Ordovician Conodont Diversity in the Northern Baltic
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Estonian Journal of Earth Sciences, 2012, 61, 1, 1–14 doi: 10.3176/earth.2012.1.01 Ordovician conodont diversity in the northern Baltic Peep Männik and Viive Viira Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; [email protected], [email protected] Received 1 June 2011, accepted 12 December 2011 Abstract. The diversity data presented in this paper are based on recently revised collections of conodonts from outcrops (northern Estonia) and core sections of Estonia and western and northern Latvia. Based on variations in the abundance of taxa, four intervals with different general trends in diversity changes can be recognized in the Ordovician part of the succession: (1) the proavus–crassus zones and (2) crassus–anserinus zones, both intervals forming quite distinct diversity cycles; (3) the variabilis– ventilatus zones, characterized in general by stable diversity values, and (4) the ventilatus Zone–the topmost Ordovician, with a steady increase in diversity up to the ordovicicus Zone, followed by a decline during the End-Ordovician Extinction Event. Diversity was lowest in the earliest and latest Ordovician but reached maximum values at the end of the Early Ordovician radiation of conodonts, in the middle and upper Darriwilian, and just before the start of the End-Ordovician Extinction Event. Four supersequences (transgressive–regressive cycles) are proposed here for the northern Baltic Ordovician succession. The diversity changes recognized in the conodont succession demonstrate general correlation with these supersequences: boundaries between supersequences are characterized by low diversity values; diversity increases more or less rapidly in the lower, transgressive parts of the supersequences and decreases in their upper parts. Key words: Ordovician, northern Baltic, biodiversity, conodonts. INTRODUCTION Russia; Fig. 1). Additionally, the distribution of cono- donts in 12 outcrop sections exposing Lower to Middle Ordovician conodonts from Estonia have been known Ordovician strata and 4 exposing Upper Ordovician since the discovery of the fossils by C. H. Pander more strata were analysed. Both published and unpublished than 150 years ago. Systematic studies of Ordovician data were used. In several earlier publications (Viira conodonts in the northern Baltic started in the middle of 1966, 1967, 1974) conodont identifications were based the last century (Viira 1966, 1967, 1974). Since then, on form taxonomy. Before using these data the collections large collections of conodonts from all stratigraphic were restudied to update the taxonomy. Other older intervals represented in the region, and from all facies identifications were checked as well. available for study, have been collected. The abundance The oldest conodonts in Estonia come from the of specimens and richness of taxa are highly variable in upper Cambrian, from sandstones and shales of the time and space. Several radiation and extinction events lower Pakerort Stage exposed in North Estonia (Heinsalu have been recognized in the succession of Ordovician et al. 1987, 1991, 2003; Viira et al. 1987; Kaljo et al. conodont faunas in Estonia (Männik 2006). Recently, 1988; Mens et al. 1989, 1996). Conodonts from the some general analyses of variations in the biodiversity clays of the Varangu Stage (middle Tremadocian) were in Ordovician faunas, including conodonts, have been studied by Viira (1970) and Viira et al. (1970). Data published (Hammer 2003; Albanesi & Bergström 2004; on conodonts in the Hunneberg and Billingen stages Wu et al. 2010a, 2010b; Goldman et al. 2011; Sheets (late Tremadocian–Floian) come mainly from the Pakri et al. 2011). The aim of the present paper is to give (in western North Estonia) and Saka (in eastern North a brief overview of the diversity changes recognized Estonia) outcrop sections (Löfgren et al. 2005; Viira et in the Ordovician conodont succession in the northern al. 2006b; Fig. 1). Detailed data on conodont faunas Baltic (in Estonia and Latvia). in the upper Tremadocian to the upper Darriwilian interval were obtained from the Mäekalda sections located close to Tallinn (Viira et al. 2001; Fig. 1). Only a small MATERIAL AND METHODS part of conodont data from the Upper Ordovician outcrops in Estonia is published (Hints et al. 2004, This study is based on material from 31 drill core sections 2005). The information on Ordovician conodonts in drill (25 from Estonia, 4 from Latvia and 2 from westernmost cores was obtained mainly from recent publications 1 Estonian Journal of Earth Sciences, 2012, 61, 1, 1–14 22 30' 25 27 30' 6 7 3 5 8 4 9 2 FINLAND Toolse(420) 1 Tapa 13 Tallinn Kerguta(565) 11 12 Taga-Roostoja(25A) 10 ESTONIA SWEDEN Vodja(H-190, H-191) RUSSIA HIIUMAA Põhjaka-Saare(H-171) Ruskavere(451) Äiamaa Kirikuküla Aidu(427) Laeva(18) RUSSIA SAAREMAA Are(171) Viljandi Kaagvere Viki Seliste(173) Mehikoorma(421) Ohesaare Ruhnu(500) Karula(320) Petseri(330) Legend Puikule Valga(10) Tsiistre(327) Sturi Dekshino(328) outcrop section Laanemetsa(70) 57 30' Hino(452) borehole 