Chitinozoans in the Wenlock(cid:150)Ludlow boundary beds of the East Baltic

. The distribution of chitinozoans in the Wenlock(cid:150)Ludlow boundary beds was studied in five drill core sections of the East Baltic. It was established that most of the typical Wenlock chitinozoan species became extinct in the uppermost part of the Jaagarahu Stage. In the lower part of the Rootsik(cid:252)la Stage the Sphaerochitina lycoperdoides Biozone was identified for the first time. The Conochitina postarmillata and Ancyrochitina desmea biozones were established in the Dubysa Formation of the Paadla Stage in the Ventspils and Pavilosta cores and in the Gorstian Stage of the Gussev-1 core. Two stratigraphically important new species, Rhabdochitina sera and Conochitina postarmillata , were described, and Eisenackitina lagena (Eisenack) was redescribed.


INTRODUCTION
Due to wide occurrence of lagoonal and shoal rocks in the very shallow-water sections of the upper Wenlock Rootsiküla Formation in Estonia (Nestor 1997), chitinozoans are very rare in that part of the stratigraphic sequence (Nestor 1994). However, in deeper-water core sections of the central part of the Baltic Basin, the distribution of chitinozoans is continuous (e.g. in the Ventspils core, see Nestor 1994).
During the last decade a large number of Silurian chitinozoan samples were investigated from the west Latvian (Kolka, Pavilosta) and Kaliningrad (Gussev-1) deep cores (Fig. 1). Additional samples were studied also from the Ohesaare core, which contained more chitinozoans than presumed earlier. This enabled distinction of the global Sphaerochitina lycoperdoides chitinozoan Biozone in the upper Wenlock of the Ohesaare core, but also better correlation of the northern and southern sections of the Baltic Basin. In the global chitinozoan biozonation (Verniers et al. 1995) an unzoned interval is marked above the S. lycoperdoides Zone. The main aim of this paper is to present more biostratigraphic information about the distribution of chitinozoans in the Wenlock-Ludlow boundary beds, but also to find stratigraphically more useful chitinozoan taxa for correlation of the East Baltic sections with other regions.
Earlier it was thought that the lowermost part of the Ludlow was lacking in the Estonian Silurian sequence (Nestor 1982;Nestor & Nestor 1991), but was represented in the West Latvian core sections (Ulst 1964). Later investigations (Viira & Einasto 2003) have shown that this gap is stratigraphically more restricted.
For independent stratigraphical control it is very important to integrate chitinozoan and graptolite biostratigraphical data. All graptolites from the Ventspils and Pavilosta drill cores, used in this study, have been identified by R. Ulst (Gailite et al. 1987). The succession of graptolites in the Gussev-1 core has been published in Koren et al. (2005), but some graptolite identifications by D. Kaljo (pers. comm. 2006) were also used. No graptolites have been found in the Rootsiküla Stage of the Ohesaare core, and only some graptolites have been identified in the Kolka core (D. Loydell, D. Kaljo, pers. comm. 2006).

LITHOLOGY, FACIES, AND STRATIGRAPHICAL NOTES
The studied core sections represent different facies belts. The studied interval of the Ohesaare core ( Fig. 2) is characterized by various carbonate rocks from open shelf limestones up to lagoonal dolomites, forming the carbonate platform of the Baltic Basin (Nestor & Einasto 1982). The Kolka core (Fig. 3) is represented by marlstones and limestones of open shelf origin. The Ventspils and Pavilosta boreholes (Figs 4,5) are located in the depression facies of the Baltic Basin (Nestor & Einasto 1982), characterized mostly by argillaceous marlstones with graptolites. The Gussev-1 (or Gusevskaya-1 by Koren et al. 2005) core (Fig. 6) in the southernmost edge of the basin contains dolomitic mudstones.
In the present paper the distribution of chitinozoans is examined in the Wenlock-Ludlow boundary beds from the topmost Jaagarahu Stage up to the Sauvere Beds of the Paadla Stage in the Ohesaare and Kolka cores, as well as in the graptolitic sections of the Ventspils, Pavilosta, and Gussev-1 cores. The distribution of selected chitinozoan species in the East Baltic drill cores is shown in Fig. 7.
In 1978 the boundary between the Wenlock and Ludlow in Estonia was fixed at the boundary of the Rootsiküla and Paadla regional stages, i.e. at the boundary of the Soeginina and Sauvere beds (Resheniya… 1978). On the basis of recent conodont and cyclostratigraphic investigations (Viira & Einasto 2003), the stratigraphical position of the Soeginina Beds in the stratotype section as well as in the Ohesaare core was specified. It appeared that the Soeginina Beds, represented by various dolostones, belonged to the Paadla Stage.
In the uppermost Wenlock of the East Baltic the most reliable level for correlation is the base of the nassa graptolite Zone (Radzevičius & Paškevičius 2005). In the Latvian graptolitic sections it coincides with the thin and more carbonate Ančia Member, treated also as the base of the Rootsiküla Stage (Kaljo et al. 1984 (Kaljo et al. 1984). This enables recognition of the base of the nassa Zone in the Pavilosta and Ventspils cores. According to indirect correlation, the Mulde Event of Gotland    with the interval of 149.8-161.5 m in the Ohesaare core, including the boundary of the Jaagarahu and Rootsiküla stages, but also three conodont extinction steps (datums 1, 1.5, and 2). The hardground at the base of the Rootsiküla Stage is regarded as the sequence boundary (Calner 1999), defined in the Ohesaare core at 155.1 m. An excellent stratigraphic marker is the Grötlingbo Bentonite , which in the Ohesaare core has been determined at a depth of 154.25-154.5 m (Kiipli & Kallaste 2006). A strong positive shift in the δ 13 C stable isotope curve, peaked at 154 m in the Ohesaare core (Kaljo et al. 1997), just above the Grötlingbo bentonite, is another good stratigraphic marker . According to Calner et al. (2006), in the Hunninge-1 core G. nassa appears about 0.6 m above the Grötlingbo Bentonite and in the Grötlingbo-1 core just below the bentonite. This obviously places the base of the nassa Zone in the Ohesaare core between 153 and 153.5 m, above the bentonite and samples containing only fragments of benthic organisms (Fig. 2).

