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Revision of Silurian vertebrate biozones and their correlation with the conodont succession/Siluri selgroogsete biotsoonide revisjon ja nende korrelatsioon konodontide jarjestusega.

INTRODUCTION

The first vertebrate-based subdivisions of Silurian strata were mainly drawn on material from outcrops in Britain (Turner 1973) and drill cores from Lithuania and Latvia (Karatajute-Talimaa 1978). Soon after, the first vertebrate biozonal scheme was compiled based on detailed studies of the vertebrate distribution in several continuous drill core sections from Estonia and Latvia (Marss 1982a, 1982b). New data obtained during later studies prompted some modifications of the original zonation, whereby the scheme became applicable to parts of the northern hemisphere (Marss 1989). Activities under IGCP Projects 328 and 406 provided additional information about Silurian vertebrates from many parts of the world, particularly from remote Arctic regions, which resulted in further revisions of the zonation and the proposal of the first standard scheme (Marss et al. 1995, 1996). New data subsequently became available from North Greenland (Blom 1999a, 1999b, 2000), Arctic Canada (Marss et al. 1998a, 1998b, 2006) and Russia (Karatajute-Talimaa & Marss 2002; Marss & Karatajute-Talimaa 2002). All the Silurian vertebrate-bearing localities in northwestern Canada (the Selwyn Basin) were revised (Soehn et al. 2000), biostratigraphically valuable thelodonts were described, a faunal succession scheme for the Avalanche Lake region (Mackenzie Mountains) was developed and correlations were made (Soehn et al. 2001). Additionally, studies of vertebrates from southern Britain (Turner 2000; Marss & Miller 2004) and Belarus (Plax & Marss 2011) have provided further taxonomical and biostratigraphical information about that group.

The first Silurian conodont zonation was published by Walliser (1964). Because a number of Walliser's zones were not recognizable outside Cellon (Austria), several alternative zonations have been proposed (e.g., Aldridge 1972; Helfrich 1975; Barrick & Klapper 1976). In addition, Aldridge & Schonlaub (1989) attempted to work out a global zonation. Over the last two decades, the Silurian conodont zonation has been revised considerably, many new units have been described and their applicability in regional and global correlations have been discussed (e.g., Jeppsson 1997; Corradini & Serpagli 1999; Viira 1999; Calner & Jeppsson 2003; Jeppsson et al. 2006; Mannik 2007a, 2007b). A simplified version of the zonation was published by Cramer et al. (2011) to encourage its wider use.

Despite the many studies on conodonts and vertebrates, only a few publications have analysed co-occurrences of vertebrates and conodonts. In the southern East Baltic, in Lithuania, studies of thelodonts and conodonts in samples from several drill core sections allowed direct comparison of their distribution and the dating of the vertebrate assemblages recognized in terms of conodont biostratigraphy (Karatajute-Talimaa et al. 1987; Karatajute-Talimaa & Brazauskas 1995). In several samples from the Severnaya Zemlya Archipelago, vertebrates were found together with conodonts (Matukhin et al. 1999). The distributions of vertebrates and conodonts in samples from Arctic Canada (Franklin Basin) were compared with data from Europe (Marss et al. 1998b). That paper also presented the first carbon isotope data from the region and correlated 513C variations with those in the Baltic area. The taxonomy and distribution of agnathans and fishes in southern Britain have been investigated, the regional vertebrate zonal scheme has been updated and characteristic vertebrate assemblages have been correlated with the conodont zonation (Miller & Marss 1999; Turner 2000; Marss & Miller 2004).

In the present study we have re-examined vertebrate distribution data from the Baltic region and have added new information from several other regions of the northern hemisphere (British Isles, Greenland, Canada, Russia, Belarus). Our aim is to give an updated Silurian vertebrate biozonation, to correlate the vertebrate and conodont biozones and to update datings and correlations of strata in the regions discussed.

The following abbreviations are used in the text and figures:

(a) zonation--CZ, Conodont Zone; CSZ, Conodont Superzone; FAD, First Appearance Datum; GVZ, Generalized Vertebrate Zonation; GZ, Graptolite Zone; VZ, Vertebrate Zone;

(b) northern East Baltic stratigraphical units--A, Aigu Beds; H, Himmiste Beds; JiP, Paramaja Member of the Jaani Formation; J2, Jaagarahu Stage; JG, Jaagarahu Formation; JM, Jamaja Formation; K, Kudjape Beds; K1, Rootsikula Stage; [K.sub,2], Paadla Stage; [K.sub,3]a, Kuressaare Stage; [K.sub,3]b, Kaugatuma Stage; [K.sub.4], Ohesaare Stage; Kn, Kuusnomme Beds; L, Loo Beds; M, Maasi Beds; Ms, Mustjala Beds; N, Ninase Beds; R, Rumba Formation; RKs, Riksu Formation; RK, Raikkula Formation; S, Sauvere Beds; Sn, Soeginina Beds; SR, Sorve Formation; T, Tahula Beds; Tg, Tagavere Beds; TM, Tamsalu Formation; TR, Torgu Formation; U, Uduvere Beds; V, Vilsandi Beds; Vents., Ventspils Formation; VL, Velise Formation; VR, Varbola Formation; Vs, Vesiku Beds; Vt, Viita Beds;

(c) names of taxa--A., Andreolepis; absid., absidata; Am., Amydrotaxis; amorph., amorphognathoides; Anc., Ancoradella; Arch., Archipelepis; Asp., Aspelundia; b., bohemica; c., confluens; Cor., Coryssognathus; Ct., Ctenognathodus; D., Distomodus; e., elegans; eopenn., eopennatus; G., Goniporus; I., Icriodella; K., Kockelella; L., Loganellia; murch., murchisoni; Neth., Nethertonodus; N., Nostolepis; o., ortus; O., Overia; Oul., Oulodus; Oz., Ozarkodina; Oz. r., Ozarkodina remscheidensis; Oz. s., Ozarkodina sagitta; p., pennatus; Pand., Panderodus; Par., Paralogania; Phl., Phlebolepis; Pol., Polygnathoides; Por., Poracanthodes; Pran., Pranognathus; Ps., Pseudooneotodus; Pt., Pterospathodus; Pt. a., Pterospathodus amorphognathoides; r., remscheidensis; rhen., rhenana; s., sagitta; sn., snajdri; staurog., staurognathoides; v., variabilis; wall., walliseri; T, Thelodus; Tr., Trimerolepis; Tu., Turinia; V, Valyalepis.

VERTEBRATE BIOZONATION

The vertebrate biozonation (hereafter zonation) described below is a summary of data from the Baltica, Avalonia, Kara and Laurentia palaeocontinents. The zonation for the lower part of the Silurian succession (Llandovery and lower part of the Lower Wenlock) is mainly based on data from Avalonia (eastern Canada and southern Britain; Fig. 1: 1, 2) and Laurentia (Scotland, northern Canada, Greenland; Fig. 1: 3-6). The upper part of the zonation, which corresponds to the upper part of the Lower Wenlock to Pridoli (incl.), is mainly drawn on information from Baltica (Fig. 1: 7-10) and Kara (Severnaya Zemlya Archipelago) (Fig. 1: 11). Some zones in these large and geographically remote areas overlap in the upper Sheinwoodian, Homerian and lowermost Gorstian, which enables us to use them for compilation of the Generalized Vertebrate Zonal Scheme. Generally, the lower boundary of each zone is defined by the first appearance of a nominal species, and its upper boundary is defined by the first appearance of the succeeding nominal species. Thus, these zones are partial range zones. The lower boundary of acanthodian zones in the Pridoli has been defined either at the level where the taxon becomes abundant or where it obtains certain specific features. The total range of the nominal taxon can extend above the upper limit of the zone or even cross one or more zones (Fig. 2A, B). Note that in this paper the lateral replacement of taxa/zones is marked by a slash in the zone names.

The Valyalepis crista Zone Marss et al., 1996

Definition. The lower boundary of the zone is defined by the FAD of V. crista (Fig. 2A).

Reference stratum and locality. Clemville Formation, Chaleurs Group, Rhuddanian, Lower Llandovery; Petit Port Daniel River section, west of Clemville, Gaspe Peninsula, Quebec, eastern Canada.

Characteristic fauna. Valyalepis crista co-occurs with Loganellia scotica-type and L. sibirica-type thelodonts.

Distribution elsewhere. Not recognized outside the type region.

Reference. Turner & Nowlan 1995.

Remark. The index species does not reach the lower boundary of the overlying zone.

The Loganellia aldridgei Zone (established herein)

Definition. The lower boundary of the zone is defined by the FAD of L. aldridgei (Fig. 2A).

