Upper Wenlock to Lower Přídolí (Silurian) conodont biostratigraphy of Saaremaa, Estonia, and a correlation with Britain

The closely related conodonts Ozarkodina bohemica, O. snajdri and O. crispa form a clade that provides useful biostratigraphical indices through the upper Silurian. Collections from boreholes and surface outcrops on Saaremaa and from a borehole at Kolka, Latvia, contain new morphotypes of O. bohemica and O. crispa. A new subspecies, O. snajdri parasnajdri, is also distinguished, occurring above O. crispa in the Kuressaare and lowermost Kaugatuma stages. Evidence from conodonts and other fossils, primarily chitinozoans, ostracods and ichthyoliths, can be used to correlate the upper Silurian succession of Saaremaa with those of the Welsh Borderland and Gotland, although some problems remain to be solved. The microfossil distribution suggests that there may be a major break at the base of the Ludlow Bone Bed Member at Ludlow, equivalent to the Kuressaare Formation on Saaremaa.


INTRODUCTION
The study of the Silurian rocks of Estonia dates back to the last century, with the establishment of a Lower Palaeozoic stratigraphical scheme by Schmidt (1858). Details were added in the earlier part of this century, and more recently scientific investigations have been considerably advanced by a group of researchers at the Institute of Geology, Estonian Academy of Sciences, working under the leadership of D. Kaljo. Modern research has been greatly facilitated by the availability of numerous boreholes drilled by the Estonian Geological Survey, and a large number of papers and monographs has been published on Lower Palaeozoic palaeontology, stratigraphy and facies distribution. General summaries of the main results of this work may be found in volumes edited by Kaljo (1970Kaljo ( , 1977, Kaljo & Klaamann (1982a,b) and Kaljo & H. Nestor (1990).
The current understanding of the Silurian stratigraphy of Estonia has been summarised by H. Nestor (1993) and the Wenlock-Piidoli correlation chart is shown in Fig. 1. The main stratigraphical units are regional chronostratigraphical stages, within which separate local lithostratigraphical units are distinguished in different parts of the Baltic basin (Bassett et al., 1989). The local units reflect facies differences: Central Estonia and most of Saaremaa (Fig. 2) are dominated by shallow-water facies, whereas in South Estonia and on the Sorve Peninsula of southern Saaremaa deeper water shelf and outer shelf sediments prevail (H. Nestor & Einasto, 1977;Bassett et al., 1989). Gradual shifts of the Facies belts occurred during the evolution of the basin.
The present paper deals with the succession representing the late Wenlock to early Piidoli interval, embracing the Rootsikula, Paadla, Kuressaare and Kaugatuma stages. In the absence of graptolites, the correlation of these stages and their representative formations with other areas is currently only approximate. The Rootsikiila Formation is considered to correspond to the late Wenlock Klinte Secundo Episode of Jeppsson et al. (1995), when very variable shallow-water sediments were deposited with characteristic cyclic alternation of limestones and primary dolomites. The total thickness is 25-32m, and the sediments display frequent discontinuity surfaces, mud cracks, ripple marks and trace fossils. Abundant stromatolites and oncolites occur, but the Fauna is restricted to eurypterids, thelodonts and conodonts. The Paadla Formation is 26-32 m thick and consists of detrital biomorphous and biohermal stromatoporoid-coral  Nestor, 1993) considered to be important fossils by Walliser (1964) in his conodont biozonation of the Silurian.
Aldridge & Schonlaub (1989) stated that 0. crispa was a direct descendant of 0. snajdri, and that the two occurred in stratigraphical succession. Miller (1995) agreed that 0. crispa originated in 0. snajdri, but showed that intermediate forms occurred in the Welsh Borderland at stratigraphically higher levels than the first occurrence of 0. crispa, so the relationship was not one of simple phyletic transition.
The succession of these conodonts on Saaremaa is comparable with that identified by Helfrich (1975). Ozarkodina bohemica bohemica, 0. aff. bicornuta, 0. snajdri snajdri and 0. crispa are all recognized, and some specimens of 0. crispa are transitional to 0. tillmani. The youngest representatives, above the last occurrence of 0. crispa, are clearly of snajdri affinities and are here referred to as 0. snajdri parusnujdri subsp. nov.
