Carbonate Shelf Development and Early Paleozoic Benthic Diversity in Baltica: a Hierarchical Diversity Partitioning Approach Using Brachiopod Data

Paleobiology, 2021, pp. 1–21 DOI: 10.1017/pab.2021.3

Article

Carbonate shelf development and early Paleozoic benthic diversity in Baltica: a hierarchical diversity partitioning approach using brachiopod data

Amelia M. Penny* , Olle Hints, and Björn Kröger

Abstract.—The Ordovician–Silurian (∼485–419 Ma) was a time of considerable evolutionary upheaval, encompassing both great evolutionary diversification and one of the first major mass extinctions. The Ordovician diversification coincided with global climatic cooling and paleocontinental collision, the ecological impacts of which were mediated by region-specific processes including substrate changes, biotic invasions, and tectonic movements. From the Sandbian–Katian (∼453 Ma) onward, an extensive carbonate shelf developed in the eastern Baltic paleobasin in response to a tectonic shift to tropical latitudes and an increase in the abundance of calcareous macroorganisms. We quantify the contributions of environmental differentiation and temporal turnover to regional diversity through the Ordovician and Silurian, using brachiopod occurrences from the more shallow-water facies belts of the eastern Baltic paleobasin, an epicontinental sea on the Baltica paleocontinent. The results are consistent with carbonate shelf development as a driver of Ordovician regional diversification, both by enhancing broadscale differentiation between shallow- and deep-marine environments and by generating heterogeneous carbonate environments that allowed increasing numbers of brachiopod genera to coexist. However, temporal turnover also contributed significantly to apparent regional diversity, particularly in the Middle–Late Ordovician.

† Amelia M. Penny and Björn Kröger. Finnish Museum of Natural History, University of Helsinki, Jyrängöntie 2, † 00560 Helsinki, Finland. E-mail: bjorn.kroger@helsinki.ﬁ. Present address: Centre for Biological Diversity, School of Biology, University of St. Andrews, St. Andrews, Fife KY16 9TF, United Kingdom. E‐mail: amp29@st‐ andrews.ac.uk Olle Hints. Department of Geology, School of Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia. E-mail: [email protected]

Accepted: 18 January 2021 *Corresponding author.

Introduction et al. 2010; Hautmann 2014; Penny and Kröger 2019). The Ordovician and Silurian were intervals The GOBE had multiple global environmen- of tremendous evolutionary and environmental drivers that were mediated by regional-scale tal upheaval, encompassing both the great processes, including carbonate platform devel- Ordovician biodiversification event (GOBE), opment, biotic invasions, and latitudinal shifts and the onset and aftermath of the Late Ordovi- due to tectonic movements (e.g., Kiessling cian mass extinction (LOME) (Webby et al. et al. 2003; Servais and Harper 2018; Stigall 2004; Servais and Harper 2018; Rasmussen 2018). The impact of these regional processes et al. 2019; Stigall et al. 2019). This has inspired on diversity is exemplified by the eastern Baltic intensive study of Ordovician–Silurian diver- paleobasin. Over the Ordovician, Baltica experi- sity patterns, because of their potential to reveal enced regional warming as it drifted from tem- the mechanisms underlying extinction and perate to equatorial latitudes, accompanied by diversification (e.g., Sepkoski 1988; Miller and the development of an extensive, reef-bearing Connolly 2001; Webby et al. 2004; Servais carbonate shelf (Dronov and Rozhnov 2007;

© The Author(s), 2021. Published by Cambridge University Press on behalf of The Paleontological Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0094-8373/21 Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.76, on 02 Oct 2021 at 04:45:59, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/pab.2021.3 2 AMELIA M. PENNY ET AL.

Kröger et al. 2017b). Baltoscandia also has an abundant, well-preserved Ordovician–Silurian fossil record combined with a well-characterized stratigraphy (e.g., Paškevičius 1997;Raukasand Teedumäe 1997;Ebbestadetal.2007;Calner et al. 2013;Bauertetal.2014). This makes it an excellent focus for investigating how the environmental heterogeneity associated with carbonate shelf development inﬂuenced regional diversity patterns through the GOBE and LOME, as well as the impact of temporal turnover on these patterns. Early Paleozoic Baltoscandian richness curves are available for several clades (e.g., Hammer 2003; Kaljo et al. 2011), but we follow

