to subtropical within Norway. situation (Jaanusson 1973, Lindstrom 1984). Re­ constructions by Ziegler et al. (1977) and Scotese et al. (1979) place Balto-Scandia on the southern Immediately overlying this highest Ordovician side of the Ordovician equator. sandstone, in some areas, is a thin nodular lime­ stone with a sparse shelly fauna which is considered to mark a brief cessation in the trans­ gression prior to deposition of the typical off­ Summary of previous research shore shales of the Solvik Formation (Brenchley The Hirnantian sequences of the Oslo-Asker dis­ & Newall 1980). To the east of Asker, in the trict have been recently described and discussed deeper parts of the basin, the shales are dark and in detail by Brenchley & Newall (1975, 1977, include graptolites, but no shelly faunas are 1980, 1984). In contrast there have been few re­ known. In Asker, coeval strata are represented cent studies of the lowest Silurian rocks of this by fossiliferous calcareous mudstones interbed­ district (Baarli 1985, Thomsen & Baarli 1982, ded with highly calcareous seams also having a Baarli & Johnson 1982). The uppermost Ordov­ rich shelly fauna. The comprehensive and metic­ ician Hirnantian sediments appear regressive ulous work of Kiær (1908) remains one of the with the development towards the top of the most important sources of reference for Silurian stage of a complicated system of barrier islands, research in the region together with those of the tidal channels, oolite shoals and slightly deeper Ordovician (Kiær 1897, 1901, 1902). On the basis areas characterised by mud deposition. To the of his wide experience of both the Ordovician west of the Asker area lay a zone of shallow wa­ and Silurian systems in the Oslo Region, Kiær ter deposition possibly bordering land or a shal­ (1908, p. 478) considered there was no real low shoal, whilst to the east offshore facies were change in fauna at the Ordovician-Silurian developed (Brenchley & Newall 1980). A com­ boundary since the most profound fauna! plex of eight benthic levet bottom communities changes had already occurred during the latest characterise the varied upper Ordovician lithofa­ Ordovician. cies (Brenchley & Cocks 1982). The Ordovician sequence ends abruptly with evidence of a rapid transgression which left a reworked sandstone layer less than one metre thick over most areas. Age of the Solvik Formation Where carbonate sediments were deposited, In Asker there are no graptolites known from the there was no reworking, and the transition up­ base of the Solvik Formati on, but a rich conodont wards in to the deeper water lower Silurian shales assemblage referable to the lowest Llandovery is fairly continuous (Brenchley & Newall 1980). Icriodella discreta-lcriodella reflecta Zone of the NORSK GEOLOGISK TIDSSKRIFT 66 (1986) Relict Ordovician brachiopod faunas 89

