‘Nodibeyrichia jurassica’ and associated beyrichiacean ostracode species and their significance for the correlation of late Silurian strata in the Baltic and Britain

Taxonomic revision reveals that the beyrichiacean ostracode ‘Nodibeyrichia jurassica’ can no longer be regarded as an index species for the uppermost (late Přídolí) ostracode ‘zone’ of the Silurian of the Baltic region embracing Estonia, Latvia and Baltic-floor derived erratic boulders. The taxon Nodibeyrichia jurassica Sarv, 1968 (non Gailite 1967) is regarded as a junior subjective synonym of Beyrichia protuberans Boll, 1862. Moreover, the material hitherto assigned to ‘Nodibeyrichia jurassica’ is herein considered to belong to two species: Nodibeyrichia protuberans (Boll, 1862) and Nodibeyrichia verrucosa Shaw, 1969. N. verrucosa occurs in England and Estonia, in the basal part and upper part of the Přídolí Series respectively. N. protuberans, as herein restricted, can be used to recognize the late Přídolí, uppermost ostracode assemblage level (N. jurassica Zone of previous literature) of the Silurian only in the central East Baltic (Latvia) and in erratic boulder material found in southern Baltic areas.

the 'species' N. jurassica required revision and its value as an index species for the recognition and correlation of the late Piidoli needed to be re-evaluted. The aim of this paper is to address both of these points. In doing so we also resolve the primary and complicated nomenclatorial problem of the availability, authorship and date of publication of the binomen Beyrichia jurassica Gailite, 1965 (nomen nudum).
We herein show that the valid binomen, authorship and date for the species in question is Nodibeyrichia jurassica Sarv, 1968, which we consider to be a junior subjective synonym of Nodibeyrichia protuberans (Boll, 1862). We also conclude that the late P'idoli Estonian ostracodes previously assigned (e.g. by Sarv, 1968) to 'Nodibeyrichia jurassica' are conspecific with the early P'i'doli British species Nodibeyrichia verrucosa Shaw, 1969. The hitherto so-called Nodibeyrichia jurassica (= Nodibeyrichia protuberans) Zone is now restricted to only the late PEdoli of the central (Latvia) and southern ('Beyrichienkalk' S.S. erratic boulders) parts of the Baltic. The 'Zone' cannot be used, as hitherto, to also embrace the late PEdoli strata in Estonia.
Of the borehole records from Latvia, it is possible to recognize N . protuberans or its synonyms with confidence only from the Piltene 1 borehole, the only locality for which there is figured material (Gailite, 1967). Other records, from similar horizons in Latvia (boreholes Piltene 31,32, Kolka 4, 54 and Ventspils), probably represent conspecific material though it is not possible to be as confident in such cases; accordingly, this qualification is reflected in the comments in both the synonymy above and 'Occurrence' below. Discussion. Morphological features characteristic of Nodibeyrichia are less evident in N . protuberans than in other congeneric species. Its syllobium is entire and its anterior lobe is not completely isolated from its other lobes nor, except in small tecnomorphs (PI. 1, figs 10, l l ) , does that lobe show any sign of an anteroventral lobule. In many respects N. protuberans shows affinity with the beyrichiines Beyrichia (Beyrichia) and Beyrichia (Simplicibeyrichia). However, in that N. protuberans differs from such taxa by having a tuberculate rather than striate subcruminal field, a lack of lobular spines and its albeit faint recall of an anteroventral lobule the species is best considered as a morphologically simple end member of Nodibeyrichia.
N. uerrucosa differs from the upper Piidoli N. protuberans (as restricted herein) chiefly by its bicuspidate syllobium and the presence on its anterior lobe of a ventral lobule and dorsal cusp. In addition, N. verrucosa tends to have a more forwardly inclined preadductorial node. The British material of N . uerrucosa occurs in sandstones and siltstones (e.g. The Whitcliffe road sections; see Siveter et al., 1989); N. protuberans, as restricted herein, is known from dolomitic marl and limestone strata from the Baltic (Piltene 1 borehole, Gailite, 1967;Kolka 4 borehole, Gailite, 1972;Ventspils borehole, Bassett et al., 1989; Beyrichienkalk ostracode association D of Hansch, 1985: 281). Thus, as both this particular British and Baltic material occur in intertidal to shallow subtidal marine to possibly marine influenced/brackish, regressive sequences, along with concomitant faunas of a broadly similar aspect (relatively impoverished, but containing non-palaeocope ostracodes and fish), it seems less likely that their morphological differences represent merely ecophenotypic, intraspecific variation. Indeed, the Estonian material herein also assigned to N. verrucosa comes from sparitic limestones of a high-energy shoal belt and from biomicritic limestones of a faunally rich and diverse, open marine shelf (Ohesaare cliff section; Nestor, 1990: 177, fig. 53) thus representing quite different environments to that of the presumed conspecific British material. Occurrence. Piidoli Series, upper Silurian; Welsh Borderland and Estonia. In the Welsh Borderland from the Ludlow (D.J.S. collections, see Siveter, 1978;Bassett et al., 1982) and Downton (Shaw, 1969) areas, Shropshire, and the Long Mountain region (Shaw, 1969) to the west. Collected by D.J.S. from the Ludlow Bone Bed Member (locs 69a, 69b of Siveter, 1980; = not 'Topmost Whitcliffe Beds' cf. Siveter, 1980: 8, in errore), Downton Castle Sandstone Formation (see Bassett et al., 1982). Shaw's (1969) Downton area material is from the Platyschisma Shale Member, Downton Castle Sandstone Formation. The Estonian material is from the upper Piidoli Ohesaare 'Stage', Isle of Saaremaa (Sarv, 1968(Sarv, , 1970(Sarv, , 1971Meidla & Sarv, 1990;Nestor, 1990), where it has also been collected from the Ohesaare cliff section by the present authors.

