An updated Telychian (Late Llandovery, Silurian) conodont zonation based on Baltic faunas
Abstract
Five conodont zones, Pterospathodus eopennatus ssp. n. 1, P. eopennatus ssp. n. 2, P. amorphognathoides angulatus, P. a. lennarti and P. a. lithuanicus, are described in the interval previously known as the P. celloni Zone. The new zones are grouped into two superzones: the first two form the P. eopennatus Superzone and the other three the P. celloni Superzone. All zones correspond to the intervals of the total ranges of the nominal taxa and to the boundaries between the zones to the levels at which one taxon was evolutionally replaced by another. The lower boundary of the P. a. amorphognathoides Zone is redefined. The P. eopennatus ssp. n. 2, P. a. angulatus and P. a. amorphognathoides zones are further subdivided into the Lower and Upper subzones. Although the zones described are mainly based on data from Estonia, they can be recognized all over the world, in most sections containing Telychian strata and from where adequate data are available. Most of the subzones can so far be applied only in a limited area.
Keywords
The first Silurian conodont zonation was published by O.H. Walliser in 1964 in his monograph on Silurian conodonts from the Cellon section, Carnic Alps. He described two zones, the ‘Spathognathodus celloni’ Zone (later known as the Pterospathodus celloni Zone) and the P. amorphognathoides Zone, from the Telychian strata. In the following years, only minor changes connected mainly with local stratigraphy were made in that part of the zonation. Aldridge (1972) published a conodont fauna dominated by icriodellids in the lower Telychian from the Welsh Borderland, Great Britain. He described a new zonal unit, the Icriodella inconstans Assemblage Zone, and correlated it with Walliser's P. celloni Zone. Later, the P. celloni and P. amorphognathoides zones were recognized all over the world (for an overview of the distribution of Pterospathodus see Jeppsson 1987 and Männik 1998a). Some attempts were made at further subdivision of the P. celloni zone (e.g. Bischoff 1986; Brazauskas 1987). Due to different concepts of the taxonomy of Pterospathodus, the position of the boundary between the P. celloni and the P. amorphognathoides zones as defined by different authors varies greatly (compare, for example, Bischoff 1986 and Männik & Aldridge 1989). About a decade ago, Jeppsson (1997) proposed a detailed zonation for the uppermost Telychian to lower Homerian interval (including the upper part of the P. amorphognathoides Zone sensu Walliser).
The conodont zonation proposed here results from long‐term study of the evolution of Pterospathodus, but also of the Telychian conodont faunas in general. Several preliminary versions of it have been published earlier (e.g. Männik 1995, 1998b, 2001). Two superzones, P. eopennatus and P. celloni, are recognized in the interval correlated earlier with the P. celloni Zone (e.g. Männik & Aldridge 1989). The P. celloni Zone sensu stricto (as originally described by Walliser in 1964) corresponds only to the P. celloni Superzone. Each superzone is subdivided into zones: P. eopennatus ssp. n. 1 and P. eopennatus ssp. n. 2 zones were recognized in the P. eopennatus Superzone and P. a. angulatus, P. a. lennarti and P. a. lithuanicus zones in the P. celloni Superzone (Fig. 1). All zones correspond to the total ranges of the nominal taxa. The boundaries between the zones are defined as the levels at which one taxon in the P. eopennatus–P. amorphognathoides lineage sensu Männik (1998a) is evolutionally replaced by another. The boundaries between the superzones are characterized also by changes in some other conodont lineages (e.g. Apsidognathodus, Aulacognathus, Ozarkodina), providing more criteria for establishing these levels. Lower and Upper subzones are recognized in the P. eopennatus ssp. n. 2, P. a. angulatus and P. a. amorphognathoides zones (Fig. 1). Changes in conodont faunas seem to allow even more detailed subdivision of some intervals (e.g. Männik 2005).
The multi‐gradational zonation (superzone‐zone‐subzone) is necessary to apply the zonation to poorly studied sections. Depending on the information available (intervals between samples, sizes of samples, etc.), units of different ranks can be recognized. To identify zones, but particularly subzones, detailed sampling of sections and good collections of conodonts are needed. A less detailed zonation including superzones may be useful if only sparse data are available. Jeppsson (1997) suggested using even wider units, zonal groups. The distribution of all taxa is dependent on ecological conditions, thus the identification of biostratigraphic units is easier in some lithologies and difficult in others.
Material
The zonation is mainly based on the data from Estonia. The material studied comes from the strata corresponding to the Adavere Stage and to the lower part of the Jaani Stage (Fig. 2). The interval is dominated by more or less calcareous marlstones. Only the Rumba Formation consists of various limestones. Conodonts from more than 30 core and eight outcrop sections in Estonia were studied (Fig. 3) together with those from several core sections of Latvia (e.g. Aizpute‐41: Loydell et al. 2003) and Poland (Gołdap: Männik & Małkowski 1998). I had also a possibility to compare my collections from Estonia with those of Lithuania and north‐western Belarus (collections of A. Brazauskas), Ukraine (collections of D. Drygant), Sweden (collections of L. Jeppsson), Norway (collections of H.A. Nakrem), Austria (collections of O.H. Walliser and H.P. Schönlaub), Great Britain (collections of R.J. Aldridge), Turkey (collections of T.T. Uyeno), China (collections of R.J. Aldridge), Greenland (collections of H.A. Armstrong), southern and mid‐eastern USA (collections of J.E. Barrick and M.A. Kleffner), Canada (collections of T.T. Uyeno, G.S. Nowlan and A.D. McCracken), and Australia (collections of G.C.O. Bischoff). In addition, data from Central Urals, Timan‐northern Ural region, Siberia and Russian Arctic islands (Severnaya Zemlya, New Siberian Islands) were considered (collections of A. Bikbaev, N. Izokh, T. Mashkova, S. Mel’nikov, M. Snigireva, and my own).