British succession includes Ozarkodina oldha­ mensis, the species considered typical of the basal Lithology of the Solvik Formation Silurian of Anticosti Island (Barnes 1982). In The Solvik Formation is 245 m thick and consists Asker this fauna occurs 8 m above the base of the of 60-90% mudstones with thin to very thin lime­ Solvik Formation at Konglungen (Grid. Ref. stones, calcareous siltstones and siltstones. The NM849347) and thus indicates an earliest Silurian whole formation may be subdivided into three age for the formation (Aldridge & Mohamed units, in ascending order the Myren, Spirodden 1982, Worsley et al. 1983). Graptolites are found and Leangen members (Baarli 1985); only the near the base of the Solvik Formation in the adja­ two lower members are considered and together cent Oslo area where they supply important in­ are some 170 m thick, essentially representing a formation regarding the precise timegap between regressive sequence. The lower of the two units the deposition of the beds containing highest Or­ has a high mudstone content with frequent, very dovician Hirnantia faunas and the overlying thin siltstone intercalations and few calcareous Llandovery shales. Howe (1982) has reported intercalations, whilst in contrast the Spirodden graptolites of early acuminatus or possibly late Member has common, highly calcareous intercal­ persculptusZone age from Ormøya about 10 km ations with fewer siltstones and a rich coral and east of Asker indicating the lower Silurian to be stromatoporoid fauna. The lower, Myren Mem­ virtually complete. ber was probably deposited mainly below storm Many of the brachiopod species in the lower wavebase, whilst the overlying unit, the Spirod­ Solvik Formation are known from lower Silurian den Member, indicates shallower depths, prob­ rocks elsewhere. For example, Schizonema sub­ ably near normal wavebase. Close inspection of plicatum has been described from the Mulloch the lowest part of the formati on suggests that the Hill Sands tone of the Craighead inlier, Girvan late Ordovician transgression continued into the (Reed 1917) and the Haverford Mudstone For­ early part of the Llandovery. In all sections mation in Dyfed, South Wales (Bancroft 1949); studied the first few metres of the Solvik Forma­ 'Leptaena' reedi from the Woodland Formation tion are characterised by diverse faunas with a of the Girvan district (Cocks 1968) and from Mei­ high proportion of limestone intercalations, but fod, Wales (Temple 1970); Katastrophomena these diminish through the next 15 m, and at Va­ scotica from the Haverford Mudstone Formation kaas (NM828357), for example, colonies of fa­ (Cocks 1968) and Streptis altosinuata from the vositid corals together with green algae (Cyclo­ Woodland Formation and Meifod (Reed 1917, crinites) are found in the first 6 m of the section Temple 1970). but are absent in the subsequent 40 m, where In the upper part of the Solvik Formation pure limestone intercalations are also lacking. ( above 95m from the base) the presence of mem­ Although the limestone content and the nature bers of the evolving Stricklandia lens lineage and diversity of these lowest faunas vary later­ (Baarli 1986) provides information as to the age ally, the re is a correlation between areas where of these levels in the formation. The transition the late Ordovician carbonate shoals were devel­ from S. lens prima to S. lens lens occurs between oped and sections where carbonate con tent in the 122 to 130 m above the base of the formation, Solvik Formation is high; it is therefore probable thus enabling a correlation with the basal A3 that the late Ordovician topography described by beds of the type Llandovery (Williams 1951). At Brenchley & Newall (1980) exerted an influence 166 m above the base is the transition from S. on the fauna! and sedimentary patterns of the lens lens to S. lens intermedia which suggests a early Llandovery sea prior to the achievement of correlation with a horizon at or near the bound­ a more even topography as the transgression con­ ary between the A and B beds of the type se­ tinued. quence, whilst the change from the latter subspe­ cies to S. lens progressa occurs near the top of the unit and accordingly indicates a correlation with the highest B strata in the type Llandovery sec­ tions. Associated conodont and graptolite faunas confirm this correlation (Worlsey et al. 1983).

7' 90 B. G. Baarli & D. A. T. Harper NORSK GEOLOGISK TIDSSKRIFT 66 (1986)

The fauna! information was gained mainly �. �1�1 ��� 1� 1� ���� ���� 1� 1� ������������ 1n from 5-10 kg bulk samples collected from units

-·· - no thicker than 20 cm in the mudstones. Casual collections made in the field supplement the , __ • - data. The bulk samples are almost all from the FfØCIIel8 IP --

-·· - - first 20 m of the formation, but to demonstrate

-·· the transition from a typical Ordovician brachio­

...... - pod fauna to one more characteristic of the Sil­

-·· - - urian, a few samples were taken from higher

- -- -- strata in the Spirodden section. The bulk samples - 1-

...,...,..., IP - - were broken up and treated in dilute hydro­

_ .. - -- - - chloric acid and thereafter all identifiable sped­

·- • - mens were registered. In this study only the bra­ - -- chiopod biofacies is considered in detail. A tuller --·· � - - ..__ - account of the brachiopod fauna together with

-- monographic descriptions will be published else­ , __ -- where by the first author.