Reference stratum and locality. Purple Shales Formation, Telychian, Llandovery; calcareous beds in the middle part of the sequence at the Devil's Dingle locality, Buildwas, Shropshire, Welsh Borderland, Britain.

Characteristic fauna. Loganellia aldridgei occurs sporadically together with another L. scotica-type loganelliid.

Distribution elsewhere. Venusbank and Minsterley formations, middle and upper Aeronian; Purple Shale Formation, lower Telychian, Shropshire, Welsh Borderland, Britain.

Reference. Turner 2000, tables 1, 2.

Remark. We exclude from L. aldridgei the specimens shown by Aldridge et al. (1996, figs 2 and 3) as L. ex gr. scotica in the Upper Telychian Kilbride Formation of Coolin Lough, western Ireland.

The Loganellia scotica Zone Ritchie, 1985

Definition. The lower boundary of the zone is defined by the FAD of L. scotica (Fig. 2A).

Reference stratum and locality. Patrick Burn Formation, Priesthill Group, upper Telychian, Upper Llandovery; Logan Water section, Lesmahagow, Scotland.

Characteristic fauna. In Scotland, the associated faunal list includes the protochordate Ainiktozoon loganense Scourfield and the agnathan Jamoytius kerwoodi White in addition to L. scotica.

Distribution elsewhere. Ree Burn Formation, Hagshaw, Scotland; Lafayette Bugt Formation, North Greenland; Wych Formation and upper part of the Purple Shale Formation, Worcester and Hereford, Welsh Borderland; Anse aux Cascons and Anse a Pierre Loiselle formations, Quebec, eastern Canada; Cape Phillips Formation, Cornwallis and Baillie-Hamilton islands, Arctic Canada.

References. Ritchie 1985; Turner & Nowlan 1995; Marss & Ritchie 1998; Marss et al. 1998a, 1998b, 2006; Blom 1999a; Turner 2000; Soehn et al. 2001.

Remarks. (1) Ritchie (1985, fig. 5) was the first to recognize the range of L. scotica in the Patrick Burn and Kip Burn formations in Lesmahagow Inlier, Scotland. (2) The total range of L. scotica in the type region of the zone in Lesmahagow includes the Patrick Burn, Kip Burn and Dunside formations (Priesthill Group, upper Telychian, Upper Llandovery). (3) The zonal taxon does not enter the overlying zone, as exemplified in the Baillie-Hamilton BH1 section. (4) Loganellia sp. (cf. L. scotica), which was identified from the Avalanche Lake section AV4 (Marss et al. 1998b), was renamed Loganellia sp. nov. 2 by Soehn et al. (2000) and L. cf. sulcata herein. Its relationships, especially with L. sulcata, still need to be clarified. (5) The L. sibirica Zone, which was a constituent of the compound L. scotica/L. sibirica Zone in the Middle-Upper Llandovery interval in the first standard scheme, is hereby excluded from the scheme for two reasons. First, the total range of L. sibirica (= L. sibirica Zone) in the Siberian Platform corresponds to the entire Llandovery (Zigaite & Blieck 2006) and therefore cannot be treated as coeval with the L. scotica Zone. Second, the interval is a single separate 'hanging' interval and does not have any vertebrate zones above or below it.

The Archipelepis bifurcata/Arch. turbinata Zone Soehn et al., 2001, modified herein

Definition. The lower boundary of the zone is defined by the FAD of Arch. turbinata or Arch. bifurcata (Fig. 2A).

Reference stratum and locality. Cape Phillips Formation, lower Sheinwoodian, lower Wenlock; sample at 90.0 m in the Baillie-Hamilton BH1 section, which corresponds to a level in the Monograptus riccartonensis-Monograptus belophorus GZ as determined by the carbon isotope curve pattern (Marss et al. 1998a).

Characteristic fauna. Archipelepis bifurcata cooccurs with thelodonts L. sulcata, Shielia parca Marss, Wilson & Thorsteinsson, S. gibba Marss, Wilson & Thorsteinsson, Phillipsilepis crassa Marss, Wilson & Thorsteinsson, putative chondrichthyan Frigorilepis caldwelli Marss, Wilson & Thorsteinsson and Lanarkia cf. horrida Traquair in the Baillie-Hamilton BH1 section; Arch. turbinata occurs with Lanarkia cf. horrida and a yet undescribed Loganellia (L. cf. sulcata herein) in the Avalanche Lake sections.

Distribution elsewhere. Cape Phillips Formation, Sheinwoodian, Cornwallis Island, and the north shore of Baumann Fiord on Ellesmere Island; Road River Formation, 1 R Member, Sheinwoodian, lower Wenlock, Avalanche Lake sections, Mackenzie Mountains, northern Canada.

References. Marss et al. 1998a, 1998b, 2006; Soehn et al. 2000, 2001.

Remarks. (1) In 1995, the IGCP 328 thelodont working group established the L. avonia Zone for the lower Sheinwoodian. A preliminary study concluded that the Loganellia that occur in the Wenlock strata in Estonia and the Welsh Borderland are the same species. However, later research revealed that the scales from these two regions possess several different features; as a result, two species, L. einari (Marss 1996), which occurs in the Tagavere Beds of the Jaagarahu Stage in Estonia (see below), and L. avonia (Turner 2000), which occurs in the Brinkmarsh Beds, lower Sheinwoodian, Lower Wenlock of Tortworth Inlier, Avon, were established. Because of the restricted distribution of L. avonia (found only in one locality), the zone was removed from the zonal scheme (Marss & Miller 2004). (2) In the original regional thelodont succession, three units were introduced for the Upper Llandovery and Wenlock in the Avalanche Lake sections: turbinata, adraini and martinssoni (Soehn et al. 2001). The Archipelepis turbinata fauna has been determined to be a zone and is combined with the Arch. bifurcata Zone herein. Both Arch. bifurcata and Arch. turbinata disappear much below the upper boundary of the zone.

The Loganellia grossi Zone Marss et al., 1995

Definition. The lower boundary of the zone is defined by the FAD of L. grossi (Fig. 2A, B).

Reference stratum and locality. Maasi Beds (except their lower part) of the Jaagarahu Stage, upper Sheinwoodian, Lower Wenlock; interval 68.4-79.0 m of the Sakla core, Saaremaa, Estonia.

Characteristic fauna. Loganellia grossi occurs together with the thelodont Thelodus sp. and sometimes with the anaspid Rhyncholepis parvula Kiaer.

Distribution in the Baltic. Units f and g, Slite Beds, Gotland Island, Sweden; Maasi Beds (except the lower part) of the Jaagarahu Stage, upper Sheinwoodian, Saaremaa, Estonia; middle part of the Wenlock, East Lithuania.

Distribution elsewhere. Sundvollen Formation, Lower Ringerike Group, Wenlock, Ringerike Area, Norway; lower part of the Kap Morton Formation, mid-Wenlock, Kap Lucie Marie, Washington Land, North Greenland; Cape Phillips Formation, upper Sheinwoodian of Baillie-Hamilton Island, Arctic Canada; Wenlock, TimanPechora Region and northern Urals; upper half of the Samojlovich Formation, Matusevich River, October Revolution Island, Severnaya Zemlya Archipelago.

References. Turner & Turner 1974; Fredholm 1990; Marss 1996; Blom 1999a; Talimaa 2000; Nestor et al. 2001; Marss & Karatajute-Talimaa 2002; Marss et al. 2006; Plax & Marss 2011.

Remarks. (1) Marss (1982a) defined the Logania taiti Zone in the Maasi and Tagavere beds of the Jaagarahu Stage, and Marss (1982b) indicated its stratotype. The taxon and two stratigraphically following species were restudied later, and the corresponding zones Loganellia grossi and L. einari were described. For that reason, a new stratotype has been chosen for both the L. grossi and L. einari zones. (2) On the Baillie-Hamilton Island BH1 section, L. grossi appears shortly after Overia adraini (L. grossi occurs between 144.5 and 163.5 m in only one sample), which might mean that these two zones are present in a similar stratigraphic time interval. (3) In several sections of the Baltic region, L. grossi enters into the L. einari Zone.

The Overia adraini Zone Soehn et al., 2001

Definition. The lower boundary of the zone is defined by the FAD of O. adraini (Fig. 2A, B).

Reference stratum and locality. Cape Phillips Formation, upper Sheinwoodian, Wenlock; interval 140.0-144.5 m in the Baillie-Hamilton 1 section, Arctic Canada, which corresponds to an interval in the Cyrtograptus rigidus-Cyrtograptus perneri GZ based on the carbon isotope curve pattern (Marss et al. 1998a).

Characteristic fauna. Frigorilepis caldwelli Marss, Wilson & Thorsteinsson, Lanarkia horrida Traquair.