CHITINOZOAN BIOSTRATIGRAPHY
The chitinozoan assemblage below the Sphaerochitina lycoperdoides Biozone Nestor (1994) distinguished the Sphaerochitina indecora chitinozoan Biozone in the uppermost part of the Jaagarahu Stage and the lower part of the Rootsiküla Stage in the Ohesaare core. Later investigations, however, showed that the occurrences of S. indecora Nestor are rather scattered. This species has not been found in the Kolka, Ventspils, Pavilosta, and Gussev-1 cores. In the global chitinozoan biozonal scheme (Verniers et al. 1995) the Conochitina pachycephala Biozone is differentiated below the S. lycoperdoides Zone (Fig. 8), but in deeper-water sections (Ventspils, Pavilosta), in the uppermost Jaagarahu Stage, the former species has only limited distribution. More frequent Conochitina cribrosa, occurring together with S. indecora (Nestor 1994), is a better index species for the biozone. The Conochitina cribrosa Biozone, corresponding to the upper part of the M. testis graptolite Zone (Nestor 1994), has been distinguished in the Ohesaare, Kolka,  (Eisenack) (Fig. 9F), Clathrochitina sp. (Fig. 9D), Eisenackitina sp. (Fig. 9R). Most of these species become extinct in the Ohesaare core at 160-161.5 m, between the datum points 1 and 1.5 of the Mulde Event (see . Of transitional species, Conochitina claviformis Eisenack ( Fig. 9A) and Con. tuba Eisenack (Fig. 10L), in the Ohesaare and Kolka cores also Con. pachycephala (Fig. 9C), are more numerous. Probably due to facies difference the genera Sphaerochitina, Ramochitina, and Plectochitina are missing in the deepest-water Ventspils section. The species diversity is higher in the Ohesaare and Kolka sections, representing the shallower-water carbonate platform environment.