Zonation
A short characterization of the zonal units recognized is presented below. The Viki core section is proposed as the reference section for all units described (Figs 3, 4). In this section the strata corresponding to the range interval of Pterospathodus have the greatest thickness known in Estonia (about 72 m) and contain rich conodont faunas. Only the data essential for the recognition of the units are given. All taxa of the genus Pterospathodus used to define biostratigraphical units below have been described earlier (Männik 1998a).
The Pterospathodus eopennatus Superzone
Definition. Corresponds to the interval of the total range of P. eopennatus Männik.
Lower boundary. Defined by the appearance of P. eopennatus.
Reference stratum and locality. The uppermost part of the Rumba Formation and the lower part of the Velise Formation of the Adavere Stage in Estonia; Viki core, 184.18–184.35 m to 169.85–170.20 m (here and below: the lower unit boundary is located between the former two depths, the upper unit boundary between the latter two) (Fig. 4).
Remarks. Two zones, P. eopennatus ssp. n. 1 and P. eopennatus ssp. n. 2, are recognized in the P. eopennatus Superzone (Fig. 1). Morphologically highly variable P. eopennatus (Männik 1998a) is the most characteristic taxon in this superzone. Other taxa of this superzone will be listed under descriptions of the P. eopennatus ssp. n. 1 and P. eopennatus ssp. n. 2 zones. The lower boundary of the P. eopennatus Superzone is exposed in Valgu ditch (point 2 in fig. 56 in Kaljo & Nestor 1990; Fig. 3). The P. eopennatus Superzone corresponds to the Aulacognathus latus Zone of Brazauskas (1987; Fig. 5).
The Pterospathodus eopennatus ssp. n. 1 Zone
Definition. Corresponds to the total range of P. eopennatus ssp. n. 1 Männik (Fig. 1).
Lower boundary. Coincides with the lower boundary of the P. eopennatus Superzone.
Reference stratum and locality. The uppermost part of the Rumba Formation and the lowermost part of the Velise Formation, Adavere Stage; Viki core, interval 184.18–184.35 m to 180.70–181.05 m (Fig. 4).
Characteristic fauna. In this zone the P. eopennatus–P. amorphognathoides lineage (Männik 1998a) is represented by its oldest form: P. eopennatus ssp. n. 1 (Fig. 6J–K, Q, T–U). At the upper boundary of the zone P. eopennatus ssp. n. 1 is evolutionally replaced by P. eopennatus ssp. n. 2 Männik (Fig. 6F–H, L). Astropentagnathus irregularis Mostler (Fig. 6: N) and Apsidognathus tuberculatus ssp. n. 1 (Fig. 6R, V) have been identified only in this zone. Apsidognathus milleri (Over & Chatterton) (Fig. 6M), Oulodus? sp. n. A Over & Chatterton (Fig. 6O–P, S) and Aulacognathus kuehni Mostler (Fig. 6B) appear in this zone but reach the strata just above its upper boundary (two former taxa) or higher (Aul. kuehni). Additionally, several long‐ranging taxa appear or reappear in this zone (Fig. 4).
Distribution and thickness in Estonia. The P. eopennatus ssp. n. 1 Zone has been recognized in most of the studied core sections. Its thickness varies from less than 0.5 m in the Ruhnu (500) core to at least 5.3 m in the Mustjala core (Fig. 3).
Other regions. The P. eopennatus ssp. n. 1 Zone is recognized in Latvia (Loydell et al. 2003), Lithuania, and in north‐western Belarus. The fauna of this zone, including Astr. irregularis, has been identified from loose pebbles collected on the beach north of Visby (Gotland, Sweden). The P. eopennatus ssp. n. 1 Zone can also be determined in the Carnic Alps (Seewarte section, samples 194/3 and 194/4 of Schönlaub 1971) and Kitzbühler Alps (Mostler 1967), Great Britain (Wenlock Edge area, Shropshire: Aldridge 1972, 1985), Australia (in New South Wales – Burly Jacky Sandstone Member, Burly Jacky and Trevena's Creek A‐A′ sections, samples 34a−39 of Bischoff 1986), Canada (Northwest Territories, lower part of the Whittaker Formation in the AV 1 section: Over & Chatterton 1987; Anticosti Island, in the lowermost Chicotte Formation: Uyeno & Barnes 1983) and northern Greenland (Armstrong 1990) (Fig. 7). Astropentagnathus irregularis was identified by T. Mashkova also from Kotel’nyj Island, New Siberian Islands (Sobolevskaya 1976).