other workers (e.g., Hints and Harper 2003; FIGURE 1. Regional facies map, showing confacies belts of Rasmussen et al. 2007, 2009; Lam and Stigall the Baltic paleobasin and the extent of the Baltic Syneclise. TTZ, Teisseyre-Tornquist Zone. Compiled from Jaanusson 2015; Hints et al. 2018) in focusing on brachio- (1976), Lazauskiene et al. (2003), and Tuuling (2019). The pods. The taxonomy and biostratigraphy of study region is outlined with a gray square. Baltoscandian brachiopods have been studied by generations of specialists, such as Armin Öpik, Tatjana N. Alikhova, Arvo Rõõmusoks, carbonates (Torsvik et al. 1996; Cocks and Tors- Madis Rubel, Linda Hints, and Petras Mustei- vik 2005; Dronov and Rozhnov 2007)(Fig. 2). kis (for recent reviews, see Harper et al. 2015; During the Middle and Late Ordovician, the Harper and Hints 2016). Because they are a Baltic paleobasin differentiated into shelf and major component of the early Paleozoic ben- basin facies zones (Fig. 3). Jaanusson (1976) dis- thos, brachiopods are also a good model clade tinguished a series of depositional zones, for investigating the impact of substrate among others the Livonian Basin in the eastern- changes on benthic diversity. most center of the basin and the North Estonian Here, we quantify how temporal turnover and Lithuanian Confacies belts as horseshoe- and environmental heterogeneity contributed like marginal zones (Fig. 1). The most distal of to brachiopod diversification in the eastern these is the Scanian Confacies belt in the part of the Baltic paleobasin during the GOBE extreme southwest of the basin comprising and also to the recovery after the LOME. parts of modern Denmark and Poland, which is dominated by graptolitic shales. The spatiotemporal development of the Bal- Geologic and Depositional Setting tic paleobasin is divided into five stages that During the early Paleozoic, the Baltic paleo- are distinct with respect to tectonic subsidence basin was an extensive epicontinental sea of rate, climate, large-scale eustatic sea-level the Baltica paleocontinent. The extent and trend, and differences in relative influx and pro- development of the Baltic paleobasin was duction of siliciclastics and carbonates (Einasto closely related to the Baltic Syneclise, a tectonic and Nestor 1973; Nestor and Einasto 1977, structure with a roughly 700-km-long south- 1997; Einasto 1986). The first two stages, the west–northeast oriented long axis (Tuuling (1) transgression and (2) unification stages, are 2019)(Fig. 1). During the Cambrian–Silurian, equivalent to a ramp situation, with a gently Baltica underwent a plate-tectonic shift from dipping seafloor slope, relatively low spatial temperate to equatorial latitudes (Torsvik and facies differentiation, and very slow deposi- Cocks 2016), with a corresponding shift in tional rates. The last three stages, the (3) differ- regional sediment deposition from cool-water entiation, (4) stabilization, and (5) infilling glauconitic carbonates and siliciclastics to stages, correspond to a carbonate platform situ- reduced siliciclastic input and tropical ation with high carbonate depositional rates,

Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.76, on 02 Oct 2021 at 04:45:59, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/pab.2021.3 EARLY PALEOZOIC BENTHIC DIVERSITY, BALTIC BASIN 3

FIGURE 2. Overview of brachiopod localities with lithologic information used in this study. Maps show extent of outcrop and subcrop of strata of the respective time bins. Hatched lines show facies boundaries. Maps are adopted from Paškevičius (1997: O1, Tremadoc, fig. 41; O2, Latorp, fig. 42; O4, Kunda, fig. 43; O5, Kukruse, fig. 44; O6, Oandu, fig. 45; O7, Ashgill, fig. 47; S1, Raikküla, fig. 59; S2, Adavere, fig. 60; S4, Ludlow, fig. 62; S5, Pridoli, fig. 63) and Raukas and Teedumäe (1997: O3, Volkhov, fig. 142). 1, brachiopod locality with lithologic information; 2, reef occurrence; 3, boundary between our “basinal,” “platform” areas; 4, facies boundary; 5, no time-bin sediments preserved; 6, extent of time-bin sediments.

high spatial bathymetric differentiation, phases development of an extensive Ordovician–Silur- of aggregation and progradation, and eventual ian facies mosaic (in the sense of Wright and termination. The change between the ramp and Burgess 2005). The different carbonate platform the platform situation was relatively abrupt substrates include: extensive hardground and occurred at around the Sandbian/Katian development (Orviku 1940; Saadre 1993;Dro- stage boundary during the regional Keila– nov et al. 2000; Vinn and Wilson 2010), shallow Oandu stages (see also Hints et al. 1989; Nestor and lagoonal soft sediments (e.g., Einasto 1986; and Einasto 1997). Teller 1997), a micritic limestone with abundant The late Sandbian–early Katian change from calcareous algae (Spjeldnæs and Nitecki 1994; ramp to platform depositional settings coin- Kröger et al. 2020), kilometer-wide shoal cided with a drastic inversion in wind and deposits of echinoderm grainstone, and reefs water currents (Kiipli et al. 2007) and a turnover and mud-mounds (e.g., Aaloe and Nestor in the biota (Ainsaar et al. 2004). This occurred 1977; Tuuling and Flodén 2013; Kröger et al. alongside the ﬁrst widespread development of 2014, 2016; Kaminskas et al. 2015). reefs (Kröger et al. 2016) and the development Hirnantian diamictites exposed in Poland of an algal-rich micritic limestone facies (Krö- demonstrate that icebergs reached the western ger et al. 2020). The massive increase in carbon- margin of Baltica, despite its position at sub- ate production was probably the main cause for tropical latitudes (Porębski et al. 2019). Carbon- the establishment of a stable, widespread, ﬂat- ate deposition continued through the topped carbonate platform and the consequent Hirnantian and into the Silurian, with ongoing

FIGURE 3. North–south cross sections showing facies distribution through the northern ﬂank of the Baltic Basin from western Estonia in the north to western Lithuania in the south. Vertical lines mark locations of drill-core sections and the distributions of different lithologies. A, Silurian strata; based on Nestor and Einasto (1982). B, Ordovician strata; based on Dronov et al. (2011). Roman numerals identify depositional intervals sensu Nestor and Einasto (1997).