-- - The brachiopods comprise about 50% of the

·-- total fauna, although proportions may vary lo­ -· - • cally from 30 to 95%. Small rugose corals are lo­ .",_,.,.... - - - - - cally dominant, commonly comprising 2�30% of

,_ ------the fauna, whilst encrusting bryozoans and trilob­ ites are also locally important. Only in the upper --- - ....",..., - part of the Spirodden Member, for example, are - .. -- - - tabulate corals, stromatoporoids, and cyclocrinid .. -. -- - - - algae particularly common. However, whilst the -- H - -- diversity of the brachiopods is relatively low, that - ·- � of the total fauna is high. The main brachiopod /Jotweglc• - - ...... pile ... taxa and their mutual quantitative relationships Myren Mb J Søn M� as a percentage of the total brachiopod fauna re­ Langoyen e Fm SOL VIK FOR MATION trieved from the bulk samples are shown on Fig. 2. The samples are arranged in stratigraphical or­ Fig. 2. Range and relative abundance data for the most com­ mon, determined brachiopods from the lower part of the Sol vik der but not to scale. An additional 26 species are Formation of the Asker area. Abbreviations in first column re­ present mainly in the lowest 20 m but are repre­ fer to sample sites with metres above the base of the Solvik For­ sented by only a single specimen or several frag­ mation: BR- Brønnøya, VA- Vakaas, KO- Konglungen, SP­ ments. These are not included on the range chart Spirodden, VB- Vettrebukta. because of their long ranges or poor taxonomic determination due to inadequacy of material. In­ spection of the range chart (Fig. 2) indicates that Faunal analysis two genera are dominant throughout the com­ The lowest 20 m of the Solvik Formation have plete sequence. These genera Eoplectodonta and been studied in detail at four different localities Isorthis occur in six different associations within east of Asker Station (Fig. 1): Brønnøya (NM- the Solvik Formation and are represented by the 862362), Konglungen (NM849347), Vakaas (NM- species E. duplicata and /. prima in the Myren 828357) and Spirodden (NM842339). At the first and Spirodden members. The associated species, three, the Ordovician-Silurian junction is ex­ however, are quite different in each assemblage. posed, but at Spirodden the lowest 10 m of the A clear fauna! change occurs within the Myren Solvik Formation are missing. At Brønnøya only Member 17-40 m from the base, whilst only the the initial 3-4 m of the formation is exposed, highest assemblages are typically Silurian. Be­ whilst at Konglungen about 25 m is present. At sides E. duplicata and l. prima, Leangella scissa Spirodden 164 m of the formation is exposed in dominates in the remainder of the Myren Mem­ an essentially undisturbed section. The sequence ber, while Stricklandia lens and Protatrypa mal­ at Vakaas is about 130 m thick; however, it is moeyensis are co-dominant in the Spirodden highly tectonised. Member. Dolerorthis plicata, Clorinda undata, NORSK GEOLOGISK TIDSSKRIFT 66 (1986) Relict Ordovician brachiopod faunas 91