Correlation with graptolite zonation. Bischoff (1986) correlated the strata corresponding to the P. eopennatus ssp. n. 1 Zone with the Streptograptus crispus Zone. Data from the Ohesaare and Aizpute‐41 cores indicate that the P. eopennatus ssp. n. 1 Zone correlates with the upper Spirograptus turriculatus (starting from the Torquigraptus proteus Subzone) and St. crispus graptolite zones (Loydell et al. 1998; Loydell et al. 2003; Fig. 5).
Remarks. The Astr. irregularis–P. pennatus Assemblage Zone of Bischoff (1986) and the lower part of the Aul. latus Zone of Brazauskas (1987) correspond to the P. eopennatus ssp. n. 1 Zone (Fig. 5).
The Pterospathodus eopennatus ssp. n. 2 Zone
Definition. Corresponds to the total range of P. eopennatus ssp. n. 2 (Fig. 1).
Lower boundary. Marked by the appearance of P. eopennatus ssp. n. 2, Astr. irregularis and Aps. tuberculatus ssp. n. 1 disappear close to this boundary (just below it?).
Reference stratum and locality. Lower part of the Velise Formation, Adavere Stage; Viki core, interval 180.70–181.05 m to 169.85–170.20 m (Fig. 4).
Characteristic fauna. In general, the P. eopennatus ssp. n. 2 Zone is characterized by the same association of conodonts as the underlying P. eopennatus ssp. n. 1 Zone. The main differences are the lack of Astr. irregularis and Aps. tuberculatus ssp. n. 1, and the appearance of Aps. tuberculatus ssp. n. 2 (Fig. 6A, C) and Pelekysgnathus? sp. n. (Fig. 6D–E) in the upper part of the zone.
Distribution and thickness in Estonia. The P. eopennatus ssp. n. 2 Zone has been identified in most of the studied sections. The thickness of this zone varies from 0.2–0.3 m in south‐western Saaremaa (Kaugatuma core) to at least 12 m in the Nurme core in mainland Estonia (Fig. 3).
Other regions. This will be discussed below, under the characterizations of subzones.
Correlation with graptolite zonation. The P. eopennatus ssp. n. 2 Zone correlates with the Streptograptus sartorius, Monoclimacis griestoniensis and lower M. crenulata graptolite zones (Loydell et al. 2003; Fig. 5).
Remarks. Two subzones, the Lower P. eopennatus ssp. n. 2 and the Upper P. eopennatus ssp. n. 2 Subzone, were recognized in this zone.
The Lower Pterospathodus eopennatus ssp. n. 2 Subzone
Definition. The Lower P. eopennatus ssp. n. 2 Subzone corresponds to the interval between the appearance of P. eopennatus ssp. n. 2 below and the extinction of Aul. kuehni above (Fig. 1).
Lower boundary. Coincides with the lower boundary of the P. eopennatus ssp. n. 2 Zone.
Reference stratum and locality. Lower part of the Velise Formation, Adavere Stage; Viki core, interval 180.70–181.05 m to 176.25–177.20 m (Fig. 4).
Characteristic fauna. The Lower P. eopennatus ssp. n. 2 Subzone is characterized by lower diversity and frequency of conodont specimens than the immediately over‐ and underlying strata. The fauna is dominated by P. eopennatus ssp. n. 2. Aulacognathus kuehni, as a rule, is continuously present in the subzone. Apsidognathus milleri and Oulodus? sp. n. A range, respectively, up to the lower and middle parts of the lower P. eopennatus ssp. n. 2 Subzone. In the uppermost part of the subzone, the Aps. tuberculatus lineage reappears.
Distribution and thickness in Estonia. The thickness of the subzone varies from about 0.2 m in the Ohesaare core to 5 m in the Nurme core (Fig. 3).
Other regions. The Lower P. eopennatus ssp. n. 2 Subzone can be recognized in several regions outside Estonia, e.g. Latvia (Loydell et al. 2003, fig. 13, samples C 97‐101 to C 97‐103), Lithuania, the Kitzbühler Alps in Austria (Mostler 1967), Australia (New South Wales, the Liscombe Pools Limestone, section A‐B in the Licking Hole Creek area, samples AB 4–6 and section X, sample X1: Bischoff 1986) and Welsh Borderland (sections Gullet 1, Gullet 2 and Hollybush: Aldridge 1972; Fig. 6).
Correlation with graptolite zonation. The Lower P. eopennatus ssp. n. 2 Subzone correlates with the St. sartorius and probably with the lowermost M. griestoniensis graptolite zones (Loydell et al. 1998, 2003).
Remarks. Although the taxonomic composition of conodont faunas of this subzone is quite similar to that of the underlying P. eopennatus ssp. n. 1 Zone, the frequency, e.g. the number of specimens per kilogram of rock, is considerably lower in this subzone. The interval between the disappearance of Astr. irregularis below and Aul. kuehni above corresponds to the Valgu Event (Männik 2005). The Lower P. eopennatus ssp. n. 2 Subzone correlates with the middle Aul. latus Zone of Brazauskas (1987; Fig. 5).
The Upper Pterospathodus eopennatus ssp. n. 2 Subzone
Definition. The Upper P. eopennatus ssp. n. 2 Subzone corresponds to the interval from the disappearance of Aul. kuehni below up to the appearance of P. amorphognathoides angulatus (Walliser) above (Fig. 1).
Lower boundary. Defined by the disappearance of Aul. kuehni. At this level, or just below it, the Aps. tuberculatus lineage, represented now by Aps. tuberculatus ssp. n. 2, reappears in most of the studied sections.