high levels of facies differentiation between lithostratigraphic unit at a single location, shallow-marine carbonate-rich and distal silici- with location coordinates matched at a preci- clastic settings (Kiipli et al. 2004). sion of three decimal places. Lithologic and stratigraphic data were obtained from data associated with the PBDB Methods and SARV occurrences, from a separate down- Data, Downloads, and Data Cleaning.—We load of drill-core data compiled within the used combined brachiopod fossil occurrence SARV, and from our own compilation from data from the literature-based Paleobiology the literature. Only brachiopod occurrences Database (PBDB, https://paleobiodb.org) and identified to genus or species level were used from the specimen-level database of the Geo- for the analysis. Lithostratigraphic units of for- science Collections of Estonia (SARV, https:// mation, member, and bed level were used. The geocollections.info; Hints et al. 2019), compris- hierarchy of the lithostratigraphic units, and in ing a dataset of more than 13,000 occurrences some cases their names, changed over the his- from more than 800 localities in the eastern Bal- torical course of stratigraphic research. We cor- tic paleobasin (Estonia, Latvia, and Lithuania) rected manually for historical name changes. (downloaded 13 January 2020). Brachiopod Because we aimed to get a proxy for spatial occurrences from the combined databases are and temporal lithologic heterogeneity, we flat- mapped in Figure 2. Because the PBDB and tened the hierarchy of lithologic names and SARV databases use different “collection” con- considered each name as reflecting a distinct cepts, we assigned all occurrences to artificial lithostratigraphic unit with equal value. collections, wherein a “collection” comprises Sequence boundaries were taken from Nielsen all brachiopod occurrences within a (2004), Johnson (2006), and Dronov et al. (2011).

Time Bins.—The Ordovician and Silurian strata of the eastern Baltic paleobasin were divided into 12 time bins with an average duration of 5.6 Myr (Fig. 4). The shortest time bin is S5 (3.8 Myr) and the longest is O4 (8.3 Myr). Time binning was based on well-resolved stratigraphic horizons and reflects a comprom- ise between time resolution and available data. Absolute ages of time-bin boundaries were taken from Gradstein et al. (2012) and Lindskog et al. (2017). The Silurian record of the eastern Baltic paleobasin is included for several reasons. By including more time bins, we could track the relationships between regional diversity and substrate change from the initiation of the carbonate shelf in the Baltic paleobasin to the eventual infilling of the paleobasin in the late Silurian–Early Devonian. This encompasses more change in substrates and biodiversity and also provides points of comparison for the Ordovician data, allowing evaluation of the influence of time-specific biodiversity change and regional spatial and facies effects. Identifying Biogenic Lithologies.—We classi- fied lithostratigraphic units into either (1) bio- dominated lithologies comprising carbonates of primarily organismic origin; and (2) non–bio- dominated lithologies comprising predominantly siliciclastic sediments and chemical sedimentary rocks. In the first category are all limestone lithologies, including marly, argilla- ceous, silty, dolomitic, and glauconitic limestone. In the second category are shale, marl, siltstone, sandstone, glauconite, dolomite, and gypsum lithologies. For simplicity, we call the bio- dominated category “carbonate-dominated” and the non–bio-dominated category “siliciclastic-dominated,” although dolomites are also carbonates, and gypsum is not siliciclastic. We include a list of lithostratigraphic units and their designations as carbonate and siliciclas-

FIGURE 4. Stratigraphic scheme showing time binning, tic dominated in Supplementary Material 1. Time depositional cycles, and stages of the depositional model for bin S2 was excluded from some analyses, because the Baltic paleobasin. Compiled from Gradstein et al. (2012), the only carbonate-dominated lithostratigraphic Männik (2014), and Lindskog et al. (2017)forabsolutetime; Kiipli et al. (2016, 2017) for climate; Calner (2008)andRasmus- unit it contains is the Rumba Formation, which sen et al. (2019) for bio-events; Nielsen (2004), Johnson (2006), contains a very low diversity assemblage domi- and Dronov et al. (2011) for depositional cycles; Nestor and nated by a single genus, which sometimes causes Einasto (1997) for depositional intervals. H, humid interval. Stage abbreviations: Daping., Dapingian; Hirn., Hirnantian; the mean diversity to decline at higher hierarch- Rhudd., Rhuddaninan; Aer., Aeronian; Shein., Sheinwoodian; ical levels (see “Diversity Partitioning” section Hom., Homerian; Gor., Gorstian; Ludf., Ludfordian. for details of the hierarchical partitions).