Skenidioides wood/andiensis and Dicoelosia os­ gill age; whilst Fascifera, Reuschel/a and Epito­ loensisoccur as minor but persistent elements in myonia are more typically identified with upper both members. Aegeria norvegica is dominant in Ordovician faunas but nevertheless have been re­ a number of samples but nevertheless is not a corded from lower Silurian strata (Boucot 1975, persistent element of the fauna as a whole. The Williams & Wright 1982). Kinne/la is known only brachiopod fauna in the Myren Member is simi­ from highest Ashgill strata (Rong 1979, 1984a,b) lar to that from the Mulloch Hill Sandstone but both Howellites and Pionodema range up­ (Lower Llandovery) of the Craighead inlier of wards from the Caradoc to the Ashgill ( Cocks the Girvan district, Scotland. The Scottish fauna 1978, Cooper 1956), but have hiterto not been re­ is dominated by Leangella scissa together with corded from the Silurian. The species Leangel/a Eoplectodonta sp., Dolerorthissp., Clorinda sp., scissaand Eospirigerina gaspeensisare known to and Skenidioides sp. (Cocks & Toghill 1973); Iso­ occur in both Ashgill and Llandovery rocks rthis prima is also common, there in its type stra­ (Boucot & Johnson 1967, Cocks 1978, Hiller tum. It is interesting to note that where S. lens 1980). and atrypoids like P. malmoeyensis are domi­ In summary the first 20 metres of the Solvik nant, L. scissa, D. osloensisand C. undata are re­ Formation contain a definite assemblage of duced in abundance; the negative relationship mixed Ordovician and Silurian elements. Many between these two groups is commonly observed of the genera represented are known to become in Llandovery faunas and has often been used to extinct dose to the Ordovician boundary. This characterise the Stricklandia and Clorinda com­ association persists until the peak of the trans­ munities of Ziegler (1965) and Ziegler et al. gression, when fauna! elements more typical of (1968). The lowest 40 m of the Solvik Formation the Silurian gradually arrive. Higher in the suc­ are dominated by L. scissa, E. duplicata and I. cession taxa typical of the Ordovician are no prima; however, the associated brachiopods are longer present, but here the assembalge is of low diverse and indude, as persistent elements, a diversity dominated by either new Silurian spe­ species of Dolerorthissimilar to D.sp.nov. Tem­ eies or long ranging forms. Both the lower and ple (1970), Skenidioides sp. (more finely ribbed higher faunas in the Solvik Formation show re­ than the stratigraphically younger S. woodland­ current species and a consistent organisation. iensis), Reuschel/a sp., Kinne/la sp., Howellites sp., and Eospirigerina gaspeensis(Fig. 2). The as­ semblages of the lowest Solvik Formation have nothing in common in terms of the associations as Discussion a whole or constituent individuals, with the un­ The fauna! extinctions at the end of the Ordov­ derlying shallow to mid-shelf communities of the ician Period have been recognised as one of five Hirnantian sequences in Asker (Brenchley & major extinction episodes (e.g. Newell 1967, Cocks 1982). The Solvik Formation, however, Raup & Sepkoski 1982, 1984, McLaren 1983). was dearly deposited in deeper water; compara­ Close scrutiny of the event reveals that the ex­ ble facies in the uppermost Ordovician succes­ tinctions occurred over a considerable period of sion are rare and not well known. Moreover the time, achieved by increments from the late Cara­ fauna! associations in the Solvik Formation are doc through the Ashgill (Brenchley & Newall not comparable, as a whole, to any described as­ 1984). Moreover the extinctions documented in sembalges from the highest Ordovician or the the various fossil groups were not synchronous Iowest Silurian. (Jaanusson 1979). Whilst the most marked disap­ Of the genera in the lowest Solvik Formation, pearance of trilobites, cystoids and graptolites Dolerorthis, Eoplectodonta, Idiospira, Leangella, occurred at the Rawtheyan - Hirnantian bound­ Leptaena, Streptisand Triplesia have become well ary, brachiopods and corals apparently suffered established by the Caradoc or early Ashgill and the most heavy losses during the early part of the continue relatively high into the Silurian. Cooli­ Hirnantian, but with a further reduction at the nia, Dicoelosia, Epitomyonia, Eospirigerina and Ordovician-Silurian junction (Brenchley 1984). Schizonema appear firstduring the Ashgill (Bou­ These fauna! extinctions have been related to a cot 1975, Sheehan 1975). In contrast, Isorthis, broadly concurrent late Ordovician glaciation Clorinda, Meifodia and Stricklandia are known which, at its dimax, created cooler surface waters only from rocks of Silurian age, although the first and a drainage of the continental shelves. The di­ and last have dose relatives in rocks of late Ash- rect cause or causes of the extinctions correlated 92 B. G. Baarli & D. A. T. Harper NORSK GEOLOGISK TIDSSKRIFT 66 (1986) with the glacial event has promoted much debate Ordovician fauna which survived the terminal (see e.g. Johnson 1984, Stanley 1984c). The lat­ Ordovician extinctions in the deeper parts of the est Ordovician extinctions have been related to a intracratonic basin of the Oslo Region. These major