Reference stratum and locality. Lower part of the Velise Formation, Adavere Stage; Viki core, interval 176.25–177.20 m to 169.85–170.20 m (Fig. 4).
Characteristic fauna. The Upper P. eopennatus ssp. n. 2 Subzone includes the same taxa as the Lower P. eopennatus ssp. n. 2 Subzone, with minor changes only. Aulacognathus kuehni disappears at the lower boundary of the subzone, and Pelekysgnathus? sp. n. appears in its upper part (Figs 1, 4). Also, the oldest specimens of Ozarkodina ex gr. gulletensis (Aldridge) in Estonia have been found from this subzone.
Distribution and thickness in Estonia. The thickness of the Upper P. eopennatus ssp. n. 2 Subzone changes from about 0.2 m in the Ohesaare core to about 6 m in the Nurme core (Fig. 3). The subzone was identified in all studied core sections.
Other regions. The Upper P. eopennatus ssp. n. 2 Subzone has been recognized in Lithuania, in the Welsh Borderland (the Gullet 4 section: Aldridge 1972) and, probably, also on Anticosti Island (in the lower part of the Chicotte Formation, samples 267 and 268A: Uyeno & Barnes 1983; Fig. 7).
Correlation with graptolite zonation. The data from the Ohesaare and Aizpute‐41 cores show that the subzone possibly corresponds to the M. griestoniensis to the lowermost M. crenulata graptolite zones (Loydell et al. 1998, 2003; Fig. 5).
Remarks. The Upper P. eopennatus ssp. n. 2 Subzone corresponds to the upper Aul. latus Zone of Brazauskas (1987; Fig. 5).
The Pterospathodus celloni Superzone
Definition. Corresponds to the interval between the first appearance of P. a. angulatus below and P. a. amorphognathoides Walliser above, and evidently correlates with the interval of total range of P. celloni (Walliser) (Fig. 1).
Reference stratum and locality. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 169.85–170.20 m to 147.50–147.60 m (Fig. 4).
Lower boundary. Defined by the appearance of P. a. angulatus (Fig. 8E–F, L–M). In the distal facies this level, and the lower boundary of the superzone, coincides with the appearance of P. celloni (Fig. 8C–D) (Loydell et al. 2003). At (or very close to) this boundary also Aps. tuberculatus ssp. n. 2 is evolutionally replaced by Aps. tuberculatus ssp. n. 3 (Fig. 1; Fig. 8Q, S, U).
Characteristic fauna. The P. celloni Superzone is characterized by the sequence of P. a. angulatus, P. a. lennarti Männik and P. a. lithuanicus Brazauskas, representing the lower part of the P. amorphognathoides lineage (Männik 1998a; Figs 1, 4). Pterospathodus celloni itself is very rare in Estonian collections coming mainly from the carbonate‐terrigenous facies.
Distribution and thickness in Estonia. The thickness of this superzone changes from about 3 m in the Kaugatuma core to more than 36 m in the Are core (Fig. 3). It has been recognized in all studied cores.
Remarks. Pterospathodus celloni has been found in some sections of the studied region, but only in the middle and upper parts of the superzone (Männik 1998a; Fig. 4). Due to rare occurrence of that species the lower boundary of the zone is defined as the level of appearance of P. a. angulatus. As it is evident from the data from the Aizpute‐41 core (Loydell et al. 2003), this level coincides with the appearance of P. celloni.
Three zones, P. a. angulatus, P. a. lennarti and P. a. lithuanicus, were identified in the P. celloni Superzone.
The Pterospathodus amorphognathoides angulatus Zone
Definition. Corresponds to the interval of the total range of P. a. angulatus.
Lower boundary. Corresponds to the lower boundary of the P. celloni Superzone.
Reference stratum and locality. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 169.85–170.20 m to 153.97–154.15 m (Fig. 4).
Characteristic fauna. Pterospathodus a. angulatus dominates in the inner shelf carbonate‐terrigeneous facies, but is sporadic in distal graptolite‐bearing facies, where Dapsilodus is common and P. celloni appears and occurs together with P. a. angulatus (i.e. Aizpute‐41 core: Loydell et al. 2003). In the proximal facies rare specimens of P. celloni have been found only in the upper part of the zone (Fig. 4). Based on the changes of fauna two parts described below as the Lower and Upper P. a. angulatus subzones were distinguished in the P. a. angulatus Zone.
Distribution and thickness in Estonia. The P. a. angulatus Zone can be recognized in all studied sections. Its thickness varies from about 0.5 m in the Kaugatuma core to 5.7 m in the Viki core (Figs 3, 4).
Other regions. Will be discussed below under the characterizations of subzones.
Correlation with graptolite zonation. The zone correlates with the upper M. crenulata and the lower Oktavites spiralis graptolite zones (Loydell et al. 2003; Fig. 5).
Remarks. The P. a. angulatus Zone corresponds to the lower and middle parts of the Llandoverygnathus pennatus Zone of Brazauskas (1987; Fig. 5).
The Lower Pterospathodus amorphognathoides angulatus Subzone
Definition. Corresponds to the interval between the appearance of P. a. angulatus below and Ozarkodina sp. n. (Fig. 8H, K) above (Fig. 1).