Diversity Partitioning.—We use the term diversity of the samples that have been pooled “richness” to refer to the number of genera (Lande 1996). In hierarchical multiplicative (which may be a raw count or an estimate diversity partitioning, beta diversity is the based on some form of standardization) and pooled diversity at a given hierarchical level “diversity” as an umbrella term when referring divided by the mean diversity of its component to diversity measures in the Hill number fam- samples (Jost et al. 2010). ily. These include richness, but also the Shan- Additive diversity partitioning is sometimes non and Simpson diversities, which are the preferred over multiplicative diversity parti- Shannon and Simpson indices converted to tioning, because it yields beta-diversity values their corresponding Hill numbers (terminology in the same units as alpha and gamma diversity after Hsieh et al. 2016). “Alpha diversity” refers (e.g., Patzkowsky and Holland 2007; Holland to diversity within hierarchically partitioned 2010). Holland (2010) also advocated using categories, “beta diversity” refers to diversity three indices of diversity in parallel—richness, difference between categories, and “gamma the Shannon index (also called the Shannon diversity” refers to the regional diversity; each entropy), and the Simpson index (also called may be measured with any of the three diver- the Gini-Simpson index)—to evaluate diversity sity measures used in this study. Where we patterns in common and rare taxa and to con- refer to alpha, beta, or gamma diversity in trol for the fact that richness is highly sensitive only one of the diversity measures, we use the to sample size (Magurran 2004). Richness, γ Greek letter with a subscript (e.g., S is regional being a simple count of the taxa in an assem- γ diversity, measured with richness; shan and blage, gives equal weight to all taxa, and conse- γ ﬂ sim would give the corresponding Shannon quently may be strongly in uenced by the and Simpson diversities, respectively). All ana- rarest taxa in an assemblage, as most of the spe- lyses were performed at the genus level. cies in a community tend to occur at low abun- Total (gamma) diversity can be partitioned dance (Whittaker 1970; Reddin et al. 2015). The into alpha and beta diversity using either addi- Simpson index is most strongly inﬂuenced by tive or multiplicative approaches. While alpha the commonest taxa in an assemblage, which and gamma diversity have the same meanings are arguably the most ecologically important in both additive and multiplicative diversity (Gaston 2010), while the Shannon index has partitioning, they differ in their concepts of an intermediate sensitivity (for a full explan- beta diversity (which is also calculated using ation, see Jost 2007; Jost et al. 2010). a range of other methods; reviewed in Koleff However, the Shannon and Simpson indices et al. 2003). In multiplicative diversity parti- do not behave additively and can give mislead- tioning, beta diversity is the ratio of gamma ing results (Jost et al. 2010). The solution, pro- diversity to alpha diversity (Koleff et al. 2003; posed by Jost et al. (2010), is to convert each Whittaker 1972), while in additive diversity index to its corresponding Hill number (Hill partitioning, beta diversity is gamma diversity 1973), also called its “effective number of spe- minus alpha diversity (Lande 1996). cies,” which is the number of species corre- Both diversity partitioning approaches can sponding to the given diversity index when be used in hierarchical sampling schemes, the species abundance distribution is perfectly which assign samples to progressively larger even. These values can easily be compared nested groups to assess how much diversity is with richness, because their units are the same. contributed at each level. Hierarchical parti- After conversion to Hill numbers, the Shan- tioning is readily applicable to fossil assem- non and Simpson indices yield the same blages, because hierarchical sampling mirrors within-group diversities whether partitioned the hierarchical nature of lithostratigraphic multiplicatively or additively (Ricotta 2005; and chronologic units (Patzkowsky and Hol- Jost et al. 2010). In this study, we used hierarch- land 2012). In hierarchical additive partition- ical multiplicative diversity partitioning with ing, beta diversity is the difference between Hill numbers, because it yields the same the pooled diversity of a group of samples at within-category diversities as additive diver- a given hierarchical level and the mean alpha sity partitioning and is easier to implement

using the R package vegan (Oksanen et al. discussion on alpha and beta diversities with 2018). However, in places we have expressed mean p-values < 0.1. the differences in mean alpha diversity between We used environmental partitioning to hierarchical levels as a percentage of gamma evaluate how faunal differences between lithol- diversity; this is not strictly equivalent to hier- ogies contributed to regional diversity patterns, archical additive diversity partitioning, but we which relates to the influence of environmental use it as an additional, intuitive indicator of preference and substrate heterogeneity on faunal difference between sampling units. assemblage heterogeneity. The basic sample We used the function multipart, within the R for alpha-diversity calculation was the lithos- package vegan (Oksanen et al. 2018), to per- tratigraphic unit, approximately equivalent to form multiplicative diversity partitioning a formation. Formations have been taken to using the Tsallis entropy, with scale factors 0, represent metacommunities, and hence may 1, and 2 corresponding to taxonomic richness, be ecologically meaningful units (Hofmann Shannon index, and Simpson index. The result- et al. 2019). We used a three-level hierarchy, ing alpha and gamma diversities are given as with lithostratigraphic units nested within their corresponding Hill numbers; we refer to carbonate-dominated or siliciclastic-dominated these as the richness, Shannon diversity, and lithologies, with the highest hierarchical level Simpson diversity, following Hsieh et al. being the whole study region. We also evalu- (2016). ated beta diversity within siliciclastic- and Data from 177 lithostratigraphic units were carbonate-dominated lithologies by perform- analyzed, of which 38 contain only a single ing separate two-level partitions of siliciclastic- collection. To avoid discarding these lithostrati- and carbonate-hosted diversity. Where we sus- graphic units, we present the raw data for pected that time binning might be affecting the all lithostratigraphic units containing five diversity curves produced, we performed second- occurrences or more. We include coverage- ary analyses at stage-level temporal resolution. standardized results, which show a substan- For temporal diversity partitioning, we used tially similar pattern, in Supplementary Mater- a three-level hierarchy to evaluate how faunal ial 2. Coverage was measured using Good’s u turnover between stages contributed to the (Good 1953), using a method from Alroy diversity curve. Again, we used lithostrati- (2014) as implemented by Brocklehurst et al. graphic units as the basic units for alpha- (2018). We coverage-standardized each lithos- diversity calculation, followed by within-stage tratigraphic unit to a Good’s u of 0.5, a value and within–time bin diversity. Brachiopod chosen to maximize the number of time bins occurrences lacking stage level temporal reso- that would run. Lithostratigraphic units with lution were excluded from this analysis. lower coverage were discarded, and those We also performed diversity partitioning whose coverage was higher were subsampled within shelf and basin facies. Brachiopod until the standardization coverage was re- occurrences were assigned to shelf or basin ached. Subsampling was repeated 100 times facies based on their geographic locations; the for each time bin, and the mean diversities R package icosa (Kocsis 2017) was used to gen- and associated p-values from null modeling erate hexagonal equal-area grids of ∼37 km per recorded. Increasing the number of subsamp- side, and occurrences were assigned to grids ling runs had a minimal effect on the results. using the locate() function. The resulting grids We used the null modeling function in multi- were used as the sampling unit and were over- part to perform individual-based randomiza- lain on a facies map of the Baltic region to tion, which randomly assigns occurrences to assign them to the shelf or basin facies for ana- samples at the lowest level of the hierarchy lysis. Comparison of shelf and basin diversity and assesses whether the diversities in the sam- patterns covers the time interval from bin O6 ple are significantly different from those of the onward, as this is when the platform differenti- randomized data. Null models were run with ation phase began (Fig. 4). 99 iterations for each subsampling run, and The meanings of different diversities within when reporting results, we focus our the partitioning schemes are summarized in