Previous records of 0. snajdvi and 0. cvispa The holotype of 0. .snajdri is from Muslovka Quarry, Bohemia, a few metres below the base of the ultimus graptolite biozone, while that of 0. crispa is from Santa Creu, Spain (Walliser, 1964). In his conodont biozonation, Walliser (1964) included a 'crispus zone' in the uppermost Ludlow, and a 'snujdri-Horizon' within the upper part of the middle/upper Ludlow siluricus zone; 0. crispa was represented in samples from his reference section at Cellon in the Carnic Alps, whereas 0. snajdri was not.
Subsequent records produced some uncertainties about the ranges of 0. snajdri, 0. crispa and the index taxon of the zone succeeding the 'crispus zone', 0. r . eosteinhornensis. Fihraeus (1969) reported the Pa element of 0. crispa on Gotland in strata above 0. r . eosteinhornensis, leading Walliser (1971) to question

Fig. 2.
Location map of outcrops (triangles) and boreholes (circles) on Saaremaa and northernmost Latvia whether the 'crispus zone' lay within the 'eosteinhornensis zone' or whether the two overlapped. In fact, the specimen illustrated by Fihraeus (1969, pl. 2, figs 13, 14) may belong to 0. snajdri, although subsequent work has resulted in records of 0. crispa and representatives of the steinhornensis group in association on Gotland (Jeppsson, 1983;Jeppsson et a/., 1994) An association of 0. snajdri with specimens referred to as primitive 0. r. eosteinhornensis was reported by Rexroad & Craig (1971) in the Bainbridge Formation of Lithium, Missouri, and the two taxa were also recorded together in the Kokomo Limestone Member of the Salina Formation in north-central Indiana by Pollock & Rexroad (1973). The specimens illustrated by Pollock & Rexroad (1973, pl. 1, figs 30-34), however, show several characteristics of the Pa element of 0. crispa and the specific assignment is equivocal. An overlap of the ranges of 0. crispa and specimens referred to 0. r . eosteinhornensis in North America was also found by Helfrich (1975) in the Central Appalachians, while Denkler & Harris (1988) reported that the range of 0. snujdri in the same region began below that of 0. crispa and terminated above it.
In Australia, De Deckker (1976) found some overlap of the ranges of 0. snajdvi and 0. crispa in the Kildrummie Formation of New South Wales, and was the first to suggest that the range of 0. snujdri might be longer than previously thought. He also considered a second possibility that the intraspecific variation of Pa elements of 0. crispa may include specimens of similar morphology to 0. snajdri. However, his specimens are rather atypical, and resemble those described by Link & Druce (1972, pl. 9, figs 22-28) from the Yass Basin, New South Wales as Spathognathodus cf. S. ranuliformis Walliser, a species that is now assigned to the genus Kockelella.
In summary, all possibilities of co-occurrence of 0. snajdri, 0. crispa and specimens referred to 0. r . eosteinhornensis are known; all three may occur together or any pairing. Only in Bohemia are they represented in clearly succeeding order, in the Muslovka and Kolednik quarries, and even here there is some overlap of the ranges.

SYSTEMATIC DESCRIPTIONS
Systematics are given here for members of the 0. bohemica lineage, based on material from Saaremaa. As noted by Miller & Aldridge (19Y7), there is some inconsistency regarding the usage of subspecies and morphotype designations in conodont taxonomy. We have followed the principle of retaining subspecies designations for populations that are separated in space and/or time; for morphological variants that occur together in some samples we have used morphotypes. The exceptions are where we subdivide subspecies or exisiting morphotypes: even if different forms seem to be chronologically or geographically separated, we have designated them as morphotypes.