Lower boundary. Coincides with the lower boundary of the P. a. angulatus Zone.
Reference stratum and section. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 169.85–170.20 m to 163.30–163.60 m (Fig. 4).
Characteristic fauna. The Lower P. a. angulatus Subzone includes Aps. tuberculatus ssp. n. 3, Aul. bullatus (Nicoll & Rexroad) (Fig. 8T), and Oz. ex gr. gulletensis (Fig. 8J, N). In Estonia, the first two species occur only in this subzone, disappearing close to its upper boundary. From this subzone also the oldest specimens of Kockelella ranuliformis (Walliser) in Estonia have been found.
Distribution and thickness in Estonia. The thickness of the subzone varies from about 2 m in the Kirikuküla core to 9 m in the Nurme core (Fig. 3).
Other regions. So far, this subzone has been identified only in Lithuania. Also, most probably, the upper part of the Oz. gulletensis range recognized elsewhere (e.g. in Great Britain) corresponds to this subzone.
Correlation with graptolite zonation. No direct correlation is known, but as the P. a. angulatus Zone corresponds to the M. crenulata and lower O. spiralis zones (Loydell et al. 2003), the Lower P. a. angulatus Subzone in general is a most probable equivalent of the middle M. crenulata Zone (Fig. 5).
Remarks. The Lower P. a. angulatus Subzone corresponds to the lowermost part of the L. pennatus Zone of Brazauskas (1987; Fig. 5).
The Upper Pterospathodus amorphognathoides angulatus Subzone
Definition. Corresponds to the interval between the appearance of Ozarkodina sp. n. below and P. a. lennarti Männik (Fig. 8: G) above (Fig. 1).
Lower boundary. Corresponds to the first appearance of Ozarkodina sp. n. In Estonia, Aps. tuberculatus ssp. n. 3 and Aul. bullatus disappear close to this level.
Reference stratum and section. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 163.30–163.60 m to 153.97–154.15 m (Fig. 4).
Characteristic fauna. The Upper P. a. angulatus Subzone is dominated by P. a. angulatus. Ozarkodina sp. n. has been found only from this subzone.
Distribution and thickness in Estonia. The thickness of the subzone varies from about 1 m in the Kirikuküla core to (at least) 9 m in the Viki core (Figs 3, 4).
Other regions. The Upper P. a. angulatus Subzone has been recognized in Lithuania.
Correlation with graptolite zonation. Probably correlates with the uppermost M. crenulata and lower O. spiralis zones (Fig. 5).
Remarks. The Upper P. a. angulatus Subzone corresponds to the middle L. pennatus Zone of Brazauskas (1987; Fig. 5).
The Pterospathodus amorphognathoides lennarti Zone
Definition. Corresponds to the interval of total range of P. a. lennarti (Fig. 1).
Lower boundary. Coincides with the appearance of P. a. lennarti.
Reference stratum and locality. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 153.97–154.15 m to 152.28–152.50 m (Fig. 4).
Characteristic fauna. The distribution of P. a. lennarti but, as a rule, also of Aulacognathus sp. n. (Fig. 8: R), is limited to this zone. However, in several sections Aulacognathus sp. n. reaches the lowermost part of the overlying P. a. lithuanicus Zone.
Distribution and thickness in Estonia. The P. a. lennarti Zone is distinct in the carbonate‐terrigeneous facies, where its thickness varies from 0.4 m in the Kaugatuma core to about 9.2 m in the Are core (Fig. 3). In the offshore graptolite‐bearing sediments recognition of this zone is problematic due to the extremely rare finds of P. a. lennarti (e.g. Loydell et al. 2003).
Other regions. Pterospathodus a. lennarti Zone has been identified in eastern Lithuania, Great Britain (uppermost Purple Shale in Ticklerton, Shropshire: Aldridge 1985), Carnic Alps and probably also in Podolia (Studenitza section, sample 15b/3: D. Drygant, unpublished data) (Fig. 7). From Carnic Alps, a nice well‐preserved Pa element of P. a. lennarti is present in the collection of H. P. Schönlaub from the Seewarte section (sample 195/1‐2) and one broken lateral process, probably also belonging to this species, in the collection of O.H. Walliser from Cellon (sample 10 H/J). In both of these collections P. a. lennarti occurs together with P. p. pennatus (Walliser) and P. celloni.
Correlation with graptolite zonation. The P. a. lennarti Zone evidently corresponds to a part of the O. spiralis Zone (Loydell et al. 2003).
Remarks. The P. a. lennarti Zone forms the middle part of the P. celloni Superzone, and corresponds to the upper L. pennatus Zone of Brazauskas (1987; Fig. 5).
The Pterospathodus amorphognathoides lithuanicus Zone
Definition. Corresponds to the interval of total range of P. a. lithuanicus (Fig. 1).
Lower boundary. Defined by the appearance of P. a. lithuanicus (Fig. 8A–B). Ozarkodina p. estonica Männik (Fig. 6I) disappears almost at the same level, and Aulacognathus sp. n. just above it.
Reference stratum and locality. Middle part of the Velise Formation, Adavere Stage; Viki core, interval 152.28–152.50 m to 147.50–147.60 m (Fig. 4).