Distribution elsewhere. Delorme Formation (Member 1D) of Avalanche Lake, Mackenzie Mountains, northern Canada.

References. Soehn et al. 2000, 2001; Marss et al. 2002, 2006.

Remark. This interval was treated as a unit in the thelodont fauna (Soehn et al. 2001) but is raised to the status of the zone herein.

The Loganellia einari Zone Marss, 1990

Definition. The lower boundary of the zone is defined by the FAD of L. einari (Fig. 2A, B).

Reference stratum and locality. Tagavere Beds of the Jaagarahu Formation (Jaagarahu Stage), Sheinwoodian, lower Wenlock; interval 58.20-62.7 m in the Sakla core, Saaremaa, Estonia.

Characteristic fauna. Thelodus sp. and L. grossi occur together with L. einari in the Riksu-803 and Vesiku-507 drill cores.

Distribution in the Baltic. The L. einari Zone is recognized in the Saaremaa (Estonia) sections and in the uppermost Slite beds (Samsungs 1 locality) of the Gotland Island (Sweden) section.

Distribution elsewhere. Sundvollen Formation, Lower Ringerike Group, Wenlock, Ringerike Area, Norway. In the Ostrovets sections 7ts, 73 and 195 in Belarus, L. grossi and L. einari have been found together.

References. Turner & Turner 1974; Marss 1996; Nestor et al. 2001; Plax & Marss 2011.

Remarks. (1) The L. einari Zone was initially established as a regional zone (Marss 1996). Because of the wide distribution of the species, the zone was later recognized elsewhere and included in the Standard Scheme (Marss et al. 1996). (2) In the Sakla core section, the boundaries of the Jaagarahu and Rootsikula regional stages, which are based on cyclostratigraphy (Einasto in Marss 1986), are in need of revision. In light of the faunal evidence found in many other cores, the boundary between the stages is situated above the last occurrence (sample 58.20 m) of the thelodont L. einari. The vertical distributions of L. einari and the next index species, Paralogania martinssoni, do not overlap in the Estonian sections or elsewhere.

The Paralogania martinssoni Zone Karatajute-Talimaa, 1978

Definition. The lower boundary of the zone in defined by the FAD of Par. martinssoni (Fig. 2A, B).

Reference stratum and locality. Viita Beds, Rootsikula Stage, upper Homerian, uppermost Wenlock; interval 143.67-153.20 m in the Ohesaare core section, Estonia.

Characteristic fauna. In this zone, Par. martinssoni occurs together with the thelodont Thelodus laevis (Pander), the osteostracans Tremataspis schmidti Rohon, Tremataspis milleri Patten, Saaremaaspis mickwitzi (Rohon), Oeselaspis pustulata Patten, Witaaspis schrenkii (Pander), Thyestes verrucosus Eichwald, and the anaspids Rhyncholepis parvula Kiaer, R. butriangula Blom, Marss & Miller, Birkenia robusta Blom, Marss & Miller, Pterygolepis nitida (Kiaer), Vesikulepis funiforma Blom, Marss & Miller and Rytidolepis quenstedtii Pander.

Distribution in the Baltic. Rootsikula and lower Paadla stages, respectively upper Homerian (uppermost Wenlock) and lower Gorstian (lowermost Ludlow), Estonia; Halla, Mulde and Klinteberg beds, and Hemse unit a, Homerian (upper Wenlock) and lower Gorstian (Ludlow), Gotland.

Distribution elsewhere. 1D Member of the Delorme Group in the Avalanche Lake sections, Mackenzie Mountains, Cape Phillips Formation, Homerian, on Baillie-Hamilton Island, the uppermost Wenlock or lowermost Ludlow, Prince of Wales Island, Northern Canada; Bed 10, upper Wenlock, Ringerike, Norway; upper Voron'ya Beds, upper Wenlock Mikhajlovsk Pond area, Central Urals, Russia; lower part of the Ust'-Spokojnaya Formation, Wenlock, Matusevich River section, October Revolution Island, Severnaya Zemlya, Russia; middle Elton Formation, lower Gorstian, lower Ludlow of Welsh Borderland, southern Britain.

References. Turner & Turner 1974; Karatajute-Talimaa 1978; Marss 1982a; Fredholm 1990; Talimaa 2000; Soehn et al. 2001; Marss & Miller 2004.

Remarks. The zone corresponds to the lower part of the Par. martinssoni total range within the Homerian (upper Wenlock) and lower Gorstian (lower Ludlow). The species reaches the lower half of the Ludfordian in northern Europe and northern North America.

The Phlebolepis ornata Zone Fredholm, 1988

Definition. The lower boundary of the zone is defined by the FAD of Phl. ornata (Fig. 2B).

Reference stratum and locality. Sauvere Beds (excluding the lowermost part) of the Paadla Stage, Gorstian, lower Ludlow, interval 111.80-112.25 m in the Ohesaare core section, Saaremaa, Estonia.

Characteristic fauna. Phlebolepis ornata occurs sporadically together with other thelodonts, such as Thelodus sp. and Par. martinssoni.

Distribution in the Baltic. The zone has been recognized only in the sections on Saaremaa and Gotland islands, in the middle and upper parts of the Sauvere Beds, Paadla Stage, and in units b and c of the Hemse Beds, respectively.

Distribution elsewhere. Not established outside the Baltic.

References. Marss 1986, 1990; Fredholm 1988.

Remarks. (1) A Phl. ornata fauna was described by Fredholm (1988) in the interval corresponding to units b and c of the Hemse Beds on Gotland. Later, a regional Phl. ornata Zone was established in the same interval (Marss 1990, 1996) and included in the Vertebrate Standard Scheme (Marss et al. 1996). (2) Phlebolepis ornata does not reach the level of appearance of Phl. elegans.

The Phlebolepis elegans Zone Marss, 1982a

Definition. The lower boundary of the zone is defined by the FAD of Phl. elegans (Fig. 2B).

Reference stratum and locality. Himmiste Beds (except their lower half) of the Paadla Stage, upper Gorstian-lower Ludfordian, Ludlow, interval 105.03-108.30 m in the Ohesaare core section, Saaremaa.

Characteristic fauna. Rich thelodont, osteostracan and anaspid fauna with Phl. elegans contains Thelodus laevis, T. carinatus (Pander), Par. martinssoni, Tremataspis mammillata Patten, Tremataspis milleri, Dartmuthia gemmifera Patten, Oeselaspis pustulata, Witaaspis schrenkii, Thyestes verrucosus, Procephalaspis oeselensis (Robertson), Vesikulepis funiforma Blom, Marss & Miller, Silmalepis erinacea Blom, Marss & Miller and Rytidolepis? quenstedtii Pander.

Distribution in the Baltic. Saaremaa, western mainland of Estonia (middle and upper Himmiste Beds and lower Uduvere Beds of the Paadla Stage), Gotland, Sweden (the uppermost part of unit c and the lower half of unit d of the Hemse limestones and the corresponding interval of the Hemse marls in NW Gotland).

Distribution elsewhere. Upper Bringewood and lower Leintwardine formations, Gorstian and Ludfordian of the Welsh Borderland, southern Britain; upper Mielnik Stage, Ludlow, eastern Poland; middle Gerd"yu Stage, upper Gorstian and lower Ludfordian, western slope of the North and Central Urals; Velikaya River Formation, Ludlow, Timan-Pechora Region; middle part of the Ust'-Spokojnaya Formation, Ludlow, Matusevich River, and lower part of the same formation in the Spokojnaya River section, October Revolution Island, Severnaya Zemlya.

References. Karatajute-Talimaa 1978; Marss 1986; Fredholm 1988; Marss 1990; Modzalevskaya & Marss 1991; Talimaa 2000; Turner 2000; Karatajute-Talimaa & Marss 2002; Marss & Miller 2004.

Remarks. (1) The Phl. elegans Zone embraces the middle part of the Paadla Stage, uppermost Gorstian-lower Ludfordian, Ludlow, but the species continues upwards into the next zone. (2) The Himmiste Quarry was indicated as a stratotype for the Phl. elegans Zone (Marss 1982b). However, during later studies, Phl. ornata was separated from Phl. elegans and, accordingly, two zones were recognized. The new stratotypes for both zones are indicated herein.

The Andreolepis hedei Zone Marss, 1982a

Definition. The lower boundary of the zone is defined by the FAD of A. hedei (Fig. 2B).

Reference stratum and locality. Mituva Formation, Paadla Stage, Ludfordian, Ludlow, interval 472.4-484.5 m in the Ventspils-D3 core section, western Latvia.

