Chitinozoan diversity in the East Baltic Silurian

Data on the diversity of the East Baltic Silurian chitinozoans have been compared to global chitinozoan biozones that are approximately correlated with the graptolite scale. The total diversity and balanced total diversity curves have three main positive peaks in the Wenlock and Ludlow and diversity lowstands in most of the Llandovery and Pridoli. The turnover ratio and species origination curves are lowest and species extinction is highest in the lower Wenlock, correlating partly with the Ireviken Event. Chitinozoan diversity is related to the transgressionregression phases of the Silurian Baltic basin, water chemistry and temperature, and evolutionary processes of the faunal group. At the level of the Silurian standard stages the trends in the East Baltic and Swedish chitinozoan diversity curves are in general outline similar to the global diversity curve.


INTRODUCTION
The regional dataset, including species diversity, and origination and extinction events of the Ordovician chitinozoans, shows similar patterns in all palaeocontinental plates (Paris et al. 2004).The main feature of chitinozoan diversification is the radiation that began in the earliest Ordovician, developed rather regularly up to the latest mid-Ordovician, and decreased progressively during the Late Ordovician (Paris et al. 2004).A similar trend characterizes East Baltic chitinozoan diversity patterns throughout the Silurian (Fig. 1).The evaluation of physico-chemical factors affecting Ordovician chitinozoan diversity and comparison of the diversification patterns with other benthic and pelagic fossil groups are given by Achab & Paris (2007).Kaljo et al. (1995) have discussed the most important Silurian bio-events and compared diversity changes in different groups and regions.The first study of Silurian chitinozoan diversity dynamics in the East Baltic sections revealed considerable changes in diversity (Nestor 1992).In that paper an attempt was made to integrate chitinozoan zonation with the graptolite scale, using graptolite data from the Ohesaare and Ventspils cores (Kaljo 1970;Kaljo et al. 1984;Gailite et al. 1987).Later studies showed, however, that several graptolite biozones are probably missing due to stratigraphic gaps in the East Baltic Silurian sequence, and besides, the precise zonal boundaries are usually indistinct due to insufficient graptolite data (Loydell et al. 1998(Loydell et al. , 2003)).In the present paper the Silurian East Baltic chitinozoan diversity diagram has been linked to the global chitinozoan bio-zonal succession (Verniers et al. 1995) (Fig. 1), correlating approximately with the graptolite scale (Fig. 2).

DATABASE AND METHODOLOGY
In this review only published materials are used, with the exception of Pridolian species, which are presently under restudy, as preliminary unpublished identifications have already been made.A total of 120 species of 22 genera derived from 2546 samples from the East Baltic drill cores (see, e.g., Nestor 1994Nestor , 2003Nestor , 2005Nestor , 2007Nestor , 2009;;H. Nestor & V. Nestor 2002;H. Nestor et al. 2003) will be discussed.Most taxa, published in open nomenclature, are excluded.For comparison, chitinozoan diversity data from Gotland (Laufeld 1974), Skåne (Grahn 1996), drill cores of Gotland (Grahn 1995) and mainland of Sweden (Grahn 1998) are considered below.
The diversity diagram (Fig. 1) has been compiled as in Paris et al. (2004).Besides total diversity of species, also balanced (or normalized) total diversity (BTD) is counted.Following Paris et al. (2004), a full score (equal to 1) is given to the species ranging through a whole time slice (biozone in this paper) or in case of short-ranging species to part of it.A half score (equal to 0.5) is given to species having their FAD or LAD within a time slice (biozone) and ranging into the succeeding or preceding time slice (biozone).
The turnover ratio has also been calculated according to Paris et al. (2004): the number of originations, minus the number of extinctions, divided by the BTD recorded in a zone.Extinction events have negative values, while  (Verniers et al. 1995).Asterisks show main chitinozoan extinction events.Rootsik., Rootsiküla.radiation events have positive values.The rates of species origination and extinction are calculated for each biozone by dividing the number of originations or extinctions by the total number of species recorded in the corresponding zone.