Characteristic fauna. In general, the number of conodont specimens and taxa is lower in the P. a. lithuanicus Zone than in the strata below and above this interval. In the carbonate‐terrigeneous facies the fauna is dominated by P. a. lithuanicus. Aspelundia f. fluegeli (Fig. 8I, O–P) is one of the most common taxa in this zone. Ozarkodina polinclinata, one of the most common taxa in the faunas below and above this interval, is missing in the main part of the zone and reappeares, together with Aps. ruginosus Mabillard & Aldridge, just below its upper boundary. Also, the most frequent occurrences of P. celloni in Estonia (in the proximal environments) are connected with the P. a. lithuanicus Zone.
Distribution and thickness in Estonia. The P. a. lithuanicus Zone can be recognized in most of the studied core sections except for the distal graptolite‐bearing facies where the species is very rare (e.g. Loydell et al. 2003). The thickness of the zone varies from about 1 m in the Paatsalu core to more than 11 m in the Are core (Fig. 3).
Other regions. The P. a. lithuanicus Zone is well established in eastern and central Lithuania, and can also be identified in Norway (Nakrem 1986; Fig. 7).
Correlation with the graptolite zonation. Most probably, the P. a. lithuanicus Zone correlates with the upper O. spiralis graptolite zone (Loydell et al. 2003; Fig. 5).
Remarks. The P. a. lithuanicus Zone corresponds to the P. angulatus Zone (in proximal facies) or to the P. celloni Zone (in distal facies) of Brazauskas (1987; Fig. 5). Earlier, this interval was dealt by Brazauskas (1983) as the lower subzone of the P. a. amorphognathoides Zone.
The Pterospathodus amorphognathoides amorphognathoides Zonal Group
Remarks. The P. a. amorphognathoides Zonal group was introduced by Jeppsson (1997) for the interval of the total range of P. a. amorphognathoides, and includes three conodont zones: P. a. amorphognathoides, Lower Pseudooneotodus bicornis and Upper Ps. bicornis.
The Pterospathodus amorphognathoides amorphognathoides Zone
Definition. Corresponds to the interval between the appearance of P. a. amorphognathoides below and Datum 1 of the Ireviken Event above (Fig. 1).
Lower boundary. Defined by the appearance of P. a. amorphognathoides. Apsidognathus walmsleyi Aldridge (Fig. 9B–C) and Ps. bicornis Drygant appear at the same level, where, or just below it, also the first Oz. p. polinclinata (Nicoll & Rexroad) (Fig. 9I) and Aps. ruginosus(Fig. 9G–H, J) are identified.
Reference stratum and locality. Upper part of the Velise Formation, Adavere Stage, and the lower part of the Jaani Formation, Jaani Stage; Viki core, interval 147.50–146.60 m to 113.90–113.95 m (Fig. 4).
Characteristic fauna. In the proximal environments, the fauna of this zone is dominated by P. a. amorphognathoides, Oz. p. polinclinata and Pand. unicostatus (Branson & Mehl). Typical species are also Aps. ruginosus, Aps. walmsleyi, and in the upper part of the zone almost continuous Nudibelodina sensitiva Jeppsson (Fig. 4). In the distal graptolite‐bearing facies P. a. amorphognathoides and Pand. unicostatus become rare and are replaced by P. p. procerus (Walliser), Pand. langkawiensis Igo & Koike and Pand. gracilis (Branson & Mehl). Aspelundia f. fluegeli, common in the P. a. lithuanicus Zone, disappears in the basal part of the P. a. amorphognathoides Zone. However, before the extinction of Aspelundia in the middle part of the zone, it reappears for a short time (Fig. 4). Now, the genus is represented by another subspecies, Asp. fluegeli ssp. n. (Fig. 9K–M), characterized by specimens with very thin and tall denticles (Fig. 10). The occurrence of this taxon allows subdivision of the P. a. amoprphognathoides Zone into the Lower and Upper subzones.
Distribution and thickness in Estonia. The zone was recognized in all studied core sections. Its thickness varies from about 1 m in the Paatsalu core to 33 m in the Viki core (Figs 3, 4).
Other regions. P. a. amorphognathoides is widely recognized all over the world. However, in many regions the upper boundary of this zone cannot be identified, mainly because of inadequate data. In that case, all samples containing P. a. amorphognathoides can be labelled as representing the P. a. amorphognathoides Zonal Group (Jeppsson 1997).
Correlation with the graptolite zonation. Based on the data from the Ohesaare and Aizpute‐41 cores, the lower boundary of the P. a. amorphognathoides Zone correlates with the lowermost Cyrtograptus lapworthi zone (Loydell et al. 1998, 2003). As the taxa allowing precise identification of Datum 1 of the Ireviken Event are very rare in the distal, graptolite‐bearing strata, the precise position of the upper boundary of the zone in the graptolite succession is uncertain. However, analysis of the distribution of conodonts and bentonites in several core sections of Estonia and Latvia suggests that the zone reaches up to the middle of the C. murchisoni Zone (Männik et al. 2002), and the P. a. amorphognathoides Zone corresponds to the interval from the C. lapworthi to the middle C. murchisoni zones (Fig. 5).
Remarks. Pterospathodus a. amorphognathoides is a morphologically highly variable and rapidly evolving taxon. Up to five different, evolutionally connected morphologies of P. a. amorphognathoides, following each other in time, were recognized (Männik 1998a). However, further studies are needed to use these different morphologies in the formal stratigraphy.