Characteristic fauna. Thelodonts Thelodus laevis, T. carinatus, Par. martinssoni, Par. kaarmisensis, Phl. elegans; heterostracan Archegonaspis sp.; osteostracan Osteostraci gen. et sp.; acanthodians Nostolepis striata and Gomphonchus sandelensis. The vertebrate fossils are strongly depauperate in the upper part of the zone.

Distribution in the Baltic. Middle and upper Uduvere Beds of the Paadla Stage, middle Ludfordian, Ludlow, Saaremaa, Estonia; upper half of units d and e of the Hemse limestones, Hemse marls and Burgsvik Sandstone (Uddvide section), Ludfordian, Ludlow, Gotland.

Distribution elsewhere. Upper (excluding the uppermost part) of the Ust'-Spokojnaya Formation, Ludlow, October Revolution, Pioneer and Komsomolets islands, Severnaya Zemlya; uppermost part of the West Khatanzej Formation, Ludlow, Khatanzej Peninsula, Novaya Zemlya; Velikaya River Formation, Gerd"yu Stage, Ludlow, Velikaya River, northern Timan; upper part of the Kuba Beds, lower Ludfordian, Ludlow, of the southern Mikhajlovsk Pond section, Mikhajlovsk Subzone, Central Urals.

References. Marss 1982a, 1982b, 2001; Valiukevicius et al. 1983; Fredholm 1988; Matukhin et al. 1999.

Remarks. (1) After the revision of A. hedei, a new species, A. petri, was described from the Tabuska Beds in the scattered Silurian bedrock blocks located on the right bank of the Ufa River, Serga Subzone in the Central Urals (Marss 2001). The beds are thought to be either late Ludlow (Modzalevskaya & Marss 1991) or early Pridoli (Shuiskij 1981; Shurygina et al. 1981) in age. (2) Andreolepis (identified as A. hedei? Gross by Marss 1986, pl. 34, figs 2, 5), found in the Long Quarry Beds (lower Pridoli) in the Chapel Horeb Main Quarry section, southern Wales, may belong to a new and yet undescribed species. (3) The ranges of A. hedei and T. sculptilis do not overlap in the Baltic.

The Thelodus sculptilis Zone Marss, 1982a

Definition. The lower boundary of the zone is defined by the FAD of T. sculptilis (Fig. 2B).

Reference stratum and locality. Lower part of the Tahula Beds of the Kuressaare Stage, upper Ludfordian, Ludlow; interval 83.10-95.17 m in the Ohesaare core section, Saaremaa.

Characteristic fauna. Other thelodonts, including

T. parvidens Agassiz, T. traquairi (Gross), Loganellia cuneata (Gross), Par. ludlowiensis (Gross), Par. perensae Marss, Nethertonodus laadjalaensis Marss, Trimerolepis tricava (Gross), Longodus acicularis Marss; heterostracan Archegonaspis sp.; osteostracan Zenaspis? sp.; anaspid Tahulalepis elongituberculata Blom, Marss & Miller; acanthodians Nostolepis striata Pander, Gomphonchus sandelensis (Pander), Poracanthodes porosus Brotzen.

Distribution in the Baltic. Lower part of the Tahula Beds of the Kuressaare Stage, upper Ludfordian in western Latvia and Estonia; upper Pagegiai Formation, upper Ludlow, Lithuania; Burgsvik Formation, Ludfordian, Gotland, Sweden.

Distribution elsewhere. Oved Sandstone Formation, Oved-Ramsasa Beds, Skane, southern Sweden; uppermost Gerd"yu Stage, upper Ludlow, Timan-Pechora Region, Tabuska Beds; upper Ludlow or lower Pridoli (see above about the Tabuska Beds), Central Urals; Greben' Stage, upper Ludlow, Novaya Zemlya Archipelago, Russia.

References. Marss 1982a, 1982b, 1986, 1990; Karatajute-Talimaa et al. 1987; Modzalevskaya & Marss 1991; Karatajute-Talimaa & Brazauskas 1995; Marss et al. 1996; Talimaa 2000.

Remarks. (1) The species is present in the overlying T. admirabilis Zone. (2) The stratotype for the T. sculptilis Zone, as given in Marss (1982b), is divided herein to represent stratotypes of two zones, T. sculptilis and T. admirabilis.

The Thelodus admirabilis Zone Marss, 1990

Definition. The lower boundary of the zone is defined by the FAD of T. admirabilis (Fig. 2B).

Reference stratum and locality. Uppermost part of the Tahula and Kudjape beds of the Kuressaare Stage, uppermost Ludfordian, upper Ludlow; interval 64.65-83.10 m in the Ohesaare core section, Saaremaa.

Characteristic fauna. As for the T. sculptilis Zone; however, the specimens are less numerous, and Nethertonodus laadjalaensis and Longodus acicularis are missing.

Distribution in the Baltic. Upper Tahula and Kudjape beds, Kuressaare Stage, upper Ludlow, and the lower Aigu Beds, Kaugatuma Stage, lower Pridoli, Latvia and Estonia; upper Pagegiai Formation, upper Ludlow, Central Lithuania.

Distribution elsewhere. Oved Sandstone Formation, Oved Ramsasa Group, lower Pridoli, Skane, southern Sweden.

References. Marss 1982a, 1982b, 1990; KaratajuteTalimaa & Brazauskas 1995; Vergoossen 1999; Talimaa 2000; Marss & Miller 2004.

Remarks. (1) The T. admirabilis Zone was originally established as a regional zone (Marss 1990; Marss & Miller 2004). Its geographically rather wide distribution and usefulness in correlations allow it to be included in the generalized scheme. (2) The Poracanthodes porosus Zone described in the deeper shelf deposits and used in a parallel zonation for the upper Ludlow-lower Pridoli interval (Marss 1997) corresponds to the T. sculptilis, T. admirabilis and N. gracilis zones in shallow shelf deposits.

The Nostolepis gracilis Zone Marss, 1982a

Definition. Corresponds to the interval between the level at which N. gracilis Gross begins to dominate the vertebrate fauna below and the domination of Poracanthodes punctatus with definite characteristic features (sensu Marss 1986, pl. 32, fig. 1) above (Fig. 2B).

Reference stratum and locality. Upper Aigu and Loo beds (excluding the uppermost part) of the Kaugatuma Stage, Pridoli, interval 55.6-94.8 m in the Sorve-514 drill core, Saaremaa.

Characteristic fauna. Abundant N. gracilis scales with rare scales of T. parvidens, T. traquairi, L. cuneata, Trimerolepis tricava and fragments of Tolypelepis undulata, G. sandelensis, Por. porosus and Lophosteus connexus.

Distribution in the Baltic. Upper Aigu and Loo beds (excluding the uppermost part) of the Kaugatuma Stage, Pridoli, Estonia and Latvia; most of the Kaugatuma Stage (excluding the lower part), Kaliningrad District; upper Minija and lower Jhra formations, Lithuania.

Distribution elsewhere. Middle Downton (Holdgate Group), Welsh Borderland, southern Britain.

References. Turner 1973; Marss 1982a, 1982b, 1997; Valiukevicius 2005, 2006.

Remarks. In his summarized biozonal chart, Valiukevicius (2006, fig. 1) gave a slightly different succession of Lithuanian acanthodians than was presented in Valiukevicius (2005, figs 7, 8) with N. gracilis appearing in the upper Ludfordian. In our opinion, the scales in the lower Kaugatuma Stage and older beds in the northern Baltic differ from the characteristic N. gracilis because they have fewer ridges on the crown and are smaller.

The Poracanthodes punctatus Zone Marss, 1982a

Definition. Corresponds to the interval starting at the level where Por. punctatus obtains the definite characteristic features (sensu Marss 1986, pl. 32, fig. 1) below and the FAD of Trimerolepis timanica (Fig. 2B) above.

Reference stratum and locality. Uppermost Kaugatuma Stage and most of the Ohesaare Stage, interval 276.2-325.2 m in the Ventspils-D3 drill core, Latvia.

Characteristic fauna. Very rich vertebrate fauna including many taxa characteristic of the Kuressaare Stage, such as T. parvidens, T. traquairi, L. cuneata, Tr. tricava, N. striata and G. sandelensis, as well as those of the Ohesaare (uppermost Silurian) and Tilze (Lower Devonian) stages, (Goniporus alatus (Gross), Tolypelepis undulata Pander, Oniscolepis dentata Pander, Gomphonchus hoppei Gross, Lophosteus superbus Pander, Tylodus deltoides Rohon).

Distribution in the Baltic. Uppermost Kaugatuma Stage and most of the Ohesaare Stage, Estonia, Latvia, Lithuania and the Kaliningrad District.

Distribution elsewhere. Barlow Inlet Formation, Pridoli, Read Bay southern section, Cornwallis Island, Arctic Canada.