Order Ozarkodinida Dzik, 1976 Family Spathognathodontidae Hass, 1959Genus Ozarkodina Branson & Mehl, 1933 Ozarkodina bohemica bohemica (Walliser, 1964) 1964 Spathognathodus sagitta bohernicus Walliser: 83,pl. 7,fig. Diagnosis. See Aldridge (1985, p. 88). Remarks. Three morphotypes of 0. b. bohemica were distinguished by Schonlaub (In Kriz et al., 1993) and numbered 1-3; morphotype 1 has low posterior denticles, similar to 0. sagitta, whereas morphotype 3 has an abrupt posterior termination and a widely flaring basal cavity. Morphotype 2 is very distinct, with an ornament of ridges of fused denticles on the basal cup. Schonlaub did not mention the specimens originally described by Walliser (1964), with a widely flaring basal cavity extending to the posterior tip, but they appear to be closest to morphotype 3. Because we cannot relate Walliser's specimens or ours to the Bohemian morphotypes we apply a different notation here. The type specimen and associates represent the a morph. Those from Saaremaa have a less flared cavity which does not extend right to the posterior tip and belong to the B morph. Description. Pa element: blade long, with 14-20 denticles that are largest posteriorly. Anterior edge gently convex, posterior edge upright. Basal cavity situated beneath posterior part of blade, terminating short of posterior tip. Small specimens shorter and relatively higher than large specimens. Large specimens usually with fused denticles above basal cavity and a longitudinal ledge in the lower third of the blade. All specimens with white matter, deep above basal cavity and narrowing anteriorly and posteriorly so that extreme denticles may be hyaline. Ozarkodina bohemica bohemica (Walliser 1964) (Helfrich 1975 Ozarkodina aff. bicornuta (Helfrich, 1975) (Pl. 1, figs 16-19) aff. 1975 Spathognathodus bicornutus Helfrich: pl. I,11,[13][14][15][16]18,pl. 2,figs 1,4,6,16. Description. Pa element: specimens small; blade rather high, denticles wide, usually 10-11 when not fused. Basal cavity beneath posterior part of blade, circular, not extending to posterior tip. Morphologically close to the smallest specimen illustrated by Helfrich (1975, pl . 1, fig. 5).

Occurrence on Saaremaa. Rootsikula Stage: Vesiku borehole
Remarks. The specimens illustrated by Helfrich (1975) are very variable, but most have a single high denticle at both ends of the blade; between these are 5-10 denticles which show no fusion on any of the figured specimens. The Estonian specimens have an enlarged posteriormost denticle, but the anterior denticle is not markedly different in size from its neighbours; the denticles above the cavity may be fused. Occurrence. Uppermost Rootsikiila Stage: Soeginina samples 1-3, Anikaitse sampes 4, 8. Material. Pa element: 9 specimens.
Ozarkodina crispa (Walliser, 1964) 1964 Spathognathodus crispus Walliser: 74, pl. 9, fig. 3, pl. 21, considerable variation, and four morphotypes, designated a , 0, x, 6, were differentiated by Walliser & Wang (1989). The p, x and S morphs are all characterized by a furrow on the oral margin of the blade; the 01 morph lacks the furrow. The holotype selected by Walliser (1964, pl. 21, fig. 12) lacks a furrow and thus belongs to the 01 morph, as do all the specimens from the northern East Baltic. However, there is major variation within this group, and we distinguish a l , a2 and a3 morphs, with the holotype belonging to a,.