TABLE 1. Meanings of different diversity terms used in the hierarchical partitions.

Alpha 1 (α1) Beta 1 (β1) Alpha 2 (α2) Beta 2 (β2) Gamma (γ) Temporal Mean diversity The mean value of alpha 2/ Mean Gamma/mean Regional partitions within mean alpha, across all diversity alpha 2. Reflects diversity lithostratigraphic stages in a time bin. within faunal difference within a units Reflects faunal difference stages in a between stages time bin between lithostratigraphic time bin in a time bin. units within stages. Lithologic/ As above The mean value of alpha2/ Mean Gamma/mean As above environmental mean alpha, across both diversity alpha 2. Reflects partitions lithological categories. within faunal difference Reflects faunal difference lithologic between between lithostratigraphic categories in lithological units within lithological a time bin categories in a categories. time bin.

Table 1. Alpha, beta, and gamma diversity are Occupancy as an Abundance Proxy.—In our referred to by their Greek characters, with sub- study, direct brachiopod abundance data were scripts denoting richness, Shannon diversity, or not available. We therefore use the number of Simpson diversity. localities with occurrences as a rough proxy of Independent Gamma-Diversity Estimates.—In abundance with the a priori assumptions that the hierarchical diversity partitioning analyses, detection in the fossil record is, at least in coverage may vary between time bins. To esti- part, dependent on abundance and that mate the richness trajectory through time, we more-abundant species have higher occupan- used independently calculated richness curves cies (see e.g., Gaston et al. 2000). produced using two methods. First, we used the estimateD function in the R package Results iNEXT (Hsieh et al. 2016), standardizing to a γ – coverage of 0.7, which is the maximum cover- Regional Richness ( S) over the Ordovician age at which time bins O3–S5 will run. Second, Silurian.—The GOBE and LOME are expressed we used the capture–recapture (CR) modeling in the regional richness patterns derived from γ approach (Nichols and Pollock 1983; Liow CR (Fig. 5). With stage-level resolution, S and Nichols 2010)byfitting the Jolly-Seber increases in two pulses, one in the Floian– model following the POPAN formulation early Dapingian (Billingen–Kunda regional (Schwarz and Arnason 1996; see also Kröger stages), with a second, larger pulse in the Sand- γ et al. [2019] for details of the method). bian. The S decline during time bin O7 is an We also produced coverage-standardized effect of the time-binning strategy used, and γ and CR modeling approach richness curves with stage-level resolution, S continues to at stage-level temporal resolution to visualize increase up until the Porkuni (Fig. 5) and then how the time binning might influence abruptly declines into the Silurian, with only richness trajectories, excluding stages without a limited recovery. The LOME is not preceded occurrences or where the estimateD function by a diversity loss and instead represents a rela- suggested large prediction bias. Because tively abrupt diversity decline between the Por- different methods can be expected to produce kuni and Juuru regional stages. different richness estimates, we focus our Temporal Taxonomic Turnover.—Taxonomic discussion on trends in the data rather than turnover between regional stages, β2 in our absolute values. To determine whether temporal partitioning scheme, is a major con- carbonate-dominated and siliciclastic-dominated tributor to within-bin diversity, though its lithologies showed the same diversity impact is highly dependent on the length of patterns, we calculated independent coverage- time bins and the diversity index used standardized richness curves for each lithologic (Fig. 6). The Sandbian–Katian (time bin O6) is category. particularly strongly influenced by this effect

FIGURE 5. Regional gamma-diversity curve based on CR modeling at time-bin and stage temporal resolution. Black ovals and solid line, gamma diversity at time-bin resolution; gray rectangles and dotted line, gamma diversity at stage resolution. Use of longer time bins masks the two pulses of regional diversiﬁcation visible at the stage level and also shifts the apparent regional diversity peak to the Katian (O6 bin), whereas with stage-level temporal resolution, the peak is in the Porkuni stage (Hirnantian; O7 bin). Stage abbreviations are: Dp., Dapingian; Sand., Sand- bian; Hi, Hirnantian; R., Rhuddanian; Ae., Aeronian; Tel., Telychian; Sh., Sheinwoodian; H., Homerian; G., Gorstian; L., Ludfordian; Pri., Pridoli.