References. Marss 1982a, 1982b, 1986, 1997; Marss et al. 1998a; Valiukevicius 2005, 2006.

Remarks. According to Valiukevicius (2006), Por. punctatus appears in the Minija Formation in the Kaugatuma Regional Stage in Lithuania. This level is slightly lower than the level of appearance in the northern East Baltic but is also lower than indicated in the generalized zonal scheme. We treat Por. punctatus in the same manner as in Marss (1986, pl. 32, fig. 1), with very fine pores only on the crown.

The Trimerolepis timanica Zone (= Katoporodus timanicus) Karatajute-Talimaa, 1978

Definition. The lower boundary of the zone is defined by the FAD of Tr. timanica (Fig. 2B).

Reference stratum and locality. Uppermost Ohesaare Stage, Pridoli; interval 269.5-276.2 m in the Ventspil-sD3 drill core, Latvia.

Characteristic fauna. Thelodonts L. cuneata, Goniporus alatus, heterostracans Tol. undulata, Oniscolepis dentata, acanthodians N. striata, N. alta Marss, Gomphonchus sandelensis, Por. punctatus.

Distribution in the Baltic. Uppermost Ohesaare Stage, Latvia (Tr. timanica) and Lithuania (Tr. lithuanica); in Estonia, the sedimentary rocks of this age are missing.

Distribution elsewhere. Uppermost Pridoli in the TimanPechora region, NW Russia (with both Tr. timanica and Tr. lithuanica present); Ledbury Formation, Welsh Borderland, southern Britain (Tr. cf. timanica, see remarks below).

References. Karatajute-Talimaa 1978; Marss 1982a, 1982b, 1986; Turner 1984; Karatajute-Talimaa & Brazauskas 1995; Talimaa 2000; Marss & Miller 2004.

Remarks. (1) Karatajute-Talimaa (1978) first identified the Katoporus lithuanicus/K. timanicus assemblage that occurs in the uppermost Downtonian (= Pridoli) strata in the European biogeographical province. Talimaa (2000, table 7) described the Par. kummerowi Zone below the combined Tr. Timanica-Tr. lithuanica Zone. (2) In the Welsh Borderland, the uppermost Pridoli is characterized by the occurrence of Par. tarranti, which is followed upwards by Par. kummerowi. The scales of Tr. timanica recognized in this region were later re-identified as Tr. cf. timanica (Marss & Miller 2004). In the Anglo-Welsh region, Tr. cf. timanica occurs in the interval from the Ledbury Formation below up to the St Maughan's Group? strata above (uppermost Pridoli to middle Lochkovian) (Turner 1984; Marss & Miller 2004).

The Turinia pagei Zone Gross, 1967

The lower boundary of this zone, as well as the lower Devonian, is marked by the FAD of the thelodont Turinia pagei (Turner 1973, 1984; Karatajute-Talimaa 1978; Marss 1986, 1997; Talimaa 2000; Marss & Miller 2004).

CONODONT ZONATION

The conodont zonation (with ranges of index species) used in this paper is shown in Fig. 3. The Llandovery-Wenlock part of the zonation is discussed in Mannik (2007b). The zonation in the Ludlow interval (excluding the uppermost Ludfordian) corresponds to that in Cramer et al. (2011), and the uppermost Ludfordian-Pridoli zonation comes from Viira (1999). Because all major revisions of the Silurian conodont zonation have been published previously, only brief comments on the distribution of some key taxa and/or faunas are provided below (Figs 4, 5).

In general, the Rhuddanian is characterized by long-ranging simple cone conodonts (e.g. Panderodus and Walliserodus). Rich fauna, including several new lineages, appears in the uppermost Rhuddanian and is more characteristic of the Aeronian. The possibility of establishing a universal (global) zonation for the Rhuddanian-Aeronian strata has been discussed, e.g., in Nowlan (1995), Mannik (2001), Cramer et al. (2011). Two lineages, Aspelundia? and Distomodus, appear to have the highest potential for such zonation. However, Distomodus is very rare in many regions and occurs sporadically in strata older than the Telychian. Instead, Aspelundia? appears to be more common in the Rhuddanian-Aeronian sections in the East Baltic region as well as in other regions (see below). In the northern Baltic, Aspelundia? appears in the lowermost Raikkula Stage in the Coronograptus cyphus GZ (Loydell et al. 2003, 2010). In the sections on Canadian Arctic islands, Aspelundia? appears in the Allen Bay Formation (Uyeno 1990) and in the Mackenzie Mountains in Member 1W of the Whittaker Formation (Over & Chatterton 1987). The level of the FAD of Pranognathus tenuis (Aldridge) is more problematic, but it most likely does not lie below the lower boundary of the Aeronian (Aldridge 1972). This conclusion agrees with the data from Estonia, where P. tenuis has been found in the Jogeva Beds (Poltsamaa and Heimtali core sections) and in the topmost Kolka Beds (Ikla core section) (Nestor et al. 2003).

A revised conodont zonation with six zones, which is based on the evolution of Pterospathodus, has been proposed for the Telychian (Mannik 2007a; Fig. 3). For ecological reasons, the Pterospathodus-based zonation cannot be applied in some regions (e.g., the Timannorthern Ural region and Severnaya Zemlya). Still, several general faunal characteristics (e.g., high frequences of specimens, a large number of taxa, occurrence of Apsidognathus together with other taxa) allow the Telychian strata to be recognized without major problems even in these regions. Apsidognathus proved to be particularly useful because it has a worldwide distribution, and its elements are morphologically very distinct. Because the Apsidognathus lineage appears in the lower Telychian (in the Pterospathodus eopennatus ssp. n. 1 Zone of Mannik 2007a) and becomes extinct at Datum 2 of the Ireviken Event (corresponding to the Llandovery-Wenlock boundary in its type section at Leasows; Aldridge et al. 1993; Jeppsson 1997), Apsidognathus can be used as the most universal indicator for the Telychian. On Severnaya Zemlya and in the Timan-northern Ural region, the strata corresponding to the interval from the Pt. eopennatus ssp. n. 1 Zone to Datum 2 of the Ireviken Event can be recognized by the occurrence of Apsidognathus and are identified in the conodont sequence as the 'Apsidognathus-interval' (Fig. 5A, B). In both regions, the uppermost Telychian and Sheinwoodian strata are evidently missing (Mannik & Martma 2000; Mannik et al. 2000; Mannik 2002).

A detailed conodont zonation has been worked out for the uppermost Llandovery and Wenlock, and its applicability to different regions has been discussed (Jeppsson 1997; Calner & Jeppsson 2003). Most of these zones can be recognized in the East Baltic (Mannik 2007b; Loydell et al. 2010). Seventeen zones were defined in the interval from the uppermost Telychian (Lower Pseudooneotodus bicornis Zone) to the topmost Homerian (Ctenognathodus murchisoni Zone). The lower six zones (Lower Ps. bicornis Zone to the Upper Kockelella ranuliformis Zone; Fig. 3) and their boundaries can easily be identified in the northern East Baltic. Ozarkodina sagitta rhenana is also quite common. Kockelella walliseri is less common, most likely due to the small size of samples from the core sections. The Oz. bohemica longa Zone corresponds to the upper Wenlock (middle Homerian) interval (Calner & Jeppsson 2003). Ozarkodina b. longa is widespread, and most reports of Oz. bohemica and Oz. b. bohemica are based on this subspecies.

As in the Late Wenlock, the distribution of conodont faunas in the northern East Baltic during the Ludlow was greatly affected by variable ecological conditions. Additionally, the correlation between the deep-water (northwestern Latvia; Fig. 4A) and shallow-water (Saaremaa; Fig. 4B) facies are complicated by a number of gaps in the outcrop area (e.g., Jeppsson et al. 1994; Kaljo et al. 1997). The Kockelella variabilis, Ancoradella ploeckensis and Polygnathoides siluricus zones in the Ludlow are only recognized in cores from western Latvia. Kockelella variabilis and A. ploeckensis have been found in the Ventspils-D3 and Pavilosta-51 core sections, while Pol. siluricus has only been found in the Pavilosta-51 core section (Fig. 4A). On Saaremaa, the intervals with the A. ploeckensis and Pol. siluricus zones most likely correspond to a gap between the Himmiste and Uduvere beds (Fig. 5A). In Arctic Canada, the Pol. siluricus Zone was recognized in the Douro Formation on Devon Island and in the Cape Phillips Formation of Ellesmere Island (Uyeno 1990; Fig. 5B).