The Lower Pterospathodus amorphognathoides amorphognathoides Subzone
Definition. Corresponds to the interval between the appearance of P. a. amorphognathoides below and the disappearance of Asp. fluegeli ssp. n. above (Fig. 1).
Lower boundary. Same as for the P. a. amorphognathoides Zone.
Reference stratum and locality. Upper part of the Velise Formation, Adavere Stage, and lower part of the Jaani Formation, Jaani Stage; Viki core, interval 147.50–147.60 m to 133.05–133.39 m (Fig. 4).
Characteristic fauna. The same as in the P. a. amorphognathoides Zone. Aspelundia fluegeli ssp. n. occurs in the uppermost part of the subzone.
Distribution and thickness in Estonia. Aspelundia fluegeli ssp. n. has been identified in most of the studied cores, in all environments represented. The thickness of the Lower P. a. amorphognathoides Subzone varies from less than 1 m in the Lõetsa (842) core to more than 15 m in the Viki core (Figs 3, 4).
Other regions. Aspelundia fluegeli ssp. n. occurs in sample C.11c from the Cellon section, Austria. Accordingly, samples C.11, C.11A, C.11B and C.11C of Walliser (1964) come from the Lower P. a. amorphognathoides Subzone. The subzone can also be identified in Latvia and Lithuania.
Correlation with graptolite zonation. The subzone evidently correlates with the C. lapworthi and lower C. insectus zones (Fig. 5).
Remarks. Restudy of collections from the Aizpute‐41 core revealed some fragments of Asp. fluegeli ssp. n. in sample C 97‐69 representing the C. lapworthi Zone (Loydell et al. 2003). This agrees with the data from the Ohesaare core, where the uppermost Asp. fluegeli ssp. n. occurs in sample 353.50–353.70 m, also from the C. lapworthi Zone (Loydell et al. 1998).
The Upper Pterospathodus amorphognathoides amorphognathoides Subzone
Definition. Corresponds to the interval between the disappearance of Asp. fluegeli ssp. n. below and Datum 1 of the Ireviken Event above (Fig. 1).
Lower boundary. Coincides with the level of disappearance of Asp. fluegeli ssp. n.
Reference stratum and locality. Lower part of the Jaani Formation, Jaani Stage; Viki core, interval 133.05–133.39 m to 113.90–113.95 m (Fig. 4).
Characteristic fauna. Similar to that of the Lower P. a. amorphognathoides Subzone, but Aspelundia is missing in the Upper P. a. amorphognathoides Subzone.
Distribution and thickness in Estonia. The same as for the Lower P. a. amorphognathoides Subzone (see above). The thickness of the Upper P. a. amorphognathoides Subzone varies from about 0.9 m in the Tori core to more than 15.4 m in the Tehumardi (863) core (Fig. 3).
Other regions. Most probably, part of the P. amorphognathoides Zone sensu Walliser (1964) in the Cellon section, Austria, (above sample C.11C) corresponds to this subzone. However, as Datum 1 of the Ireviken Event is not located in that section, the strata corresponding to the Upper P. a. amorphognathoides Subzone in Cellon remain problematic. The subzone can be recognized in Latvia and Lithuania.
Correlation with graptolite zonation. Corresponds to the interval from the uppermost C. lapworthi Zone below up to the middle C. murchisoni Zone above (Fig. 5).
Remarks. The Lower and Upper P. a. amorphognathoides Subzones can be recognized only in sections where Asp. fluegeli ssp. n. has been found.
The Pseudooneotodus bicornis Superzone
Remarks. Jeppsson (1997) distinguished the Ps. bicornis Superzone in the uppermost part of the P. a. amorphognathoides range, indicating the first and third datums of the Ireviken Event as its boundaries. He recognized the Lower Ps. bicornis and Upper Ps. bicornis zones, with the boundary between them corresponding to Datum 2 of the Ireviken Event.
Distribution and thickness in Estonia. The uppermost part of the P. a. amorphognathoides range and the Ireviken Event interval have been studied thoroughly in the Viki core (Jeppsson & Männik 1993). In this section the Lower Ps. bicornis Zone is about 0.2 m and the upper one about 0.4 m thick. In most of the other Estonian sections at the moment only the Ps. bicornis Superzone can be recognized, reaching in thickness up to more than 3 m in the Pahapilli (675) core in northern Saaremaa (Fig. 3).
Correlation with the graptolite zonation. Jeppsson (1997) proposed a correlation of the Ps. bicornis Superzone with an interval from the Stomatograptus grandis to C. insectus graptolite zones. Recent studies indicate that the superzone is younger and correlates with an interval in the C. murchisoni Zone (Männik et al. 2002).
The Pterospathodus pennatus procerus Superzone
Remarks. Jeppsson (1997) defined this superzone with its lower boundary coinciding with Datum 3 of the Ireviken Event and upper boundary with Datum 6 of the Ireviken Event. Datum 4 of the event divides this superzone into the Lower and Upper P. p. procerus zones.
Distribution and thickness in Estonia. The P. p. procerus Superzone was recognized only in some of Estonian sections. Its thickness changes from a few tens of centimetres in the Uulu (345) core to more than 2.5 m in the Tehumardi (863) core (Fig. 3).