Ozarkodina crispa a?, morph Description. Pa element: specimens small; blade high, short with fused denticles towards posterior end. Denticle height decreases posteriorly from anterior end to fused portion. Posterior termination of blade concave to vertical. Basal cavity with wide subcircular lips, underlying posterior half of blade and extending to posterior tip. White matter fills denticles, extending down to tip of cavity beneath fused portion of blade. Remarks. These specimens differ from the a1 morph in the subcircular, rather than asymmetrical, shape of the basal cavity.  -16, 19, 20, P1. 3, figs 5, 6, 9-11) Ozarkodina crispa a3 morph (PI. 2, figs 17, 18, 21, 22, P1. 3, figs 1 4 , 7, 8) Description. Pa element: similar to a2 morph but with posterior denticles unfused or only partly fused. Basal cavity variable in position, normally extending to posterior tip. White matter occupies one-third to one-half of blade. Remarks. Specimens in which the basal cavity does not extend beyond the posterior end of the blade are similar to 0. s. snajdri, but differ in the smaller size of the cavity and the shorter blade. The specimens illustrated by Helfrich (1975)  Diagnosis. Pa element with large subquadrangular basal cavity extending to posterior end of blade; denticles above cavity fused. Other elements unknown. Description. Blade commonly long and low, with small denticles, fused above cavity; anterior end higher, posterior portion arched. Posterior termination of blade vertical or oblique concave. Basal cavity with tip a little posterior of centre of blade; lips commonly quadrangular and extending to posterior tip of blade; cavity extends as a narrow groove to anterior tip. White matter deepest above cavity, rising anteriorly and posteriorly; anterior denticles may lack white matter. Remarks. There is variation in the length of the fused part of the blade and in the position of the basal cavity tip. However, this subspecies differs from 0. s. snajdri in always possessing fused denticles and in the more central position of the cavity tip. The cavity lips always extend to the posterior tip of the blade. Ozarkodina snajdri cf. snajdri (Walliser 1964) (PI. 1, figs 20,21) Remarks. Seven specimens in the Ohesaare and Kolka boreholes are close to 0. s. snajdri. All have fused denticles above the basal cavity, but on unbroken specimens the cavity can be seen to terminate just short of the posterior end of the blade.

CONODONT BIOSTRATIGRAPHY OF SAAREMAA
Samples have been processed for conodont recovery from seven boreholes and nine outcrops (Fig. 2). Spacing of samples in the  (Viira) and Oulodus elegans (Walliser) (Viira, 1982(Viira, a,b. 1994. The 0. bohemica lineage is most completely represented in the Ohesaare borehole (Figs 5 and 6). In the lower part of the Viita In the Riksu borehole further to the north, the highest strata are assigned to the Uduvere Beds. The 0. bohemica lineage is represented only by 0. crispa a2 and a3 morphs at 33. 5-36.0m in the Sauvere Beds. The most northerly borehole sampled from West Saaremaa is at Vesiku, where 0. b. bohemica ,!? morph has been recovered from two samples in the Viita and Kuusnomme Beds.
The Sakla borehole is situated in East Saaremaa, where the strata are generally dolomitized. Nevertheless, 0. b. bohemicu / 3 morph has been recovered from the Viita Beds and 0. crispa a2 and cy3 morphs occur in the Sauvere Beds and in a single sample from the Uduvere Beds (Fig. 7).
In Formation of Ohesaare. In the Kolka borehole, 0. crispa occurs with 0. Y. eosteinhornensis s. 1 in the Kuressaare Stage. with the most typical specimens at a depth of 270.8 m. In general, there is a pattern of increasingly high occurrence of 0. crispa in the lithostratigraphical sequence from north to south, i.e. towards deeper water facies. The exposure of Himmiste Beds or uppermost Sauvere Beds at Karala has yielded more than 50 specimens of the Pa element of 0. crispa, with a2 morphs dominant, but with some a3 morphs and transitional forms (Fig. 8). A few specimens of the a2 morph have also been recovered from KBrla (Sauvere Beds) and Paadla (Himmiste Beds). One specimen from Roopa Cliff (Himmiste Beds) is identified as an a3 morph.
Small specimens with short blades and large posterior and anterior denticles have been recovered from the Soeginina cliff section (uppermost Rootsikiila Stage) of West Saaremaa and from the outcrop at Anikaitse in East Saaremaa. They are identified as 0. aff. bicornuta. 0. bicornuta was placed in the bohemicu lineage between 0. b. bohemicu and 0. snajdri by Helfrich (1975).