The Oz. snajdri Zone corresponds to the upper Paadla Stage in Estonia (= middle Ludfordian; Fig. 3). Ozarkodina crispa has also been reported from several sections (Viira & Aldridge 1998; Viira 1999). However, restudy of collections demonstrated that (1) the specimens identified as Oz. crispa in Estonia are morphologically different from those described and illustrated by Walliser (1964, pl. 21: 7-13) from Cellon and (2) Oz. crispa-type specimens are rare in samples, have sporadic distribution and always occur together with specimens of Oz. snajdri. Hence, it is possible that, in reality, they represent some specific morphs in the Oz. snajdri lineage. Further detailed taxonomical studies of these specimens and direct comparison of them with type material from Cellon are needed to prove the occurrence/absence of Oz. crispa in Estonia. In this paper, the Oz. crispa-type specimens are considered as elements of Oz. snajdri s.l. As our Oz. crispa-type specimens differ morphologically from the type material, we think that the strata yielding this species are missing in Estonia and the Oz. crispa Zone sensu Walliser (1964) corresponds to a gap between the Paadla and Kuressaare stages.

Traditionally, only one zone has been identified in the Pridoli, based on the distribution of the Ozarkodina remscheidensis-Oz. eosteinhornensis Group. Different names have been used for this zone (e.g., Oz. remscheidensis Interval Zone in Nowlan 1995; Oz. r. eosteinhornensis Zone in Aldridge & Schonlaub 1989; Oz. Steinhornensis s.l. Interval Zone in Cramer et al. 2011). In the uppermost part of the Pridoli, the Oulodus elegans detorta Zone was introduced by Jeppsson (1988). The abundant Ozarkodina fauna in the northern East Baltic has allowed four zones in the Oz. remscheidensis Superzone to be recognized (from below): Oz. r. baccata-Oz. s. parasnajdri, Oz. r. eosteinhornensis, Oz. r. canadensis and Oz. r. remscheidensis (Viira 1999). The lowermost of these zones, the Oz. r. baccata-Oz. s. parasnajdri Zone, is latest Ludlow in age. Oulodus e. detorta occurs in a short interval in the uppermost (excluding the topmost portion) Oz. r. remscheidensis Zone (Viira 1999, 2000; Fig. 3). The Oz. remscheidensis Superzone has also been recognized in Arctic Canada (identified as the Oz. eosteinhornensis Zone in Uyeno 1990), where it lies in the Devon Island Formation on Devon Island and in the Cape Phillips Formation on Ellesmere and Baillie-Hamilton islands and corresponds to the uppermost Ludlow and Pridoli (Uyeno 1990).

CORRELATIONS OF VERTEBRATE AND CONODONT ZONES

The Valyalepis crista Zone

In the type region of the zone (Gaspe Peninsula, Quebec, eastern Canada), V. crista occurs in a number of localities in strata corresponding to the Distomodus kentukyensis CZ (the A3-A4 brachiopod zones; Turner & Nowlan 1995) (Fig. 5B).

The Loganellia aldridgei Zone

The L. aldridgei Zone is found in several localities in Wales and the Welsh Borderland in the middle and upper Aeronian Venusbank and Minsterley formations and in the lower part of the Telychian Purple Shales Formation (Turner 2000, table 1, pl. 1). The zone most likely correlates with an interval extending from the middle Aspelundia? conodont Superzone (CSZ) to the Pt. a. amorphognathoides CS (Fig. 5A, B).

The Loganellia scotica Zone

On Baillie-Hamilton Island (Arctic Canada), L. scotica appears together with another Loganellia, L. sulcata, in the strata that yield the conodonts Apsidognathus lobatus Bischoff, Ozarkodina paraconfluens Jeppsson and Pterospathodus cf. rhodesi Savage (Marss et al. 1998a, 1998b, 2006; in the first two papers, L. sulcata was treated among L. scotica) (Fig. 5B). This conodont assemblage is characteristic of the Pt. a. amorphognathoides and Lower Ps. bicornis CZs and indicates the late Telychian age of these strata. In the Kilbride Formation (Co. Galway, western Ireland), scales similar to those of L. sulcata, which were identified earlier as L. aldridgei (Turner 2000; Marss et al. 2007) but are reidentified here as L. aff. sulcata, co-occur with the conodonts Icriodella aff. I. deflecta Aldridge, Panderodus sp., Ozarkodina ex gr. excavata (Branson & Mehl), Ozarkodina sp. and Distomodus sp. (Aldridge et al. 1996). The fauna comes from a bed packed with shells of the brachiopod species Eocoelia curtisi curtisi Ziegler, suggesting a late Telychian age for this level. The conodont fauna does not identify the zone unequivocally, but the data indicate that this fauna also comes from strata not older than the Pt. celloni CSZ (Fig. 5A). In the Avalanche Lake section AV 4 from the Mackenzie Mountains in Canada, some thelodont scales similar to L. scotica and identified as L. cf. scotica (Marss et al. 1998b) were found in the upper(most) Pt. a. amorphognathoides CZ. These specimens were later considered to belong to a new, yet undescribed Loganellia sp. nov. 2 (Soehn et al. 2000). However, it is possible that these scales might belong to L. sulcata (herein as L. cf. sulcata; Fig. 5B), which is also found in Arctic Canada (Marss et al. 2002, 2006).

The Archipelepis bifurcata/Arch. turbinata Zone

In the Avalanche Lake sections from the Mackenzie Mountains, the lowermost Arch. turbinata was reported from strata corresponding to the upper half of the Ps. bicornis CZ (Soehn et al. 2001), which today is treated as the Upper Ps. bicornis CZ. This interval corresponds to the lowermost Sheinwoodian. The genus Archipelepis is also known from the sections on Canadian Arctic islands (Baillie-Hamilton, Cornwallis and Ellesmere) and is represented here by another species, Arch. bifurcata (Thelodonti gen. et sp. nov. 1 in Marss et al. 1998a), which occurs there in the Pt. pennatus procerus CSZ (Fig. 5B).

The Loganellia grossi Zone

In the Riksu-803 core section (Estonia), L. grossi appears in the uppermost part of the Kockelella walliseri range but occurs in the interval between the K. walliseri range and the lowermost K. ortus ortus. This suggests a correlation between the L. grossi Zone and the uppermost Upper K. walliseri and the lowermost K. o. ortus CSZs (Fig. 4B). On Gotland (Sweden), L. grossi is known from units f and g of the Slite Beds (Fredholm 1990, p. 63). Because the lower part of unit f corresponds to the Middle K. walliseri CZ (the lower part of the Upper K. walliseri CSZ; Jeppsson 1997), the lower boundary of the L. grossi Zone likely lies within this conodont zone.

Loganellia grossi occurs in samples 5-2-62 to 5-2-68 from section 2 of the Matusevich River succession (October Revolution Island, Severnaya Zemlya Archipelago, Russia) together with abundant specimens of the conodont Ozarkodina confluens bucerus Viira and less frequently with poorly preserved specimens identified as Kockelella cf. ortus (Walliser) (Mannik 2002; Marss & Karatajute-Talimaa 2002, Fig. 5B).

The Overia adraini Zone

In the Avalanche Lake region of the Mackenzie Mountains, O. adraini occurs in the interval 242-331 m of section AV 2 (Soehn et al. 2001). Overia adraini first appears in the lower part of Member 1D of Over & Chatterton (1987) in the interval of Kockelella patula Walliser (i.e., the K. patula CZ of Jeppsson 1997, Fig. 5B). Overia adraini was also found on Baillie-Hamilton Island in a thick complex in samples at 140.0 and 144.5 m in the BH1 section (Sheinwoodian) and upwards to 28.5 m in the BH2 section (Homerian), but diagnostic conodonts are absent. The carbon isotope curve pattern allows the correlation of its lower occurrence with the Cyrtograptus rigidus-Cyrtograptus perneri GZ level and the upper occurrence with the Pristiograptus parvus-Gothograptus nassa GZ level (Marss et al. 2006, text-figs 3 and 6).

The data from Gotland, in which the thelodont L. grossi appears in Slite beds f, the conodont K. patula occurs in Slite beds g, and O. adraini occurs together with K. patula in the Avalanche Lake section AV2, indicate that the lower boundaries of the L. grossi and O. adraini zones are rather close to each other.

The Loganellia einari Zone

In Estonia, L. einari is found in the Kipi, Paadla, Riksu803 and Sakla core sections from Saaremaa (Marss 1996; Nestor et al. 2001). Conodonts have been studied from the Riksu-803 and Sakla core sections (Nestor et al. 2001 and herein). In both sections, L. einari has a short range below the level of appearance of Oz. bohemica longa Jeppsson (Fig. 4B). In the Riksu-803 core section, L. einari appears above the K. walliseri range in the interval corresponding to the 'post-K. walliseri interregnum' according to Jeppsson (1997), and reaches the lower part of the K. o. ortus range; this suggests that the L. einari Zone most likely correlates with the K. o. ortus CSZ (Figs 4B, 5A). On Gotland, L. einari occurs in the Samsungs 1 locality in the uppermost Slite beds, which is correlative with the upper(?) part of the K. o. ortus CSZ.