BIOSTRATIGRAPHICAL SIGNIFICANCE OF TAXONOMIC REVISION
These new taxonomic opinions regarding N . protuberans ('N. jurassica') and N. uerrucosa necessitate a revision of part of the informal ostracode 'zonal' scheme which has long been applied to the Upper Silurian of the East Baltic area and associated Beyrichienkalk drift boulder sequences. Fig.  1 outlines such a revised scheme for the various parts of Baltoscandia and the faunally associated area of Britain.
In the Baltoscandian area in general the oldest and most widespread ostracode assemblage of the basal part of the Piidoli Series is the Frostiella groenvalliana association, which can be traced into correlatives in Britain and eastern North America ( Fig. 1; see Siveter, 1989 andHansch et al., 1990 for a summary of its distribution). The F. groenvalliana association is succeeded by a number of ostracode faunas which encompass the Beyrichienkalk S.S. faunas of traditional literature. Of the latter, the most widespread ostracode associations in the literature are the successive faunas characterized by Nodibeyrichia tuberculata and, in the late Piidoli, either Nodibeyrichia pustulosa (gedanensis) or 'N. jurassica' depending on the area of Baltoscandia in question (for example, see Martinsson, 1967, Siveter, 1978, 1989, Hansch, 1985, 1993). The two faunas with F. groenvalliana and N . pustulosa respectively are both recognized from Britain, the former from outcrops of
Neoheyrichia regnans in the upper Ludlow Siedlce beds of Poland (cf. Tomczykowa & Witwicka, 1974) is based on Zbikowska (1973) and Martinsson (1964Martinsson ( , 1967 possible 'quasi-marine' deposits in an overall regressive the Silurian of the northern and central East Baltic (Estonia sequence and the latter from fully marine deposits in a and Latvia respectively; land outcrop and boreholes) and borehole.
The taxonomic revisions herein have the following consequences for the Upper Silurian ostracode biostratigraphy in the Baltic-British ostracode faunal region: 1. The 'Nodibeyrichia jurassica Zone' should be termed the Nodibeyrichia protuberans Zone. 2. Nodibeyrichia protuberans, and therefore its 'Zone', can still be identified from the central East Baltic (Latvia) and from the submarine floor of the Baltic (Beyrichienkalk S.S. erratic boulder sequence). 3. Nodibeyrichia protuberans, and therefore its 'Zone', can no longer be identified from the Silurian of the northern East Baltic (Estonian) sequence . 4. Nodibeyrichia verrucosa is no longer confined to the lower Piidoli Downton Castle Sandstone Formation of the Welsh Borderland. Conspecific material also characterizes the late Piidoli Ohesaare Formation (Ohesaare Regional 'Stage') of Estonia. 5. Considering the ostracode faunal succession of the East Baltic area in general, it is possible to recognize three, broadly defined, successive faunas which are relatively widespread: characterized by the beyrichiaceans F. groenvalliana, N. tuberculata and N. pustulosa repectively (Fig. 1 herein; see also Hansch, 1993, in press for a summary). The oldest and youngest of these three associations are also present in Britain (Siveter, 1978(Siveter, , 1989.