The relative completeness of the 0. bohemica lineage in the Ohesaare borehole is complemented by evidence that Coryssognathus dubius has its longest range in this section. A more   Fig. 5. Distribution of conodonts, icthyoliths (Marss, 1986), chitinozoans (Nestor, 1982) and ostracods (Sarv, 1971)  complete section in this borehole than in others is also confirmed by occurrences of ichthyoliths and ostracods (Mirss, 1986(Mirss, , 1990Sarv, 1971). The distribution of conodonts in the Paadla Formation, in particular, suggests the presence of gaps in the Saaremaa succession. V. Nestor (1982) has previously suggested on the basis of chitinozoan data that there is a gap between the Rootsikiila and Paadla stages, and an increase in the scale of this gap northwards has been recognized (Nestor, V., 1990;Nestor, H., 1993). Conodont correlations with Gotland confirm a hiatus at this level (Jeppsson et al., 1994), and the absence of 0. s. snqjdri in all studied sections except Ohesaare provides consistent evidence. A break between the Paadla and Kuressaare stages was also postulated by Einasto (1991) and Marss (1992), and is corroborated by the fact that overlap between 0. crispa and 0. r. eosteinhornensis s. 1. is apparent in the Kolka section, but has not been found anywhere on Saaremaa. Alternatively, the restricted nearshore facies represented by the Uduvere Beds  may be a factor in producing the differences in the observed stratigraphical distributions of the conodonts.
In the Ludlow type area, Britain, C. latifrons is restricted to the Gorstian, with the highest record at the top of the Lower Bringewood Formation, while A . elonguta occurs throughout the Gorstian from the uppermost Middle Elton Formation and extends into the Lower Leintwardine Formation (Sutherland, 1994). Sutherland (1994) also showed no overlap between the ranges of E. lagenomorpha and E. philipi, with the former spanning the Lower Bringewood Formation to Lower Leintwardine Formation and the latter ranging from the Upper Leintwardine Formation to the Upper Whitcliffe Formation. On Gotland, C. latifrons and A . elongata appear in the topmost Klinteberg Beds, and E. lagenomorpha and E. philipi appear together in the upper part of the Hemse Beds coincident with the highest occurrence of C. latfrons (Laufeld, 1974).

Ostracods
The distribution of ostrdcods in the Ohesaare borehole has been documented by Sarv (1971), and the ranges of key species are shown in Fig. 5 . Ostracods are rare in the Rootsikula Stage and absent from the lowermost Paadla Stage, but in the Paadla Stage above a depth of 115m, rich assemblages occur. Sarv (1971) reported that five species, including Neoheyrichiu nutam Martinsson, had not been found in the northern East Baltic other than in the Ohesaare borehole, but included a N . nutans biozone in the upper Paadla Stage of Saaremaa (Sarv, 1982). In Britain, N . nutans occurs in the Lower Leintwardine Formation (Siveter, 1989), but other species recorded in the Ohesaare borehole are not represented. lchthyoliths A microvertebrdte biozonation for the Silurian of the northern East Baltic has been established by Marss (1982Marss ( , 1990, and all the zonal species for the Rootsikiila, Paadla and Kuressaare stages are represented in the Ohesaare borehole (Mars, 1986;see Fig. 5). Logania martinssoni (Gross) occurs in the Rootsikula and lowermost Paadla stages, and has also been found in the Halla, Mulde, Klinteberg and H e m e beds of Gotland (Fred->50 specimens 1 3 6 5 0~ holm, 1990). Phleholcpis ornuta MCrss characterizes strata assigned to the upper Sauvere Beds by M a r s (1986); this species has not been found in the other boreholes, but occurs in the lower Hemse Beds on Gotland (Fredholm, 1988). Phlebo-Iepis elegans Pander ranges from the upper Sauvere Beds to the lowermost Uduvere Beds, and has been recorded widely in Estonia and Latvia. Andreolepis hedei Gross is restricted to the Uduvere Beds, and is also known from the upper Hemse Beds on Gotland (Fredholm, 1988(Fredholm, , 1989 and the Long Quarry Beds (lowermost Piidoli) of South Wales (Marss, 1982).

CORRELATION
The evidence given by conodonts, chitinozoans and icthyoliths for correlations is not always consistent. An attempt to provide the most parsimonious correlation between the succession on Saaremaa and those of the Ludlow type area and of Gotland is shown in Fig. 9. The correlation with Gotland is based on the recent conodont work by Jeppsson et al. (1994), who confirmed the presence of gaps in the exposed Saaremaa sequence. One of these gaps is between the Jaagarahu and Rootsikula stages, embracing strata referred to the upper Slite Beds, Mulde Beds and Halla Beds on Gotland. This results in only part of the 0. b. bohemica Biozone being recognized on Saaremaa, here characterized by the p morph.