The Paralogania martinssoni Zone

In the Kolka-54 core section (Latvia), Par. martinssoni appears in a sample at 396.7-397.0 m, which is 6.1 m above the level of appearance of Kockelella ortus absidata Barrick & Klapper at 402.8-403.1 m (i.e., the K. o. absidata CZ; Loydell et al. 2010; Fig. 4A). The lowermost Oz. b. longa in that section comes from a sample at 399.8-400.0 m. In the Kaugatuma and Riksu803 core sections (Estonia), Par. martinssoni appears above the level of appearance of Oz. confluens densidentata Viira in the Ctenognathodus murchisoni CZ. The oldest (based on conodont biostratigraphy; see also Viira 1982a, 1982b) known Par. martinssoni in the studied region comes from the Oz. b. longa CZ in the Ohesaare core section. In the Sakla core section, Par. martinssoni is present only in the middle part of the Paadla Stage. Based on these data, the level of appearance of Par. martinssoni and the lower boundary of the Par. martinssoni Zone lie in the Oz. b. longa CZ.

In the Avalanche Lake region, Par. martinssoni occurs in section AV 4 (Soehn et al. 2001) with Oz. bohemica (Over & Chatterton 1987) (Fig. 5B). On Baillie-Hamilton Island in the Canadian Arctic Archipelago, the level with Par. martinssoni lies within the Cape Phillips Formation in the interval of Erika range (Marss et al. 1998a). On Gotland, Erika occurs in the unzoned interval above the C. murchisoni CZ and in the lower part of the K. crassa CZ (upper Klinterbeg Formation and lower Hemse Group; Jeppsson et al. 2006).

The Phlebolepis ornata Zone

In Estonia, Phl. ornata appears in the lower part of the Paadla Stage, below the level of appearance of Oz. snajdri s.l. in the Ohesaare core section and above it in the Riksu-803 core section (Fig. 4B). Phlebolepis ornata and Oz. roopaensis occur together in these two sections. On Gotland in Sweden, Phl. ornata is present in units b and c of the Hemse Beds (Fredholm 1988). In terms of conodont biostratigraphy, these strata correspond to a transition interval between the K. crassa and K. variabilis CZs, which is characterized by Oz. excavata n. spp. S, post-Oz. excavata n. spp. S and Oz. e. hamata faunas (Jeppsson et al. 2006). These strata evidently correspond to a gap in Estonia, and only the upper parts of the Phl. ornata range and the Phl. ornata Zone are observed. Based on the data from Gotland, the lower boundary of the Phl. ornata Zone lies in the K. variabilis CZ.

The Phlebolepis elegans Zone

The lowest (in terms of conodont biostratigraphy) Phl. elegans in the East Baltic was discovered from the Ventspils-D3 core section (Fig. 4A). Here, Phl. elegans occurs in an interval below the level of appearance of Oz. snajdri s.l. Kockelella cf. variabilis has also been identified from the interval with Phl. elegans. In the Ohesaare and Kaugatuma core sections, Phl. elegans appears in the Oz. snajdri s.l. range. In the more proximal section (Riksu-803, Sakla), Phl. elegans occurs above the interval with Oz. snajdri s.l. (Fig. 4B). In the Goldap drill core from eastern Poland, Phl. elegans occurs at 1184.6-1202.0 m in the upper Mielnik Stage, Ludlow (pers. observation by T. M. in the collection of Dr K. Malkowski, Warsaw). This interval lies in the Polygnathoides siluricus CZ (Mannik & Malkowski 1998). These data indicate that the lower boundary of the Phl. elegans Zone lies in the K. variabilis CZ but certainly not higher than the basal Pol. siluricus CZ. In southern Britain, the lowermost Phl. elegans has been found together with Pol. siluricus from the Aymestrey Formation (uppermost Gorstian; Marss & Miller 2004). This level is unusual for Pol. siluricus, as this taxon is mainly found in strata of early Ludfordian age. However, rare specimens of Pol. siluricus are known to occur in strata older than the Pol. siluricus CZ and even older than the underlying Ancoradella ploeckensis CZ (Jeppsson & Aldridge 2000; Jeppsson 2005).

The Andreolepis hedei Zone

In Estonia, A. hedei has been found in the Sakla and Ohesaare core sections (Fig. 4B). It appears above the interval with Oz. snajdri s.l. in the Sakla core section and with Oz. snajdri s.l. in the Ohesaare core section. In the Ventspils-D3 core section (Latvia), A. hedei lies below the level of the FAD of Oz. snajdri s.l. (Fig. 4A). On Gotland, A. hedei is present in the upper part of unit d and in unit e of the Hemse Beds (Fredholm 1988). In the Gogs section (located near Lau, SE Gotland), A. hedei was recovered from strata corresponding to the Monograptus leintwardinensis GZ (Gross 1968), which correlates with the Anc. ploeckensis and lower Pol. siluricus CZs (Cramer et al. 2011). Andreolepis hedei is also known from the Uddvide section (Burgsvik Sandstone), which corresponds to the lower Oz. snajdri CZ (Jeppsson et al. 2006). Andreolepis hedei appears slightly below the Pol. siluricus CZ and disappears above this zone on Gotland.

The Thelodus sculptilis Zone

In several sections (Kaugatuma, Ohesaare, Kolka-54, Ventspils-D3) (Fig. 4A, B), T. sculptilis appears at almost the same level as Oz. remscheidensis baccata, whereas in the Pavilosta-51 core section its level of appearance lies about 15 m below the lowermost occurrence of Oz. remscheidensis (Fig. 4A). In the Ohesaare core section, the level of appearance of T. sculptilis lies below Oz. remscheidensis, but it appears to correlate with the base of the Oz. r. baccata-Oz. s. parasnajdri CZ in the Kaugatuma, Ventspils-D3 and Kolka-54 sections (Fig. 4A, B). In Lithuania, the range of T. sculptilis is within the upper Pagegiai Formation (Karatajute-Talimaa et al. 1987; Karatajute-Talimaa & Brazauskas 1995), the lower part of which (an interval below the appearance of Oz. remscheidensis) most likely corresponds to at least a part of the Oz. crispa CZ. The data from Lithuania suggest that T. sculptilis appears in the Oz. crispa CZ. Accordingly, the T. sculptilis Zone correlates with the Oz. crispa CZ and with the lower Oz. r. baccata-Oz. s. parasnajdri CZ. In Estonia, only the upper part of the T. sculptilis Zone is preserved. Its lower part, which corresponds to the Oz. crispa CZ, is missing due to a hiatus between the Paadla and Kuressaare stages, noticed earlier by Marss (1992).

The Thelodus admirabilis Zone

In the Ventspils-D3, Ohesaare and Kaugatuma core sections (Fig. 4A, B), T. admirabilis appears in the middle part of the Oz. r. baccata-Oz. s. parasnajdri CZ. It appears somewhat earlier, in the lower part of the zone, in the Kolka-54 core section (Fig. 4A). However, in all the studied sections, T. admirabilis begins within the Oz. r. baccata-Oz. s. parasnajdri CZ below the level of appearance of Oz. r. eosteinhornensis. According to Karatajute-Talimaa & Brazauskas (1995), T. sculptilis and T. admirabilis appear together in the upper Pagegiai Regional Stage in some sections in Lithuania.

The Nostolepis gracilis Zone

In most of the sections discussed in this paper, N. gracilis appears in the middle or upper part of the Oz. r. eosteinhornensis CZ and is abundant in the samples (Fig. 4A, B). In eastern Lithuania, it begins in the upper Pagegiai Regional Stage, Upper Ludfordian (Valiukevicius 2005). The beginning of abundant occurence in the sections is treated here as the lower boundary of the N. gracilis VZ.

The Poracanthodes punctatus Zone

In the studied sections, Por. punctatus (sensu Marss 1986) has been found in the Ventspils-D3 and Kolka-54 core sections. In both sections, the lowermost specimens occur slightly below the appearance of Oz. r. remscheidensis (Fig. 4A), which indicates that the lower boundary of the Por. punctatus Zone lies in the uppermost Oz. r. canadensis CZ (Fig. 5A).

The Trimerolepis timanica Zone

Trimerolepis timanica has been identified in the Ventspils-D3 and Kolka-54 core sections, where it occurs together with Oz. r. remscheidensis above the range of Oulodus elegans detorta (Fig. 4A). Accordingly, the Tr. timanica VZ corresponds to the topmost Oz. remscheidensis CSZ, to the interval above the Oul. e. detorta range (Fig. 5A).