TAXONOMIC DIVERSITY AND TURNOVER
As the duration of global chitinozoan biozones, compared with the graptolite scale, is different (Fig. 2), and duration of global stages in millions of years (Fig. 3) is also different, we can speak only about egneral diversity trends in the chitinozoan succession of the East Baltic sections.The total diversity and BTD curves show a similar trend throughout the Silurian (Fig. 1).Both curves have three main positive peaks, in the Angochitina longicollis, Conochitina pachycephala, and Angochitina elongata biozones.The highest total diversity established in the Margachitina margaritana Biozone might be related to the high number of the studied samples ( 461), but in the other cases (for example Euconochitina electa and Conochitina alargada biozones) no such dependence of high diversity on the high number of samples was observed.Besides the diversity lowstands in the Rhuddanian and in the upper part of the Pridoli, the most significant fall of diversity is documented at the Wenlock-Ludlow boundary, in the global Sphaerochitina lycoperdoides and in the East Baltic Ancyrochitina desmea and Conochitina postarmillata biozones (Nestor 2007).It is worth mentioning that the diversity curve of pelagic graptolites (Melchin et al. 1998) reveals a different pattern to that of chitinozoans.Intense radiation of graptolite taxa starts in the Rhuddanian, with a diversity peak in the early Aeronian, and is followed by an overall diversity decline from the early Telychian through the rest of the Silurian (Melchin et al. 1998, fig. 4).Contrary to graptolites, chitinozoan diversity increases from the early Telychian up to the late Homerian.Though, some increase in the diversity of both fossil groups is observed in the late Gorstian.The reason for the different patterns of chitinozoan and graptolite diversity trends is unclear, but the levels of large diversity drops (extinction events) are almost coincident, related to the Ireviken, Mulde, and Lau events (Fig. 1, see also V. Nestor et al. 2002;Nestor 2007Nestor , 2009)).The turnover ratio, species origination and extinction curves (Fig. 1) are more or less regular except the high peak in the M. margaritana Biozone (Fig. 1), correlating partly with the Ireviken Event level.On the contrary, the Mulde Event at the boundary of the C. pachycephala and S. lycoperdoides biozones (Nestor 2007) and the Lau Event at the boundary of the E. lagenomorpha and E. barrandei biozones (Nestor 2009) are not expressed in the diagram.The turnover ratio values close to 0 indicate that the numbers of originations and extinctions are more or less identical.The curve shows decline only in the upper part of the Silurian, in the Pridoli, related to the major extinction of chitinozoan species.

DIVERSITY AND ENVIRONMENTAL CYCLICITY
In Kaljo et al. (1998) mean values of the carbon isotope curves, global bathymetry, and glaciations have been tied to the generalized graptolite zones.To study the influence of these factors on chitinozoan diversity (Fig. 2), global biozones of the latter group were correlated with the generalized graptolite zones given in Koren et al. (1996).This correlation was compiled after Verniers et al. (1995), Loydell et al. (2003), and Nestor (2007, 2009).The eustatic curve from Kaljo et al. (1998, modified from Johnson et al. 1991) exhibits sea-level high-and lowstands in the Silurian (Fig. 2).The diversity of chitinozoans is clearly related to the transgressionregression phases.The high diversity values of chitinozoans from the Telychian to the lower Homerian and in the Gorstian are coeval with highstands of the global eustatic curve.The short-time sea-level fluctuations, connected with glaciations (Fig. 2), affected chitinozoan diversity, but are not expressed in the generalized diversity curve.
Carbon cycling is one feature of environmental cyclicity.In the East Baltic Silurian four main positive excursions in carbon isotopes have been recorded (Fig. 2).They occur usually during a low-diversity interval, after important extinctions of planktic organisms (Kaljo et al. 1998).This fits well with the severe chitinozoan extinction levels in the M. margaritana Biozone, followed by the Ireviken Event and in the uppermost C. pachycephala Biozone, followed by the Mulde Event at the lower boundary of the S. lycoperdoides Biozone.The Ludfordian extinction of chitinozoans takes place at the boundary of the E. philipi and E. barrandei biozones and is followed by the Lau Event (Nestor 2009).Loydell (2007) has pointed out several causes of positive δ 13 C excursions.Increased carbon burial, which is related to increased carbonate subareal weathering associated with glacial periods, as well as other causes of positive excursions, influenced chitinozoan biodiversity.These phenomena, however, have not been studied in detail yet.

THE REGIONAL DIVERSITY VERSUS GLOBAL DIVERSITY OF CHITINOZOANS
The East Baltic diversity pattern of chitinozoans, tied to the Silurian global standard stages and series, is shown in Fig. 3.The Silurian global chitinozoan diversity curve has been compiled by Paris using the CHITINOVOSP database, which was designed for recording all chitinozoan taxa described until the end of 1997 (Paris & Nõlvak 1999).In addition, in the present paper the distribution data of new chitinozoan species published between 1998 and 2008 have been considered.Absolute ages (My) of stage boundaries from the home page of the ICS (www.stratigraphy.org) have been used (Fig. 3).
The chitinozoan diversity curve for the Silurian of Sweden was compiled using chitinozoan distribution data from Gotland (Laufeld 1974;Grahn 1995), Skåne (Grahn 1996), and other localities on the mainland of Sweden (Grahn 1998).At the level of the Silurian standard stages the diversity curves of the East Baltic and Swedish chitinozoans are similar (Fig. 3).The diversity increases from the Rhuddanian to the Sheinwoodian and decreases in the Homerian and Gorstian.A slight increase follows in the Ludfordian.The global diversity curve shows the same trend with the exception of the Pridoli, where the number of species increases considerably, contrary to the East Baltic.
At the end of the Pridoli the Baltic basin suffered from severe regression.The clastic sediments of the lowermost Devonian lack chitinozoans.

CONCLUSIONS
Chitinozoan diversity is related to the transgressionregression phases of the Silurian Baltic basin, which are linked to glaciations, global sea-level fluctuations, water temperature and chemistry.The turnover ratio and species origination and extinction are more or less regular throughout the Silurian, except the high peak in the M. margaritana Biozone, correlating partly with the Ireviken Event.At the level of the Silurian standard stages the East Baltic chitinozoan total diversity and global diversity curves show similar trends with the main positive peak in the Sheinwoodian.Chitinozoan diversity is also related to the evolutionary processes of the faunal group, but this aspect of diversity is still poorly studied.