Revision of Silurian vertebrate biozones and their correlation with the conodont succession

The first vertebrate-based subdivisions of Silurian strata were mainly drawn on material from outcrops in Britain and drill cores from the southern Baltic. Nearly twenty years ago the first vertebrate biozonal scheme was developed on the basis of vertebrate distribution in several continuous drill core sections in the northern Baltic. This paper presents a new scheme in which many new data on vertebrate distribution from the Baltica (Baltic region, NW Russia), Avalonia (southern Britain, eastern Canada), Laurentia (northern Canada, Greenland, Scotland) and Kara (Arctic Russia) palaeocontinents have been used. All the zones have been defined, and the geographical distribution and the reference stratum and locality for each zone have been given. The Llandovery part of the succession contains the Valyalepis crista, Loganellia aldridgei and L. scotica zones; the Wenlock part is represented by the Archipelepis bifurcata/Arch. turbinata, L. grossi, Overia adraini, L. einari and Paralogania martinssoni zones. The Par. martinssoni Zone continues in the Ludlow and is followed by the Phlebolepis ornata, Phl. elegans, Andreolepis hedei, Thelodus sculptilis and T. admirabilis zones. The last zone continues in the lower Přidoli and is followed by the Nostolepis gracilis, Poracanthodes punctatus and Trimerolepis timanica zones. The L. aldridgei and Arch. bifurcata zones are new, and the Arch. turbinata and O. adraini faunas have been raised from faunal succession units to zones. The geographically widely distributed L. grossi Zone in the upper Sheinwoodian and the Par. martinssoni Zone in the upper Homerianlowermost Gorstian allow the integration of regional successions into one Generalized Vertebrate Zonal Scheme. Possible correlations of conodont and vertebrate biozones, and gaps in sedimentation in the northern East Baltic Silurian sequence have been discussed, the most extensive hiatus being between the Paadla and Kuressaare stages.


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 (Karatajūtė-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 (Märss 1982a(Märss , 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 (Märss 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 (Märss et al. 1995(Märss et al. , 1996)).New data subsequently became available from North Greenland (Blom 1999a(Blom , 1999b(Blom , 2000)), Arctic Canada (Märss et al. 1998a(Märss et al. , 1998b(Märss et al. , 2006) ) and Russia (Karatajūtė- Talimaa & Märss 2002;Märss & Karatajūtė-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;Märss & Miller 2004) and Belarus (Plax & Märss 2011) have provided further taxonomical and biostratigraphical information about that group.
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 (Karatajūtė-Talimaa et al. 1987;Karatajūtė-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 (Märss et al. 1998b).That paper also presented the first carbon isotope data from the region and correlated δ 13 C 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 & Märss 1999;Turner 2000;Märss & 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.

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;.The upper part of the zonation, which corresponds to the upper part of the Lower Wenlock to Přidoli (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 Přidoli 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 Märss et al., 1996
Definition.The lower boundary of the zone is defined by the FAD of V. crista (Fig. 2A).
Distribution elsewhere.Not recognized outside the type region.
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).
(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 (Märss 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 (Märss & 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) References.Turner & Turner 1974;Fredholm 1990;Märss 1996;Blom 1999a;Talimaa 2000;Nestor et al. 2001;Märss & Karatajūtė-Talimaa 2002;Märss et al. 2006;Plax & Märss 2011. Remarks.
(1) Märss (1982a) defined the Logania taiti Zone in the Maasi and Tagavere beds of the Jaagarahu Stage, and Märss (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 140.0-144.5m 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 (Märss et al. 1998a).
Distribution elsewhere.Delorme Formation (Member 1D) of Avalanche Lake, Mackenzie Mountains, northern Canada.
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 Märss, 1990 Definition.The lower boundary of the zone is defined by the FAD of L. einari (Fig. 2A, B).
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.
In the Ostrovets sections 7ts, 73 and 195 in Belarus, L. grossi and L. einari have been found together.
(1) The L. einari Zone was initially established as a regional zone (Märss 1996).Because of the wide distribution of the species, the zone was later recognized elsewhere and included in the Standard Scheme (Märss et al. 1996).
(2) In the Sakla core section, the boundaries of the Jaagarahu and Rootsiküla regional stages, which are based on cyclostratigraphy (Einasto in Märss 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 Karatajūtė-Talimaa, 1978 Definition.The lower boundary of the zone in defined by the FAD of Par.martinssoni (Fig. 2A, B).
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).