cooling event (Stanley 1984a,b), a restrict­ species were subsequently able to create and par­ ion of the area of the continental shelves by ticipate within new community structures during drainage (Sheehan 1973, 1979, Jaanusson 1979, the early Silurian transgression. As the trans­ Brenchley 1984) or to major tectonic events gression proceeded, immigration occurred of (Campsie et al. 1984). The present study, how­ more offshore elements originating around un­ ever, demonstrates that extinction of the brachio­ stable continental margins or achipelagos where pods did not cease at the boundary but continued both isolation and speciation may have taken for some time into the earliest Silurian. As a con­ place during the late Ordovician regression (see sequence a further cause for these later extinc­ e.g. Eldredge 1974, Fortey 1984, Neuman 1984 tions must be sought, though this is difficult since Bruton & Harper 1985). In due course the relict the majority of sections across the Ordovician - Ordovician elements were displaced gradually by Silurian boundary exhibit either a hiatus, a competition with the immigrant species whilst a marked change in facies from shelf deposits to different less diverse recurrent assemblage graptolitic shales, or merely unfossiliferous evolved consisting of a mixture of the more eu­ strata. Jaanusson (1979) has noted the lack of rytopic native species and successful immigrants. data on earliest Llandovery faunas, while Leggett We therefore conclude that a period of competi­ et al. (1981) noted that the lowest Llandovery tion is possibly a further aspect of the late Or­ fauna in Great Britain exhibits an extremely low dovician-early Silurian extinction event which re­ fauna! diversity compared with that of the upper quires consideration. Ashgill and moreover concluded that this trend was also worldwide. They attributed this to slow Acknowledgements- We thank Professor Markes Johnson and Dr. Alan W. Owen for comments on the maunscript and Drs. recolonisation of the shelf environment following David L. Bruton and David Worsley for encouragement. Baarli the latest Ordovician regression. This explana­ acknowledges financial support from the Norwegian Research tion does not seem valid in the central Oslo Re­ Council for Science and Humanities (NAVF), and Harper from gion where diverse associations are already pres­ the Natura! Environment Research Council (NERC, U.K.). ent during the course of the early Llandovery transgression and are only later replaced after the References peak of the transgression by a typical earl y Llan­ dovery assemblage, of low diversity and compris­ Aldridge, R. J. & Mohamed, L 1982: Conodont stratigraphy of the early Silurian of the Oslo Region. In: D. Worsley (Ed.), ing eurytopic species. Field Meeting, Oslo Region 1982. Pal. Contr. Univ. Oslo Jaanusson (1979) has suggested that furtherex­ 278, 109-120. tinctions during the early Llandovery may have Baarli, B. G. 1985: The stratigraphy and sedimentology of the been caused by the development of large euxinic early Llandovery Solvik Formation, central Oslo Region, Norway. Nor. Geo/. Tidsskr. 65, 229-249. basins where bottom conditions were inimical to Baarli, B. G. 1986: A biometric re-evaluation of the Silurian the colonisation of benthic organisms. In contrast brachiopod lineage Stricklandia lens/ Stricklandia laevis. Pal­ Leggett et al. (1981) argue that with transgres­ aeontology 29, 187-205, pl. 21. & sion, large shelf areas suitable for colonisation Baarli, B. G. Johnson, M. E. 1982: Lower Silurian biostrat­ igraphy of stricklandiid and pentamerid lineages in the Oslo are created, whilst those faunas of the deep eu­ Region. In: D. Worsley (Ed.) Field Meeting, Oslo Region xinic basins continue largely unaffected. 1982. Pal. Contr. Univ. of Oslo 278, 91-104. The sections of the Sol vik Formation examined Bancroft, B. B. 1949: Welsh Valentian brachiopods. Quarry J., have yielded important information regarding the Managers' 1-10. Barnes, C. R. & McCracken, A. D. 1981: Early Silurian chro­ hitherto poorly documented earliest Llandovery nostratigraphy and a proposed Ordovician-Silurian boundary shelly faunas and events which in the lowest part stratotype, Anticosti Island, Quebec. In: Lesperance, P. J. of the formation may be summarised as follows: (Ed.) Field Meeting, Anticosti-Gaspe, Quebec 1981, Vol. Il, the establishment of a relict Ordovician fauna Stratigraphy and Paleontology, 71-79. (l) Berry, W. B. N. & Boucot, A. J. 1973: Glacio-eustatic control persisting during the early Silurian transgression; of Late Ordovician-Early Silurian platfonn sedimentation (2) new eurytopic elements arrive as the trans­ and faunal changes. Geo/. Soc. Am. Bull. 84, 275-284. gression continues until finally, (3) a typical low­ Boucot, A. J. 1975: Evolution and Extinction Rate Controls. density lower Llandovery fauna is firmly estab­ Elsevier, Amsterdam. Boucot, A. J. and Johnson, J. G. 1%7: Silurian and Upper Or­ lished. We suggest that the initial fauna consists dovician atrypids of the genera Plectatrypa and Spirigerina. of the more eurytopic species of the regressive Nor. Geo/. Tidsskr. 47, 79-101. NORSK GEOLOGISK TIDSSKRIFT 66 (1986) Relict Ordovician brachiopod faunas 93