FIGURE 6. Temporal diversity partitioning, which empha- (Fig. 6B,C), though it is reduced when using the sizes the contribution of temporal turnover to regional Shannon and Simpson diversities (Table 2). diversity. Curves are calculated using genus richness and fi — Hill numbers corresponding to the Shannon and Simpson Environment-Speci c Diversity Trajectories. indices (Shannon and Simpson diversity). A three-level As the Estonian and Lithuanian shelves dev- hierarchy is used, with mean diversity within lithostrati- eloped, they created broadscale substrate het- graphic units (alpha1, gray diamonds), mean diversity in regional stages within a time bin (alpha2, white diamonds), erogeneity in the eastern Baltic paleobasin, and regional diversity (gamma, black circles). The differ- and differentiation between shallow-water ence between alpha1 and alpha2 reflects the extent of faunal and deep-marine facies. Shelfal localities typic- differences between lithostratigraphic units within regional stages, while the difference between alpha2 and gamma ally have higher richness than basinal ones, and reflects faunal differentiation between regional stages. A, platform and basin facies also show distinct Raw genus richness curves. B, Shannon diversity curves. richness trends (Fig. 7). C, Simpson diversity curves. Stage abbreviations are: Dp., Dapingian; Sand., Sandbian; Hi, Hirnantian; R., Rhudda- Differences in richness trajectories between nian; Ae., Aeronian; Tel., Telychian; Sh., Sheinwoodian; shelf and basin are also reflected in differences H., Homerian; G., Gorstian; L., Ludfordian; Pri., Pridoli. in the diversity trajectories of carbonate- and siliciclastic-dominated lithologic categories, as carbonate-dominated lithologies predominantly depositional environments. Siliciclastic- occur on the shelf, at least after the Early Ordovi- dominated lithostratigraphic units show a cian (Fig. 3). Independent richness curves for more rapid diversification than carbonate- carbonate- and siliciclastic-dominated litholo- dominated ones (Fig. 8) but are dominated by gies (Fig. 8) indicate that in the Ordovician, just a few brachiopod genera until O5 (Uhaku– carbonate lithologies typically hosted higher Haljala stages, Darriwilian–Sandbian) (see Sup- richness than siliciclastic ones. However, parallel plementary Material 3). The marked increase in increases in carbonate- and siliciclastic-hosted siliciclastic-hosted richness in O6 reflects the richness over the Ordovician may suggest regional faunal turnover event during this inter- diversification drivers that operated across val (Ainsaar et al. 2004 and references therein).

TABLE 2. Taxonomic turnover between stages (β2) measured as a percentage of γ diversity and the associated p-values. β2 was measured using richness, the Shannon diversity, and the Simpson diversity. The values used to produce this table are given in Supplementary Material 3A. Data where p-values were below 0.1 are highlighted in gray. NAs denote time bins with insufﬁcient data for analysis, or where bins were not subdivided into multiple stages.

Richness Shannon diversity Simpson diversity β β β β β β 2S 2S p-value 2shan 2shan p-value 2sim 2sim p-value CNANANANANANA O1 NA NA NA NA NA NA O2 1.57 0.09 1.33 0.51 1.17 0.97 O3 NA NA NA NA NA NA O4 1.73 0.01 1.68 0.01 1.70 0.01 O5 1.88 0.01 1.68 0.01 1.67 0.01 O6 2.43 0.01 1.94 0.01 1.65 0.01 O7 1.45 0.01 1.4 0.01 1.42 0.01 S1 1.28 0.07 1.12 0.25 1.1 0.59 S2 NA NA NA NA NA NA S3 1.48 0.01 1.27 0.05 1.17 0.65 S4 1.92 0.17 1.67 0.15 1.53 0.55 S5 1.19 1 1.1 0.93 1.09 0.97