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 (Märss 1990(Märss , 1996) ) and included in the Vertebrate Standard Scheme (Märss et al. 1996).( 2) Phlebolepis ornata does not reach the level of appearance of Phl.elegans.
The Phlebolepis elegans Zone Märss, 1982a Definition.The lower boundary of the zone is defined by the FAD of Phl.elegans (Fig. 2B).
(1) The species is present in the overlying T. admirabilis Zone.
(2) The stratotype for the T. sculptilis Zone, as given in Märss (1982b), is divided herein to represent stratotypes of two zones, T. sculptilis and T. admirabilis.

The Thelodus admirabilis Zone Märss, 1990
Definition.The lower boundary of the zone is defined by the FAD of T. admirabilis (Fig. 2B).
(1) The T. admirabilis Zone was originally established as a regional zone (Märss 1990;Märss & 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 Přidoli interval (Märss 1997) corresponds to the T. sculptilis, T. admirabilis and N. gracilis zones in shallow shelf deposits.

The Nostolepis gracilis Zone Märss, 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 Märss 1986, pl. 32, fig. 1) above (Fig. 2B).
Remarks.According to Valiukevičius (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 Märss (1986, pl. 32, fig.1), with very fine pores only on the crown.
(2) In the Welsh Borderland, the uppermost Přidoli 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 (Märss & 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 Přidoli to middle Lochkovian) (Turner 1984;Märss & 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 Männik (2007b).The zonation in the Ludlow interval (excluding the uppermost Ludfordian) corresponds to that in Cramer et al. (2011), and the uppermost Ludfordian-Přidoli 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 longranging simple cone conodonts (e.g.Panderodus and Walliserodus).Rich fauna, including several new lineages, appears in the uppermost Rhuddanian and is more charac-teristic of the Aeronian.The possibility of establishing a universal (global) zonation for the Rhuddanian-Aeronian strata has been discussed, e.g., in Nowlan (1995), Männik (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 Raikküla Stage in the Coronograptus cyphus GZ (Loydell et al. 2003(Loydell et al. , 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 Jõgeva Beds (Põltsamaa 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 (Männik 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 Männik 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 'Apsidognathusinterval' (Fig. 5A, B).In both regions, the uppermost Telychian and Sheinwoodian strata are evidently missing (Männik & Martma 2000;Männik et al. 2000;Männik 2002).

B
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 (Männik 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 Přidoli, 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 & Schönlaub 1989;Oz. steinhornensis s.l. Interval Zone in Cramer et al. 2011).In the uppermost part of the Přidoli, 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(Viira , 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 Přidoli (Uyeno 1990).

CORRELATIONS OF VERTEBRATE AND CONODONT ZONES The Valyalepis crista Zone
In the type region of the zone (Gaspé Peninsula, Québec, 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 (Märss et al. 1998a(Märss et al. , 1998b(Märss et al. , 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;Märss 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 (Märss 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 (Märss et al. 2002(Märss et al. , 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 Märss 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.

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 (Märss 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, Riksu-803 and Sakla core sections from Saaremaa (Märss 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 396.7-397.0m, 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 399.8-400.0m.In the Kaugatuma and Riksu-803 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 1982aViira , 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 (Märss 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 Gołdap drill core from eastern Poland, Phl.elegans occurs at 1184.6-1202.0m in the upper Mielnik Stage, Ludlow (pers.observation by T. M. in the collection of Dr K. Małkowski, Warsaw).This interval lies in the Polygnathoides siluricus CZ (Männik & Małkowski 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; Märss & 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 (Karatajūtė- Talimaa et al. 1987;Karatajūtė-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 Märss (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 Karatajūtė-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 (Valiukevičius 2005).The beginning of abundant occurrence 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 Märss 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 (Märss 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.
Two species of the genus Nethertonodus have been described by Märss & Miller (2004) and Märss (2006).One of them, Neth.prodigialis, is known from the uppermost Ludfordian and lowermost Přidoli 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 94.45-94.48m).Nethertonodus and Par.ludlowiensis, which are characteristic of the Ludlow and Přidoli 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 Přidoli age in the East Baltic (Märss 1986;Karatajūtė-Talimaa 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 Přidoli) and N. alta (uppermost Přidoli) zones, which are both characteristic of sediments formed in deep-shelf environments (Märss 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 Přidoli strata of Lithuania and Latvia (Valiukevičius 2003(Valiukevičius , 2004a(Valiukevičius , 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 (Valiukevičius 2005(Valiukevičius , 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 Valiukevičius, N. gracilis and Por.punctatus appear slightly earlier than in the northern East Baltic sections (Valiukevičius 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 (Märss et al. 1995(Märss et al. , 1996) ) has mainly been directed against the acanthodian zonation in its Přidoli 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 Přidoli 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 Přidoli, while G. alatus is abundant in the Ohesaare Stage, upper Přidoli, and in the Lochkovian, Lower Devonian.Trimerolepis tricava and G. alatus were also discovered in the Barlow Inlet Formation, Přidoli, on Cornwallis Island in the Canadian Arctic Archipelago (Märss et al. 1998a(Märss et al. , 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 Přidoli.
-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.