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Appendix PLATE l.

Orthacea and Enteletacea from the Solvik Formation, Asker Schizonema subplicatum (Reed) a,b. External mould and latex east of brachial valve, PMO 107590, x2. c. Internal mould of pedicle valve, PMO 107591, x2. d,e. Internal mould and latex east of brachial valve, PMO 107592, x2.

Dolerorthis sp. f. 1nternal mould of brachial valve, PMO 107593, x2. g,k. lnternal mould and latex east of pedicle valve, PMO 107594, x2. h,l. External mould and latex east of pedicle valve, PMO 107595, x2.

Kinnella sp. i,j. lnternal mould and latex east of brachial valve, PMO 109735, x2. p,q. Internal mould and la tex east of pedicle valve, PMO 107597, x2. [specimens assigned to Ptychopleurel/a sp. e.g. pl.l, figs 2,3 in Thomsen and Baarli 1982 are determined herein as Kinnel/a sp.]

Reuschella sp. m. Internal mould brachial valve, PMO 109732, x3.5. n. Internal mould of pedicle valve, PMO 111738, x2. o. Internal mould of brachial valve, PMO 111713, x3. NORSK GEOLOGISK TIDSSKRIFf 66 (1986) Relict Ordovician brachiopod faunas 95 96 B. G. Baarli & D. A. T. Harper NORSK GEOLOGISK TIDSSKRIFT 66 (1986)

PLATE 2.

Enteletacea and Plectambonitacea from the Solvik Formation, Eoplectodonta duplicata (J. de C. Sowerby) Asker g,h. lnternal mould and latex east of brachial valve, PMO Kinne/la sp. 107606, x2. a,b. External mould and latex east of brachial valve, PMO i,j. Ventral and posterior views of interna! mould of pedicle 107598, X2. valve, PMO 107607, X2.

Leange/la scissa (Davidson) Epilomyonia sp. c. lnternal mould of brachial valve, PMO 107601, x3. k. lnternal mould of pedicle valve, PMO 108287, x4. [figured as Dicoelosia cf. inghami Wright in Thomsen and /sorthis prima Walmsley & Boucot Baarli 1982, pl.l, fig. 16) d. lnternal mould of pedicle valve, PMO lll7ll, x4. f. lnternal mould of brachial valve, PMO 111709, x4. Dicoelosia osloensis Wright l. Latex east of external mould of pedicle valve, PMO Fascifera sp. 107605, x4. e. lnternal mould of brachial valve, PMO 109739, x4. NORSK GEOLOGISK TIDSSKRIFT 66 (1986) Relict Ordovician brachiopod faunas 97

PLATE3.

Skenidioides sp. a. Intemal mould of pedicle valve, PMO 107609, X4. b. Intemal mould of brachial valve, PMO 107610, X4. ldiospira sp. d,e. Ventral and dorsal views of conjoined pair, PMO 107611, x4.

'Leptaena' reediCocks f. Intemal mould of brachial valve, PMO 105211, x2.

Eospirigerina gaspeensis (C ooper) c,g,h. Dorsal, posterior and ventral views of conjoined pair, PMO 107613, x4.

Repository of specimens: Paleontologisk museum, Oslo (PMO).