With stage-level temporal resolution, The Silurian decline in faunal differentiation β fl carbonate- and siliciclastic-hosted richness between lithostratigraphic units ( 1S)reects curves are decoupled beginning during the regional-scale homogenization of brachiopod Katian, during the Oandu Stage (in bin O6), assemblages across environmental gradients and continuing into the Silurian (Fig. 8). In accompanied by alpha-diversity increase, con- the Silurian, siliciclastic-hosted brachiopod sistent with relaxed environmental preferences richness begins to recover from the LOME earl- in Silurian brachiopods. Both siliciclastic- and ier than carbonate-hosted richness, though carbonate-dominated lithologies become both decline toward the end of the Silurian dominated by relatively few brachiopod gen- (Fig. 9). However, richness within carbonate- era, present at high abundances (e.g., Leptaena dominated lithologies closely reflects the and Atrypa in bin S3, Dayia in S4, Shaleria and regional richness pattern, suggesting that car- Microsphaeridiorhynchus in S5; see Supplemen- bonate lithologies are dominating the regional tary Material 4). richness trajectory, particularly in the Ordovi- To evaluate whether metacommunity faunal cian (compare Figs. 5 and 8A). heterogeneity differed between siliciclastic- Lithologic Diversity Partitioning.—The large and carbonate-dominated lithostratigraphic difference between α1 and α2 suggests that units, we performed multiplicative diversity faunal differences between lithostratigraphic partitioning on the two lithologic categories units are a major contributor to regional diver- separately. Comparison of alpha and gamma sity through most of the Ordovician; this trend diversity trajectories (Figs. 10A, 11A) suggest seems particularly strong during O4-O7, when that beta diversity among carbonate- β 1S is at its highest (Table 3, Fig. 9A). Neverthe- dominated lithologies is generally higher than less, differentiation between carbonate- and among siliciclastic-dominated lithologies and β siliciclastic-dominated lithologies ( 2S) remains that this contrast is especially pronounced an important contributor to diversity (Table 3, when the Simpson diversity is used, emphasiz- α Fig. 10A). Most features of the trends in S ing the most abundant genera (Fig. 10B,C, and β and S are also shown by the Shannon and 11B,C). A one-sided Mann-Whitney test using Simpson diversities (Fig. 9B,C), demonstrating stage-level Simpson diversity data shows that that faunal differences apply to the entire abun- beta diversity between lithostratigraphic units β fi dance distribution of the brachiopod assem- ( 1sim) is signi cantly higher in carbonate- blages within lithostratigraphic units and are dominated than in siliciclastic-dominated not restricted to rare taxa. lithologies ( p = 0.03).

FIGURE 8. Independent coverage-standardized genus richness curves for carbonate- and siliciclastic-dominated FIGURE 7. Diversity partitioning in platform and basinal lithostratigraphic units, calculated using iNEXT, using localities, deﬁned based on the regional facies boundaries both the regional stages (gray rectangles, dotted line) and in Fig. 1. Diversity curves are only shown from O6 onward, coarser time-binning (black ovals, solid line) scheme. All as this marks the start of the platform differentiation curves are coverage-standardized to 0.7. A, Regional depositional interval (Fig. 3). A two-level hierarchy is genus richness in carbonate-dominated lithologies. B, used, with genus richness within lithostratigraphic units Regional genus richness in siliciclastic-dominated localities. (gray diamonds) and regional richness in the relevant set- Stage abbreviations are: Dp., Dapingian; Sand., Sandbian; ting (black circles). A large difference between the two Hi, Hirnantian; R., Rhuddanian; Ae., Aeronian; Tel., Tely- diversity curves represents a larger contribution from beta chian; Sh., Sheinwoodian; H., Homerian; G., Gorstian; L., diversity. Error bars represent 95% conﬁdence intervals, Ludfordian; Pri., Pridoli. though these are generally very small. A, Raw genus richness in platform environments. B, Raw genus richness in basinal environments. Stage abbreviations are: Hi, Hirnan- tian; R., Rhuddanian; Ae., Aeronian; Tel., Telychian; Sh., Sheinwoodian; H., Homerian; G., Gorstian; L., Ludfordian; This pattern suggests that in siliciclastic- Pri., Pridoli. dominated lithostratigraphic units, high beta diversity is typically generated among the rarer genera, while in carbonate-dominated Beta diversity in genus richness among lithologies, high beta diversity also affects the carbonate-dominated lithostratigraphic units commonest genera, implying important differ- β ( 1S) increases consistently through the Dapin- ences in assemblage composition between gian–Katian (time bins O3–O6), though to a lithostratigraphic units. While time bin O6, slightly lesser extent when the Shannon and where beta diversity peaks, is also the time Simpson diversities are used (Table 4, Supple- bin showing highest temporal turnover, we mentary Material 3C). In siliciclastic lithologies, expect this effect to be related to environmental β 1S is lower than in carbonate-dominated lithol- change rather than depth-related differences in ogies, and the difference between α1andγ is evolutionary rates, because onshore and off- markedly lower when the Simpson diversity is shore environments have similar origination used (Table 4, Supplementary Material 3D). and extinction rates (Franeck and Liow 2019).

