Ordovician eunicid polychaetes of Estonia and surrounding areas: review of their distribution and diversification

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Abstract

Scolecodonts, the jaws of polychaete worms, are common and diverse palynomorphs in the Ordovician rocks of Estonia and surrounding areas. Some 120 apparatus-based species representing about 40 genera have been recorded thus far. Relatively long stratigraphical ranges of the majority of species reflect a low rate of evolution of jawed polychaetes. However, some individual species, as well as structural changes in the assemblages, appear to be useful for stratigraphical purposes. Environmental events like those in the middle Caradoc and late Ashgill had some impact on polychaete faunas, but less than on several other groups. In order to study the spatial distribution of eunicids, faunas of particular intervals of the Ordovician were investigated. Quantitative analysis revealed that polychaete assemblages with a very consistent qualitative and quantitative composition were widespread over long distances within the belts of similar facies conditions in the Baltoscandian Palaeobasin. On the other hand, it appears that species of Ordovician jawed polychaetes were strongly influenced by particular facies, and accordingly well differentiated along the palaeobasin gradient. The decrease in diversity and abundance towards the deeper-water part of the palaeobasin indicates that the majority of Ordovician eunicids preferred relatively shallow-water conditions. The increase in differentiation of environments is accompanied by an increase in differentiation of polychaete assemblages.

Introduction

Scolecodonts, the jaws of polychaete annelids of the Order Eunicida, are abundant and diverse acid-resistant microfossils beginning from the Ordovician. Although they represent the main palaeontological evidence of fossil polychaetes, the knowledge of scolecodonts is relatively scarce. Thus the aspects of the distribution and diversification of fossil polychaetes have so far been discussed in few papers (e.g. Kielan-Jaworowska, 1966, Bergman, 1989, Hints, 1998).

One of the reasons why the scolecodonts have gained rather little attention is the misconception that their systematics are hopelessly confused. The modern classification of fossil jaw-bearing polychaetes introduced by Lange, 1947, Kozlowski, 1956 and especially by Kielan-Jaworowska, 1961, Kielan-Jaworowska, 1962, Kielan-Jaworowska, 1966 is based on the architecture of the jaw apparatus; in every species this comprises several differently shaped elements. Although more complicated than the alternative element-based form classification, the apparatus-based taxonomy is close to the natural classification of polychaetes, as revealed by comparison with Recent material, and therefore the only approach to study the evolution and distribution of fossil polychaetes. It has also been shown in several papers (Bergman, 1989, Bergman, 1991a, Eriksson, 1997, Eriksson and Bergman, 1998, Eriksson, 1999) that the problems when following the nomenclature rules (ICZN) in fossil polychaete taxonomy are not as critical as previously suggested by e.g. Kielan-Jaworowska, 1968, Jansonius et al., 1971, Szaniawski and Wrona, 1973.

The Baltic area has been among the primary study areas for many fossil groups, and polychaete jaws are not an exception — some of the first reports on scolecodonts came from Estonia and Gotland (Eichwald, 1854, Pander, 1856, Hinde, 1882). Ordovician polychaete jaws are comparatively well known in the Baltic region. Polish researchers (e.g. Kozlowski, 1956, Kielan-Jaworowska, 1961, Kielan-Jaworowska, 1962, Mierzejewski, 1978) studied mainly the material from erratic boulders, but scolecodonts have also been encountered in Estonia and NW Russia (Hints, 1996, Hints, 1998, Hints et al., 1997), Sweden (Bergman, 1989), Norway (Harper et al., 1984) and subsurface of Poland (Szaniawski, 1970). Apart from northern Europe, Ordovician scolecodonts have been described in large numbers from North America (Stauffer, 1933, Eller, 1942, Eller, 1945, Eller, 1969). For other parts of the world the knowledge of Ordovician scolecodonts is as yet remarkably poor, though their occurrence in particular regions has usually been corroborated.

More than 30 years of investigation of Ordovician organic-walled microfossils in Estonia has resulted in a large collection of dispersed scolecodonts, and numerous articulated jaw-apparatuses. The material was investigated by Ralf Männil, but nothing on scolecodonts was published at that time. In recent years, I have re-studied parts of the old collection, and as well have sampled many additional sections with particular focus on the polychaete jaws. These latter samples have made possible the application of quantitative study methods, and thus a more detailed look at particular polychaete assemblages. The first results of this study have already been published (Hints, 1998, Hints, 1999b).

The aim of this paper is to provide an extended version of the overview of Ordovician scolecodonts of Estonia and surrounding areas that was initially presented in the Eighth International Symposium on the Ordovician System and published in the corresponding abstracts volume (Hints, 1999a). It should be emphasized, however, that the study of the present collection, as well as of additional material from the Baltic area, is currently still in progress. Several taxonomic groups need specific study, and certain intervals of the Ordovician are yet inadequately investigated. Therefore, the data presented below should be viewed as preliminary rather than a solidly based final analysis.

Section snippets

Geological setting

The studied localities are located within the limits of the Baltoscandian Palaeobasin, a large epicontinental sea which occupied a wide area of the Baltica craton during the Ordovician. This palaeobasin provided a variety of different environments, ranging from lagoons and near-shore carbonate build-ups to deeper-shelf depressions. According to the large-scale zonation of the palaeobasin by Männil (1966), northern and central Estonia, the St. Petersburg area and eastern Lithuania constituted

Material and methods

Most of the material here summarized derives from 10 drill cores and two outcrops in Estonia. The Estonian material is compared with that of specific stratigraphical intervals of some localities from the surrounding areas in NW Russia, Latvia and Poland. The most important study localities are shown in Fig. 1. Altogether more than 50 000 detached polychaete jaws and many tens of differently preserved jaw apparatuses were extracted from the samples collected especially for scolecodonts. The

Notes on the taxonomic composition

Approximately 120 apparatus-based species of eunicids belonging to some 40 genera and 13 families have been recorded from the Ordovician of Estonia and the neighbouring areas thus far. The overall number of species is likely to grow when the group will be studied further, possibly up to about 200. All five types of polychaete jaw apparatuses so far distinguished (for review see Szaniawski, 1996) are represented in the collection studied.

The most common genera, in terms of the number of species,

Acknowledgements

D. Kaljo, M. Eriksson and C.F. Bergman kindly commented upon the initial manuscript. H. Szaniawski provided access to his collection from the Mielnik borehole and also made useful suggestions about the manuscript. J. Jansonius and B. Courtinat acted as reviewers and improved the paper in different ways. I am indebted to all of them. This study is a contribution to IGCP Project 410 and was supported by the grants No. 3751 and No. 3516 of the Estonian Science Foundation.

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