REMARKS ON SOME TAXA POTENTIAL FOR CORRELATIONS

Loganellia aldridgei scales from the upper Telychian of western Ireland (Turner 2000) are morphologically similar to loganelliid scales that were identified as L. sulcata in the Llandovery-Wenlock boundary beds from Baillie-Hamilton and Cornwallis islands in Arctic Canada (Marss et al. 2006). The Ireland scales are identified here as L. aff. sulcata (Fig. 5A). The scales similar to L. sulcata from the Mackenzie Mountains are named herein as L. cf. sulcata. These scales need to be studied in detail and identified to determine the biostratigraphical value of this taxon.

Phlebolepis elegans co-occurs with Paralogania kaarmisensis Marss in Estonia (in the Himmiste Beds of the Paadla Stage; Fig. 5A, letter 'K') and the Welsh Borderland (Upper Bringewood, Gorstian, and Lower Leintwardine Group, Ludfordian) (Marss 2003; Marss & Miller 2004). In the Matusevich River section in the Severnaya Zemlya Archipelago, Par. kaarmisensis was found in sample MF 11-1 (Ust'-Spokojnaya Formation, Ludlow; Karatajute-Talimaa & Marss 2002, fig. 3G-I, L) from a level just above the interval with Phl. elegans (samples 2-10, 2-11 and 2-19) and within the same formation (Fig. 5B). It appears that within these three regions, Par. kaarmisensis with Phl. elegans is useful for correlations of the Gorstian/Ludfordian strata.

Two species of the genus Nethertonodus have been described by Marss & Miller (2004) and Marss (2006). One of them, Neth. prodigialis, is known from the uppermost Ludfordian and lowermost Pridoli in the Welsh Borderland. The other, Neth. laadjalaensis, occurs in the lower part of the Tahula Beds (Kuressaare Stage, upper Ludfordian) in the Thelodus sculptilis Zone in Estonia. In Britain, the interval with Nethertonodus is dominated by Par. ludlowiensis, which is extremely rare in Estonia (only one scale from the basal Tahula Beds is known from the Ohesaare core at depths of 94.45-94.48 m). Nethertonodus and Par. ludlowiensis, which are characteristic of the Ludlow and Pridoli boundary beds, provide additional criteria for correlation of this interval in the East Baltic and Britain.

Because acanthodians are most common in the strata of Pridoli age in the East Baltic (Marss 1986; KaratajuteTalimaa et al. 1987), vertebrate zonation for this interval was mainly based on these fossils. The Nostolepis gracilis and Poracanthodes punctatus zones were established based on acanthodians but the suitability of these zones was proved later. The scheme was complemented with Por. porosus (upper Ludlow-lower Pridoli) and N. alta (uppermost Pridoli) zones, which are both characteristic of sediments formed in deep-shelf environments (Marss 2000). Detailed studies of acanthodians with several new taxa described demonstrated that the group had high diversity in the Upper Silurian in the Ludlow and particularly in the Pridoli strata of Lithuania and Latvia (Valiukevicius 2003, 2004a, 2004b). Investigations of acanthodian biostratigraphy resulted in two different zonal schemes: one for the deeper shelf, mainly composed of argillaceous graptolitic facies, and another for the shallow shelf to lagoonal calcareous facies (Valiukevicius 2005, 2006). The N. gracilis Zone was recognized in both environments, whereas the N. alta Zone is only applicable to deeper shelf strata. According to the scheme of Valiukevicius, N. gracilis and Por. punctatus appear slightly earlier than in the northern East Baltic sections (Valiukevicius 2006). In our (T. M.) opinion, the morphology of scales of N. gracilis differs between the northern and southern Baltic.

Criticism of the Vertebrate Biozonal Standard (Marss et al. 1995, 1996) has mainly been directed against the acanthodian zonation in its Pridoli part. The main argument is that the zonation must be based on the evolutionary lineage(s) of only one group (genus or family). Pure thelodont zonation was preferred, for example, by Talimaa (2000, table 7). She proposed a subdivision of Pridoli age strata into two zones: the Trimerolepis tricava (= Katoporodus timanicus) and Goniporus alatus zones. However, in Estonia and Latvia, these two thelodont species occur at some levels only; Tr. tricava occurs in the lowermost Kuressaare Stage, Ludlow, and in the Ohesaare Stage, upper Pridoli, while G. alatus is abundant in the Ohesaare Stage, upper Pridoli, and in the Lochkovian, Lower Devonian. Trimerolepis tricava and G. alatus were also discovered in the Barlow Inlet Formation, Pridoli, on Cornwallis Island in the Canadian Arctic Archipelago (Marss et al. 1998a, 2006; Fig. 5B), but their very rare occurrence in samples complicates their use in the dating of strata. Therefore, for the time being, we retain our acanthodian zonation, although we realize that acanthodians require further investigation.

RESULTS

Over the last several decades, many new taxa of Silurian vertebrates and conodonts have been described and their distributions in time and space have been characterized. On the basis of the new data, the Generalized Vertebrate Zonation has been presented. The zonation is mainly based on data from the Avalonia (eastern Canada and southern Britain) and Laurentia (Scotland, northern Canada, Greenland) palaeocontinents, in the Llandovery and lower Sheinwoodian, Lower Wenlock, but on data from the Baltica (Baltic Sea region, Timan, Novaya Zemlya) and Kara (Severnaya Zemlya) palaeocontinents in the upper Sheinwoodian to the end of the Pridoli.

--Two previously informal units, the turbinata and adraini faunas in the thelodont faunal succession of the Mackenzie Mountains, have been raised to the rank of zones. The Arch. turbinata Zone has been combined with the Arch. bifurcata Zone.

--Our study resulted in the revision of several vertebrate zones. Some boundaries were re-defined (Andreolepis hedei, Thelodus sculptilis), index species were named for some new zones (Loganellia aldridgei), and reference stratum and locality were selected for new zones and zones in which the index species were divided (L. grossi and L. einari; Phlebolepis ornata and Phl. elegans; T. sculptilis and T. admirabilis).

--The Llandovery part of the zonation contains the Valyalepis crista, Loganellia aldridgei, L. scotica zones; the Wenlock part is represented by the Archipelepis bifurcata/Arch. turbinata, L. grossi, Overia adraini, L. einari and Par. martinssoni zones. The Par. martinssoni Zone continues in the Ludlow and is followed by the Phl. ornata, Phl. elegans, A. hedei, T. sculptilis and T. admirabilis zones. The last zone continues into the lower Pridoli and is followed by the N. gracilis, Por. punctatus and Tr. timanica zones.

--The L. grossi Zone in the upper Sheinwoodian and the Par. martinssoni Zone in the upper Homerianlowermost Gorstian are geographically widely distributed in the Baltica, Laurentia and Kara palaeocontinents and allow uniting regional zonations into one Generalized Vertebrate Zonation.

--Vertebrate zones were correlated with conodont zones, and the ages of the lower boundaries were specified (noticeably in the A. hedei and T. sculptilis VZs) based mainly on co-occurrences of taxa. In Estonia, the Oz. crispa CZ probably corresponds to a gap between the Paadla and Kuressaare stages.

--Acanthodians are the most common taxa in Pridoli age strata, and the zones based on this group are easily recognizable in the Baltic region. Some acanthodians, including Poracanthodes porosus and Por. punctatus, have wider distributions; they have been found in Arctic Canada as well. Poracanthodes porosus is found in Severnaya Zemlya. Although the taxonomy and distribution of some acanthodian taxa need to be studied in more detail, it is evident that this group has good potential in biostratigraphy.

doi: 10.3176/earth.2013.15

Acknowledgements. This work was supported through targeted financing by the Estonian Ministry of Education and Research (project SF0140020s08) and by the Estonian Science Foundation (grant No. 8907). G. van der Brugghen provided us with additional data on the distribution of L. scotica in Scotland and K. Soehn explained some lithostratigraphical details of the Avalanche Lake sections in the Mackenzie Mountains. We are grateful to D. Kaljo for reading an early version of the manuscript and for his advice, to V. Viira for providing the Silurian conodont collection from the northern Baltic for revision, and to our referees, O. Bogolepova, University of Cambridge, U.K., and O. Lehnert, University of Erlangen, Germany.

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Tiiu Marss and Peep Mannik

Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; Tiiu.Marss@ttu.ee, Peep.Mannik@ttu.ee

Received 18 July 2012, accepted 26 October 2012
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Author:Marss, Tiiu; Mannik, Peep
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Geographic Code:1CANA
Date:Dec 1, 2013
Words:12632
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