BIOSTRATIGRAPHY AND TAXONOMY OF THREE-DIMENSIONALLY PRESERVED NEMAGRAPTIDS FROM THE MIDDLE AND UPPER ORDOVICIAN OF BALTOSCANDIA

Journal of Paleontology, Mar 2007 by N�lvak, Jaak, Goldman, Daniel

ABSTRACT-

Biostratigraphically significant finds of two important species of Nemagraptus in the Middle and Upper Ordovician carbonate sections of Baltoscandia are reviewed. Nemagraptus subtilis is found in the lower part of the Uhaku Regional Stage, which is correlated with the Hustedograptus teretiusculus graptolite zone. Nemagraptus gracilis is restricted to the upper part of the Kukruse Stage in the Central and South Estonian and western Latvian sections. This level correlates with the Nemagraptus gracilis graptolite Zone. The vertical distribution of these taxa is similar to, although less complete than, their ranges in the classic black shale succession in Scania, southern Sweden. The first three-dimensionally preserved specimens of N. subtilis are illustrated and described. Nemagraptus subtilis can be differentiated from N. gracilis by its lack of cladial branches, more slender stipes, shorter sicula, and upward orientation of the first two thecae.

INTRODUCTION

NEMAGRAPTID GRAPTOLiTES are a particularly important group of fossils for Ordovician biostratigraphy. Recently, the International Subcommission on Ordovician Stratigraphy designated the first appearance datum (FAD) of Nemagraptus gracilis (Hall, 1847) as the level of the base of the Upper Series of the Ordovician System (Bergstr�m et al., 2000). Although Nemagraptus gracilis is a cosmopolitan taxon that has been recorded from Upper Ordovician black shale successions on nearly every continent, it and other nemagraptids are very rare in carbonate facies. Finney (1985) provided the only detailed descriptions of three-dimensionally preserved nemagraptids based on specimens isolated from the Athens Shale of central Alabama, USA.

Nemagraptids are well known from cores and outcrops in Scania, southern Sweden, where the classic black shale section at Page! sang [the Global Stratotype section and Point (GSSP) for the base of the Upper Ordovician Series] contains a rich graptolite fauna that spans the Hustedograptus teretiusculus to Diplograptus foliaceous (=D. multidens) graptolite Zones (Hede, 1951; Nilsson, 1977; Bergstr�m et al., 2000). Northeast of Scania, however, the dark shales grade rapidly into the predominantly carbonate facies of the North Estonian and Central Baltoscandian confacies belts (Fig. 1). Thus, correlations between the East Baltic region and Scania are complicated by biofacies differences.

The distribution of Nemagraptus Emmons, 1855 in the carbonate rocks of the East Baltic area has never been comprehensively reviewed. M�nnil (1976, figs. 2-4; 1986, fig. 2.1.1) provided some preliminary data on occurrences of Nemagraptus from some Estonian and Latvian sections, but without species-level identifications. The senior author has dissolved several thousand Middle and Upper Ordovician limestone samples from numerous sections in the North Estonian and Central Baltoscandian confacies belts (Fig. 1) to obtain acid-resistant microfossils (mainly chitinozoans). During this work, rare fragments of two species of Nemagraptus were also isolated. These specimens are important for a variety of reasons. They are useful in correlating the East Baltic carbonate sections with the black shale successions in Scania and other parts of the world. They help establish biostratigraphic ties between graptolites and the other fossil biozones that are more commonly used in platform-facies rocks, primarily chitinozoan and conodont zones. Perhaps most importantly, they also serve as the basis for only the second description of three-dimensionally preserved nemagraptids, and the first isolated three-dimensional specimens of the oldest species of Nemagraptus, N. subtilis Hadding, 1913.

BIOSTRATIGRAPHIC SIGNIFICANCE OF THE EAST BALTIC OCCURRENCES OF NEMAGRAPTUS

Conodonts, chitinozoans, graptolites, ostracods, and to a lesser degree trilobites, cephalopods, and brachiopods have all been used with varying success for the correlation of the Ordovician strata of Baltoscandia (N�lvak and Grahn, 1993). One of the most intractable problems has always been establishing correlations between the carbonate facies (North Estonian and Central Baltoscandian confacies belts) of the East Baltic area with the black shales of the Scanian confacies belt (see Fig. 1), and with other graptolite successions in general. Thus, the discovery of Nemagraptus specimens in the carbonate facies of the East Baltic area is significant in that it allows us to directly test previous correlations between the primarily dark shale graptolite biozonations, the regional Estonian stages, and the chitinozoan and conodont biozonations.

Specimens of Nemagraptus collected from the carbonate rocks of the East Baltic region occur at three different stratigraphie levels within the Uhaku and Kukruse regional stages, and are biostratigraphically important for the local subdivision (Figs. 1, 2). The lowermost samples that contain Nemagraptus are from the uppermost Gymnograptus linnarssoni-beanng beds and from just above them (Fig. 2). These specimens of Nemagraptus are all referable to N. subtilis Hadding (Table 1 ). At F�gels�ng, and other sections in Scania, N. subtilis and G. linnarssoni (Moberg, 1896) are restricted to the H. teretiusculus graptolite Zone (Hede, 1951; Nilsson, 1977; Bergstr�m et al., 2000). In the East Baltic region and the Siljan District of Sweden, Gymnograptus linnarssoni has a range that is restricted to the lower Uhaku Stage (M�nnil, 1976, fig. 2). Thus, the lower part of the Uhaku Stage in North Estonia, western Latvia, the Siljan District of Central Sweden, and the Leningrad District of Russia (all localities that have yielded specimens of W. subtilis) can be correlated with H. teretiusculus Zone rocks in Scania and other black shale successions (Jaanusson, 1960).