Correlation with the graptolite zonation. Jeppsson (1997) correlated the P. p. procerus Superzone with the upper C. insectus and the C. centrifugus zones. Data from Latvia (Aizpute‐41 core) gave a younger age for this superzone and correlate it with an interval from the upper C. murchisoni Zone to the lowermost Monograptus riccartonensis Zone (Männik et al. 2002).
Concluding notes
Pterospathodus, a morphologically highly variable and rapidly evolving genus, provides a good basis for high‐resolution stratigraphy for Telychian strata. Rich conodont faunas in the Pterospathodus interval allow additional criteria for recognizing the zones. After analyzing the information available on the Telychian conodont faunas it became evident that two main factors hinder the application of the zonation. (1) Usually the most common problem is inadequacy of data, too scarce sampling of sections, too small samples, bad treatment of samples in laboratory (e.g. conodonts are broken, partly dissolved), etc. In case of published information, problems arise also from subjectivity in identifications of taxa and inadequate illustrations. Often, restudy of the existing collections improves the results considerably. (2) A more complicated problem is the ecological dependence of the distribution of faunas. It has been shown earlier that at least two, but probably more (e.g. those illustrated by Nowlan 1983) different ecologically restricted lineages of Pterospathodus existed (Männik 1998a). One of them (P. a. angulatus–P. a. lennarti–P. a. lithuanicus–P. a. amorphognathoides) dominated open shelf carbonate‐terrigeneous environments, the other one (P. p. pennatus–P. p. procerus) the deeper basinal, graptolite‐bearing facies. Both lineages probably originated from a common ancestral taxon at the end of the P. eopennatus time. The zones described above are based on the succession of taxa in the first lineages and are difficult to identify in off‐shore graptolite‐bearing environments (e.g. Loydell et al. 2003). In this case the multi‐gradational zonation (subzone‐zone‐superzone) becomes handy: although the identification of zones is problematic, superzones can usually be easily recognized.
Analysis of the Telychian conodonts in different regions of the world revealed that, despite variation in the composition of faunas, the zonation can be applied in many of the studied sections (Fig. 7). As noted above, the main problem is the lack of adequate information. The P. eopennatus Superzone, particularly its lower part, the P. eopennatus ssp. n. 1 Zone, is the easiest to recognize all over the world. On the basis of morphologically very distinct Astr. irregularis occurring in this interval, the zone can be traced even in poorly studied sections. Aulacognathus kuehni, characteristic of the Lower P. eopennatus ssp. n. 2 Subzone, helps to recognize also this subzone elsewhere quite easily.
The lower boundary of the P. a. amorphognathoides Zone is very distinct but still somewhat problematic in some regions. Comparison of data from all over the world revealed that, in general, two main types of Late Telychian faunas existed (Männik 1998b). One of them is dominated by P. a. amorphognathoides and is well known from the Baltic region, Central Europe and central North America. The other fauna, containing P. rhodesi Savage, has been described from the northern marginal areas of Laurentia (modern Arctic Canada and North Greenland) and Australia (New South Wales). As a rule, P. a. amorphognathoides and P. rhodesi do not occur together. The origin of P. rhodesi and its relationship to other Pterospathodus taxa are not yet known. The data available suggest that the distribution intervals of P. a. amorphognathoides and P. rhodesi coincide in general although the last taxon seems to appear somewhat later in the sections. Additional detailed studies are needed in many regions to identify the upper boundary of the P. a. amorphognathoides Zone.
Identification of the P. a. angulatus, P. a. lennarti and P. a. lithuanicus zones outside Baltic is more complicated either due to lack of information (respective intervals are not sampled, not exposed in the studied sections or correspond to gaps) or for ecological reasons, and therefore further studies are needed.
Specific conodont faunas characterize the Telychian strata in Arctic Russia (Timan–Pechora region and Severnaya Zemlya: Mel’nikov 1999; Männik 1983, 2002). Typically, the faunas here lack ‘real’Pterospathodus and contain several undescribed representatives of Ozarkodina and Ctenognathodus, also Gamachignathus? macroexcavatus Wang Cheng‐yuan & Aldridge, Aps. aff. tuberculatus and Pterospathodus? spp. The last taxon has been reported also from eastern Canada (Gaspé Peninsula), where it is represented by three different morphologies identified as Pterospathodus n. sp. A, Pterospathodus n. sp. B and P. pennatus by Nowlan (1983). Some of these taxa (e.g. G.? macroexcavatus, and also several ozarkodinids) have been found together with ‘real’Pterospathodus in Australia (New South Wales, Bischoff 1986) and South China (Wang & Aldridge 1996). The zonation described in this paper is not yet applicable to this type of sections. However, based on the general composition of taxa (particularly on the occurrence of Apsidognathus), the strata of Telychian age can be recognized without particular problems. On the basis of the disappearance of Apsidognathus (= Datum 2 of the Ireviken Event), the boundary between the Lower Ps. bicornis and Upper Ps. bicornis zones can be identified also in these regions.
Acknowledgements
I am grateful to all my colleagues who provided free access to their collections of conodonts. Also, I would like to thank the referees Professor J.E. Barrick and Dr A.D. McCracken for valuable comments, Mrs. Anne Noor for linguistic revision of the text and Dr. Valdek Mikli for SEM assistance. The study was supported by the Estonian Science Foundation (grants no. 5406 and no. 5920).
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