The Sphaerochitina lycoperdoides Biozone
The S. lycoperdoides Biozone was erected by Paris (1981) in Portuguese sections. Definition of this total range global biozone is presented in Verniers et al. (1995) and Grahn (1996). The biozone in the East Baltic sections is first described in the present paper. Earlier (Nestor 1994) this interval was distinguished as Interzone. It is relevant to note that the zonal species itself is rather sparsely represented throughout the biozone, whereas it was not found in the Ventspils core. The lower boundary of the biozone coincides roughly with the lower boundary of the Viita Beds and the Siesartis Formation of the Rootsiküla Stage, lying a few metres above it. Together with Sphaerochitina lycoperdoides Laufeld ( (Laufeld) (Fig. 10B), and R. tabernaculifera (Laufeld) (Fig. 10A) in Ohesaare. As appendices were frequently broken, it was difficult to identify precisely the species of Ancyro-, Ramo-, and Plectochitina, but Ancyrochitina gutnica Laufeld (Fig. 9N, V) was established in all studied sections. In the Ohesaare core the last Wenlock species disappear within this biozone. In more off-shore sections some Wenlock species (Conochitina claviformis, Con. tuba, Con. pachycephala Eisenack) range up to the Ludlow. It is important to note that in deep-water sections, where Sphaerochitina is lacking, the range of Rhabdochitina sera usually coincides with the upper part of the S. lycoperdoides Biozone.
According to graptolite data from Pavilosta (Gailite et al. 1987), the Sphaerochitina lycoperdoides Biozone corresponds to the nassa and ludensis graptolite zones. The graptolite finds from Kolka (Fig. 3) also refer to the correspondence of the S. lycoperdoides and nassaludensis zones. Concerning the position of the boundary between the colonus and scanicus zones, some graptolite data from the Gussev-1 core (Koren et al. 2005) seem to be insufficiently detailed and there was no evidence presented to place the bottom of the scanicus Zone at a depth of 1464 m (Fig. 6). Some unpublished graptolite data by D. Kaljo refer to a 7-8 m higher position of this boundary. Considering these data, S. lycoperdoides may be present also in the nilssoni graptolite Zone. It should be noted that the zonal graptolites Pristiograptus praedeubeli and P. deubeli have not been identified in the Ventspils and Pavilosta cores (Gailite et al. 1987).
In the Pavilosta core the S. lycoperdoides Biozone corresponds to the uppermost Wenlock Siesartis Formation, in the Ohesaare and Kolka cores to the Viita Beds of the Rootsiküla Stage. In the last two sections the uppermost Wenlock (Kuusnõmme and Vesiku beds of the Rootsiküla Stage) is mostly represented by lagoonal dolomites, containing an impoverished assemblage of only scarce specimens of long-ranging species of Ancyroand Conochitina. An exception is the occurrence of Conochitina rudda Sutherland (Fig. 10J) in the Ohesaare core, in the middle of the Vesiku Beds. This interval could be treated as a continuation of the S. lycoperdoides Zone or treated as an Interzone (see Nestor 1994).

The Conochitina postarmillata Biozone
This biozone is an interval zone of Conochitina postarmillata sp. nov. (Fig. 11A-C). The base of the zone is defined at the first occurrence of the index species and the top by the first occurrence of the index species of the succeeding biozone. It is distinguished only in the southern, deeper-water Ventspils, Pavilosta, and Gussev-1 sections and characterizes the lowermost beds of the Ludlow. In the Ohesaare and Kolka cores this biozone is lacking due to sparse, only scattered distribution or absence of chitinozoans. Beside longranging species (Con. claviformis, Con. tuba), only a few species associate with Con. postarmillata: Sphaerochitina scanicus Grahn (in the Ventspils core), Cingulochitina wronai Paris & Křiž and Eisenackitina lagena (Eisenack) (Fig. 11 F-H; Ventspils and Gussev-1 cores), Conochitina rudda (Pavilosta core), Cingulochitina convexa, Cin. gorstyensis, Cin. baltica, and Cin. wronai (Gussev-1 core). It is worth mentioning that Grahn (1996) dis-tinguished the Sphaerochitina scanicus Subzone as the uppermost part of the S. lycoperdoides Zone in Skåne, southern Sweden.
In the Ventspils and Pavilosta cores the Conochitina postarmillata Biozone corresponds to the lower part of the Dubysa Formation, and to the nilssoni graptolite Zone (Gailite et al. 1987).
According to graptolite data (Gailite et al. 1987), in the Pavilosta and Ventspils cores the Ancyrochitina desmea Biozone corresponds to the lower part of the scanicus graptolite Zone, which coincides with the middle part of the Dubysa Formation.
Many new species appear in the succeeding Angochitina elongata Biozone (Verniers et al. 1995). Besides the zonal species (Fig. 11 P) also Ancyrochitina brevispinosa Eisenack (Fig. 11 M), Ramochitina valladolitana Schweineberg (Fig. 11 I), and Sphaerochitina cf. impia Laufeld (Fig. 11 L) appear in the Torgu Formation of the Paadla Stage, as well as in the upper part of the Dubysa Formation. This zone, however, is not considered in this paper.