The gap between the Rootsikula and Paadla stages on Saaremaa corresponds to the uppermost Klinteberg Beds and lower Hemse Beds of Gotland (Jeppsson et al., 1994). Chitinozoan evidence, particularly the presence of C. latgfions in the Ohesaare borehole, indicates that at least the upper part of the Gorstian Stage is represented on Saaremaa; the occurrence of Phlebolepis ornata is consistent with this. The hiatus is probably of greater significance in shallower facies away from Ohesaare, where P. ornata has not been found, and Kaljo (1990, p. 26) has suggested that the entire Gorstian may be lacking in the shoreward region. The conodont 0. s. cf. snajdri and the ostracod N . nutans have also been found only in the Ohesaare borehole.
Jeppsson et al. (1994) also determined the presence of a gap between the Paadla and Kuressaare stages, equivalent to the upper Hemse and Eke beds of Gotland. Part of the evidence for this is the absence of Polygnathoides siluricus Walliser on Saaremaa, although this species is rare in most facies and its distribution may have been environmentally controlled. However, there is a marked change in conodonts at the Paadla/ Kuressaare boundary on Saaremaa, with C. dubius, 0. excavata and 0. crispa disappearing, and a fauna characterized by 0. r. eosteinhornensis, s. I., 0. confluens, Oulodus elegans (Walliser) and 0. snajdri parasnajdri appearing. Uppermost Ludlow and lowermost Downton conodont faunas from the Welsh Borderland have recently been documented in detail by Miller (1995). He noted that the top of the Upper Whitcliffe Formation in the Ludlow area is characterized by C. dubius and 0. s. snajdri, with less common 0. r . eosteinhornensis and 0. cf. crispa. At Ludlow itself, 0. cf. crispa rnges 30-15cm below the top of the formation; the specimens compare with the a3 morph described in this paper and found in the upper Paadla Stage on Saaremaa. Miller (1995), however, also recovered 0. crispa from the lower part of the Whitcliffe Formation at Tite's Point, Gloucestershire, and from the lower part of the Upper Perton Beds at Woolhope; these specimens have a narrow furrow on the upper surface and are similar to the p morph of Walliser & Wang (1989).
There is a problem in using the appearance of 0. crispa in correlating the Saaremaa succession with that of the type Ludlow, as the species is found throughout the upper Sauvere Beds, the Himmiste Beds and the Uduvere Beds. This part of the sequence is usually correlated with the middle part of the Ludlow Series. A revision of this correlation would contradict the evidence from the thelodont P. elegans, which has been found by RJA in the Upper Bringewood Formation of the Welsh Borderland, but co-occurs with 0. crispa in the Sakla Borehole. An explanation may be that the fluctuating nearshore environment of Saaremaa was suited to the earlier evolution of populations of 0. crispa, which spread into the Welsh Borderland with the onset of appropriate conditions. Further stratigraphical evidence, perhaps from acritarchs or from chemostratigraphy, is required to clarify this question.
The base of the Piidoli Series is commonly identified by the appearance of the ostracod Frostiella groenvalliana Martinsson (Hansch & Siveter, 1994), which occurs in the basal Downton Castle Sandstone Formation of Ludlow (Bassett et al., 1982;Miller, 1995) and in the lower part of the Kaugatuma Stage on Saaremaa (Sarv, 1982). However, the conodont fauna of the Kuressaare Stage suggests a Piidoli age, or at least an age postdating the highest Upper Whitcliffe Formation at Ludlow. One explanation that fits all the fossil data would be the existence of a gap in the succession at Ludlow at the top of the Upper Whitcliffe Formation, as intimated by Jeppsson (1983, p. 137), equivalent to the Kuressaare Stage of Estonia and encompassing the highest part of the Ludlow Series. Bassett et al. (1982, p. 6), however, while noting that the basal contact of the Downton Castle Sandstone Formation is sharp and scoured, considered there to be no indication of a major break in sedimentation at this level.