Fig. 2 .
Fig. 2. Ranges of selected vertebrate taxa:A, in the Avalonia and Laurentia palaeocontinents; B, in the Baltica palaeocontinent.The Baltic regional stratigraphy is modified fromNestor (1997).Vertical striping in the regional stratigraphic column indicates the main gaps, i.e., their known maximum extent in the outcrop area of the Silurian sections in Estonia.The horizontal line at the lower end of the vertical distribution of a taxon indicates the beginning of its range, i.e., the first appearance datum (FAD) and/or the zone boundary; dotted line -range of a taxon outside Estonia; solid line -known range in Estonia; horizontal lines with arrows in the transitional Sheinwoodian-Homerian interval show the shift of zone boundaries in an adjacent palaeocontinent.Rh., Rhuddanian; Aeron., Aeronian; Gorst., Gorstian.For abbreviations of northern East Baltic stratigraphical units and names of taxa see p. 182.
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.

Fig. 3 .
Fig. 3. Ranges of selected conodont taxa.Only the known total ranges of the taxa are indicated; u, upper subzone; l, lower subzone.For abbreviations of northern East Baltic stratigraphical units and names of taxa see p. 182.

Fig. 4 .Fig. 4 .
Fig. 4. Distributions of selected conodont and vertebrate species in some studied core sections in Latvia (A) and Estonia (B); vertical solid line -distribution of conodonts, vertical dotted linedistribution of agnathans and fishes.Boundaries of stratigraphic units (stages, formations, beds) of sections and distributions of recognized conodont zones are indicated.Arrows mean that a taxon is also known from strata below the interval shown in the figure.Solid horizontal line -reliable location and correlation of zonal boundaries; dotted line -possible location and correlation of zonal boundaries.Thick dented lines correspond to main (dated) gaps in the sections.Stratigraphic subdivisions of the sections are mainly based on lithostratigraphy (after Einasto in Märss 1986) and not on fossil data.Conodont distribution data are from Viira & Männik (1997) (conodont collection revised herein by P. M.).For abbreviations of northern East Baltic stratigraphical units and names of taxa see p. 182.

The Archipelepis bifurcata/Arch. turbinata Zone Soehn et al., 2001, modified herein
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
(Märss 1982bThe 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)TheHimmiste Quarry was indicated as a stratotype for the Phl.elegansZone(Märss 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 lower boundary of the zone is defined by the FAD of T. sculptilis (Fig.2B).
Uppermost part of the Tahula and Kudjape beds of the Kuressaare Stage, uppermost Ludfordian, upper Ludlow; interval 64.65-83.10m in the Ohesaare core section, Saaremaa.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 Äigu Beds, Kaugatuma Stage, lower Přidoli, Latvia and Estonia; upper Pagegiai Formation, upper Ludlow, Central Lithuania.

Poracanthodes punctatus Zone Märss, 1982a
Upper Äigu and Lõo beds (excluding the uppermost part) of the Kaugatuma Stage, Přidoli, Estonia and Latvia; most of the Kaugatuma Stage (excluding the lower part), Kaliningrad District; upper Minija and lower Jūra formations, Lithuania.)withN.gracilisappearing 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.Definition.Corresponds to the interval starting at the level where Por.punctatus obtains the definite characteristic features (sensuMärss 1986, pl.32, fig.1)below and the FAD of Trimerolepis timanica (Fig.2B) above.
Distribution in the Baltic.Uppermost Ohesaare Stage, Latvia (Tr.timanica) and Lithuania (Tr.lithuanica); in Estonia, the sedimentary rocks of this age are missing.