Gotland and Skåne
The chitinozoan assemblages of Gotland (Laufeld 1974) and Skåne (Grahn 1996) partly coincide with those determined from the Wenlock-Ludlow boundary beds in the East Baltic area. Sphaerochitina lycoperdoides was found in the dubius-nassa Zone in the När-1 core (graptolites identified by Jaeger 1991) and in the ludensis Zone in the Järrestadsån-4 core (Grahn 1996, fig. 2). On Gotland, Laufeld (1974, fig. 77) recorded this species from the upper part of the Mulde and throughout the Klinteberg beds. According to the correlation of Jeppsson et al. (2006), this interval coincides also with the nassa-ludensis zones, similarly to the East Baltic core sections. Sphaerochitina scanicus occurs in the nilssoni Zone in the Ventspils core, but has also been established in the colonus Zone in the sections of Skåne (Grahn 1996 , fig. 3). Conochitina postarmillata sp. nov. is not found in Swedish sections. Ancyrochitina desmea has been recorded in the middle part of the Hemse Beds (Laufeld 1974) of the Gotland sequence, correlating with the scanicus Zone (Jeppsson et al. 2006), and also in the sections of Skåne (Grahn 1996, fig. 4), where its occurrences correspond to the chimaera Zone.
There are some differences in the stratigraphical ranges of several species in Gotland (Laufeld 1974) and East Baltic sections. For example, the range of Anc. gutnica ends in the uppermost Wenlock on Gotland, but in the lower Ludlow in the East Baltic sections. Cingulochitina convexa has been recorded on Gotland only from the Ludlow (Hemse and Eke beds), whereas in the Ventspils and Pavilosta sections this species appears already in the upper Wenlock (Siesartis Formation). The appearance level of Belonechitina latifrons and Angochitina elongata at the top of the Klinteberg Marl on Gotland (Laufeld 1974), corresponding to the top of the ludensis Zone (Jeppsson et al. 2006), does not coincide with the appearance of these species in the Ventspils and Pavilosta cores in the middle of the Dubysa Formation, correlating with the middle part of the scanicus Zone. In the Ohesaare and Kolka cores these species appear at the base of the Torgu Formation of the Paadla Stage, which is barren of graptolites.
In Skåne, Grahn (1996, fig. 5) identified B. latifrons and Ang. elongata at the base of the Klinta Formation from the Bjärsjölagård 1 section, above the scanicus Zone.
Such differences are difficult to explain. These may be induced by the lithology and facies of the sampled beds, but also by gaps in sedimentation.

Ludlow type area
Chitinozoans from the Ludlow type area were recorded by Dorning (1981) and Sutherland (1994). The former author listed 35 taxa from the type Wenlock and Ludlow localities and gave their stratigraphical ranges. The most important events are the disappearance of Cin. cingulata and Anc. gutnica in the Much Wenlock Limestone Formation (uppermost Wenlock) and the appearance of B. latifrons and Ang. elongata in the Middle Elton Formation (lower Ludlow).
In his monograph Sutherland (1994) described chitinozoans from the type Ludlow Series, showing also the species coming from the Much Wenlock Limestone Formation (Sutherland 1994, text-fig. 36). The most important species is Conochitina rudda, which is present also in the Lower Elton Formation and at 130.0-130.20 m in the Ohesaare core, in the Vesiku Beds of the Rootsiküla Stage. Conochitina sp. A Sutherland (pl. 9, figs 12, 13) from the Lower Elton Formation (nilssoni graptolite Zone) is probably identical with the new zonal species Con. postarmillata. In the uppermost part of the Middle Elton Formation, corresponding to the scanicus graptolite Zone, there appear B. lauensis, Ang. elongata, and E. toddingensis. These data correlate well with the occurrences of the same species in the East Baltic core sections. Differences are observed in the stratigraphical ranges of some species as well. In the type Ludlow area Cingulochitina gorstyensis and Cin. convexa appear in the Middle Elton Formation (scanicus graptolite Zone), but in the uppermost Wenlock in the East Baltic cores (nassa and ludensis zones). However, we should note here that not all members of the species assemblage, contained in the studied beds, may be recorded if too small rock samples (15-25 g) are used for analysis (Hints et al. 2006).

Prague Basin (Bohemia)
The Wenlock-Ludlow boundary beds in the Prague Basin have been studied in great detail by Křiž et al. (1993). Chitinozoans (identified by P. Dufka) are represented by a rather few species. Among some long-ranging species Eisenackitina pregranosa and Cingulochitina wronai were determined. The former species, identical with E. lagena, occurred in the dubius parvus and chimaera graptolite zones. Cingulochitina wronai was reported from the colonus and chimaera zones. The ranges of these species partly coincide with the ranges of the same species in the East Baltic cores.
Palencia Province (Spain) Schweineberg (1987) investigated chitinozoans in the uppermost Wenlock to lowermost Devonian sediments of North Spain, including the Wenlock-Ludlow boundary beds. In addition to many exotic species, missing in the East Baltic sections, there occur also more widespread species, useful for interregional correlation. The range of Anc. desmea coincides with the nilssoni Zone and the range of Ang. elongata with the scanicus Zone. In the Gussev-1 core Ancyrochitina valladolitana Schweineberg was found, appearing here together with Ang. elongata as in Palencia. Verniers (1982) and Verniers et al. (2002) present the results of chitinozoan studies in the Brabant Massif. Sphaerochitina lycoperdoides was recorded in the nassa and lower-middle part of the ludensis graptolite zones in the Mehaigne area (Verniers 1982). Verniers et al. (2002) recorded a number of species in the Ludlow sections, corresponding to the undifferentiated nilssoni and scanicus zones, but presented no data on the topmost Wenlock, probably lacking chitinozoans. Besides the long-ranging species, appearing jointly in the lowermost Ludlow, a more important event is the occurrence of Cin. wronai (zone F of Verniers et al. 2002).