0 50 100 km LATVIA Stirnas(18) Riga Adze(6) 22 30' 25 27 30' Fig. 1. Location of the studied sections (only those exposing Ordovician strata are indicated). Numbers in circles: 1, Pakri (I) and Pakri (II); 2, Mäekalda (I) and Mäekalda (II); 3, Tõnismägi; 4, Suhkrumägi; 5, Ülgase; 6, Vihula; 7, Varangu; 8, Saka (I) and Saka (II); 9, Orasoja; 10, Paluküla; 11, Vasalemma; 12, Porkuni; 13, Ussimägi. (Taga-Roostoja (25A): Viira & Männik 1999; Valga (10), (Männik 2001, 2003; Hints et al. 2010). In northern Ruhnu (500), Viki: Männik 2001, 2003, 2010; Mehi- Estonia this interval corresponds to a gap. In the koorma (421): Männik & Viira 2005; Kerguta (565): Stirnas (18) core section (Hints et al. 2010), the taxa Viira et al. 2006a; Stirnas (18): Hints et al. 2010). occurring in this interval but represented by redeposited Additionally, unpublished material from several core specimens were not counted. The stratigraphical chart sections (Fig. 1) was used. Although the current paper used in this study is based on Nõlvak et al. (2006). For deals mainly with Ordovician faunas, data from the general correlation time slices sensu Webby et al. lowermost Silurian were included to highlight the faunal (2004) were also included in the figure. However, as the changes caused by the End-Ordovician Event. location of the time slice boundaries in a particular In this paper the focus of the diversity study is on section is often problematic, these units were not used taxonomic richness as adequate abundance data of the as a basic stratigraphical framework in our study. taxa are not available at the moment (specimens are not The diversity measures used in this paper follow counted). The total numbers of species and genera, and Cooper (2004). Total diversity is the number of species the numbers of the appearing and disappearing species occurring in a stratigraphical unit recognized. Normalized were counted per stratigraphical unit recognized (Fig. 2). diversity is the sum of species that range from the To subdivide the succession, the smallest available, and interval below to the interval above, plus half the recognizable all over the studied region, conodont-based number of species that originate and/or become extinct biostratigraphical units (zone, subzone or an informal within the interval, plus half of those that are confined unit: e.g. ‘Uppermost Baltoniodus range’, ‘Noixodontus to the interval itself. The origination and extinction fauna’; Männik 2007) were applied. In some intervals ‘rates’ are calculated for each interval by dividing the application of stage slices sensu Bergström et al. (2009) number of species appearing or disappearing in the in addition to the conodont-based units allowed a more interval with the total number of them recorded in the detailed subdivision of the strata (e.g. in the ordovicicus unit. We did not apply any method which requires Zone). Data for the uppermost Ordovician, for the geochronological dating of strata, as such datings of interval above the ‘Noixodontus fauna’ come from finer units are still too speculative. drill cores from southern Estonia and western Latvia 2 P. Männik and V. Viira: Ordovician conodont diversity TRENDS IN THE FAUNAL CHANGES Acodus, Drepanodus, Drepanoistodus, Oistodus, Scolopodus) and by the appearance of new, more In total, the Ordovician conodont fauna in the northern complicated types of apparatus in which elements have Baltic comprises, as presently known, 177 species denticulated processes (Prioniodus, Oepikodus, Periodon, belonging to 64 genera (Fig. 2). The distribution of taxa Trapezognathus) (Fig. 2). Also Baltoniodus that appears in the stratigraphic succession is variable, in abundance higher in the section, in the triangularis–navis Zone, of specimens as well as in the number of taxa. Diversity represents this type of apparatus. At the end of the Early is lowest in the lowermost and topmost Ordovician Ordovician radiation, after its maximum in originalis (Fig. 3). The total diversity of species reaches maximum time, normalized species diversity decreased rapidly and values at the end of the Early Ordovician radiation of reached a minimum for this interval in the crassus Zone conodonts (in the evae Zone), in the suecicus Zone and (Fig. 3). At the same time, decline in normalized genus just before the start of the End-Ordovician Extinction diversity was minor. Total species and genus diversities Event (in the ordovicicus Zone). Diversity is lower started to decrease earlier, after their maximums in between these three levels, lowest in the lower the upper evae Zone, and reached minimum values, Darriwilian (in the crassus Zone) and somewhat higher respectively, in the crassus Zone and in the anti- and slowly but continuously increasing in the main variabilis–variabilis interval. This is the largest decline part of the Upper Ordovician (from the variabilis Zone in conodont species diversity recorded in the conodont below up to the ordovicicus Zone above). Changes in succession between the Tremadocian and the Katian species and genus diversity follow similar trends, although in the northern Baltic.