Facies dependence of chitinozoans is well expressed
by the scarcity or lack of chitinozoans in shallowwater sparitic limestones and dolomites (Kuusnõmme, Vesiku, and Soeginina beds of the Ohesaare and Kolka cores), compared to their diverse and continuous presence in deeper-water marl-and mudstones (Siesartis and Dubysa formations in the Ventspils and Pavilosta cores).   (Laufeld 1974;Grahn 1996), corresponding also to the scanicus Zone. It is also known from the sections of the Ludlow type area (Dorning 1981) and from Palencia, North Spain (Schweineberg 1987). Ancyrochitina cf. desmea has been identified in the Ludlow of western Gondwana (Grahn 2006). Considering the wide distribution of the zonal species, the Anc. desmea Biozone might be a good candidate for a global biozone in the future, filling partly a gap in the chitinozoan biozonal succession. 6. The appearance of Angochitina elongata, marking the lower boundary of the respective global chitinozoan biozone in the middle Ludlow, was observed in all studied East Baltic core sections.
All figured specimens of chitinozoans are deposited in collection No. 527 of the Institute of Geology (GIT) at Tallinn University of Technology, Estonia.
Conochitina postarmillata sp. nov. Figure 11A-C Diagnosis. Long cylindrical vesicles with convex base, which may be indented. Basal edge is widely rounded or inconspicuous. Mucron wide and button-like. Vesicle wall smooth.
Remarks. The mucron is usually not seen in flattened specimens and in those with an indented base.
Rhabdochitina conocephala Eisenack has a flat base and it is shorter. Rhabdochitina magna Eisenack is long, but lacks a mucron.
Occurrence. Uppermost Wenlock and Lower Ludlow of the East Baltic: Viita Beds of the Rootsiküla Stage in the Kolka core, 359-400 m; Siesartis Formation of the Rootsiküla Stage and the lowermost part of the Dubysa Formation in the Ventspils core, 655-667 m; Siesartis Formation in the Pavilosta core, 862-883.3 m; boundary beds of the Homerian and Gorstian stages in the Gussev-1 core, 1459.8-1467.8 m.
Family DESMOCHITINIDAE Eisenack, 1931 Subfamily DESMOCHITININAE Paris, 1981Genus Eisenackitina Jansonius, 1964 Eisenackitina lagena (Eisenack 1968) Figure 11 F-H Description. Barrel-shaped vesicles with more or less developed flexure. The centre of the base is slightly concave, with a wide and low mucron. The basal margin in inconspicuous. The neck is not developed, but the aperture is slightly widening. The porous vesicle wall is densely covered by granules or tubercles.
Remarks. In earlier papers the author misidentified the species Eisenackitina lagena (Eisenack 1968) and described it from the middle Wenlock sequence of Estonia and North Latvia (Nestor 1994). High-resolution SEM study, however, revealed differences in the ornamentation of the vesicle wall in the Wenlock and Ludlow material of two species that are usually identical in the overall shape of the vesicles. Eisenack (1968) described the original material of Conochitina lagena from the graptolitic erratic boulders of Early Ludlow age. Swire (1990) described Eisenackitina spongiosa from the Middle Wenlock Coalbrookdale Formation, Shropshire. It had spongy ornamentation of the vesicle wall like that in well-preserved Wenlock specimens of the East Baltic (Nestor 1994, pl. 15, fig. 2). Thus, not Eisenackitina lagena but E. spongiosa occurs in the East Baltic Wenlock sections and the regional biozone between the Cin. cingulata and Con. pachycephala zones (see Nestor 1990Nestor , 1994 has to be renamed the E. spongiosa Biozone.
Occurrence. Lower Ludlow of the East Baltic: middle part of the Dubysa Formation of the Paadla Stage -in the Ventspils core at 617-635 m, in the Pavilosta core at 836.40-836.60 m; lower-middle part of the Gorstian Stage in the Gussev-1 core, 1431-1441 m.