TABLE 3. Taxonomic turnover between lithostratigraphic β units ( 1S) and between carbonate- and β siliclastic-dominated lithologies ( 2S), measured using richness, and the associated p-values. Data used to produce this table are given in Supplementary Material 3B. Data where p-values were below 0.1 are highlighted in gray. NAs denote time bins with insufﬁcient data for analysis, or with data from only one lithological type. β β β β 1S 1S p-value 2S 2S p-value CNANANANA O1 1.08 0.91 1.45 0.01 O2 1.5 0.01 1.57 0.11 O3 1.43 0.45 1.77 0.19 O4 2.40 0.01 1.80 0.01 O5 3.2 0.01 1.66 0.01 O6 5.25 0.01 1.36 0.01 O7 3.74 0.01 1.72 0.01 S1 1.97 0.01 1.4 0.01 S2 1.38 0.77 1.87 0.01 S3 2.97 0.33 1.18 0.29 S4 2.74 0.63 1.37 0.01 S5 1.54 0.01 1.24 0.15

The decline in siliciclastic-hosted genus richness and beta diversity from time bins O6 to O7 may be attributable to a decline in the number of exposed siliciclastic rock units. In time bin O6, siliciclastic-hosted genus richness and beta diversity are far higher than in time bin O7, but the number of lithostratigraphic units is also higher (10 in time bin O6, compared with 4 in O7). Responses of Alpha, Beta, and Gamma Diversity to the GOBE and LOME.—The GOBE has relatively little impact on alpha diversity in the eastern Baltic paleobasin. The genus richness α of lithostratigraphic units ( S) shows a small increase over the Ordovician, but this diversification comprises only a small proportion of the FIGURE 9. Lithologic diversity partitioning, which empha- regional diversity, which instead is principally sizes how faunal differences between lithostratigraphic fl units contribute to regional diversity. Diversity curves generated by environmentally in uenced were calculated using genus richness and Hill numbers faunal heterogeneity and temporal turnover. corresponding to the Shannon and Simpson indices. The principal Ordovician diversification occurs A three-level hierarchy is used, with mean diversity within lithostratigraphic units (alpha1, gray diamonds), within both siliciclastic and carbonate environ- mean diversity within carbonate- and siliciclastic- ments, and the faunal differences between the dominated lithostratigraphic units (alpha2, white dia- carbonate- and siliciclastic-dominated litholo- monds), and regional diversity (gamma, black circles). The difference between alpha1 and alpha2 reflects the gies are a relatively small contributor to extent of faunal differences between lithostratigraphic regional diversity. However, the LOME seems units, while the difference between alpha2 and gamma to have a differential impact on siliciclastic reflects faunal differentiation between carbonate- and siliciclastic-dominated facies. A, Genus richness curves. and carbonate facies. There was only a limited B, Shannon diversity curves. C, Simpson diversity recovery from the LOME overall and in curves. Stage abbreviations are: Dp., Dapingian; Sand., carbonate-dominated lithostratigraphic units, Sandbian; Hi, Hirnantian; R., Rhuddanian; Ae., Aero- nian; Tel., Telychian; Sh., Sheinwoodian; H., Homerian; though the recovery in siliciclastic lithostrati- G., Gorstian; L., Ludfordian; Pri., Pridoli. graphic units is more pronounced.

FIGURE 11. Diversity partitioning within siliciclastic- dominated facies, calculated using genus richness and Hill numbers corresponding to the Shannon and Simpson indices. Curves represent mean diversity within siliciclastic-dominated lithostratigraphic units (alpha, gray diamonds) and regional diversity in siliciclastic-dominated facies (gamma, black circles). A large difference between FIGURE 10. Diversity partitioning within carbonate- the two curves reﬂects the degree of faunal differentiation dominated facies, calculated using genus richness and between siliciclastic-dominated lithostratigraphic units. A, Hill numbers corresponding to the Shannon and Simpson Genus richness curves. B, Shannon diversity curves. C, indices. Curves represent mean diversity within Simpson diversity curves. Stage abbreviations are: Dp., carbonate-dominated lithostratigraphic units (alpha, Dapingian; Sand., Sandbian; Hi, Hirnantian; R., Rhudda- gray diamonds) and regional diversity in carbonate- nian; Ae., Aeronian; Tel., Telychian; Sh., Sheinwoodian; dominated facies (gamma, black circles). The difference H., Homerian; G., Gorstian; L., Ludfordian; Pri., Pridoli. between the two curves reﬂects the degree of faunal differentiation between carbonate-dominated lithostratigraphic units. A, Genus richness curves. B, Shannon Discussion diversity curves. C, Simpson diversity curves. Stage abbreviations are: Dp., Dapingian; Sand., Sandbian; Hi, Diversity patterns in this study show some Hirnantian; R., Rhuddanian; Ae., Aeronian; Tel., Tely- chian; Sh., Sheinwoodian; H., Homerian; G., Gorstian; generalities, such as a Darriwilian diversity L., Ludfordian; Pri., Pridoli. increase and further increase in the Sandbian,

TABLE 4. Beta diversity within carbonate- and differences between the regional Keila–Vormsi siliciclastic-dominated lithostratigraphic units, measured using richness. Full values used as a basis for this table and stages, which are unconformity-bounded units the Shannon and Simpson diversities are given in on the Estonian and Lithuanian shelves. The Supplementary Material 3C and 3D. Beta diversities with base Oandu, base Rakvere, and base Nabala p-values lower than 0.1 are highlighted in gray. “NA” denotes time bins with insufficient data for analysis, or stages are karstic over large areas of the North containing only a single lithostratigraphic unit in the Estonian shelf (Calner et al. 2010), contrasting relevant lithological category. with the more complete Livonian basin sec- Carbonate-dominated Siliciclastic-dominated tions. Although exceptionally high turnover rates from this interval are not known from glo- β1 β1 p-value β1 β1 p-value S S S S bal (Kröger et al. 2019)or CNANANANA O1 NA NA 1.17 1.00 paleocontinental-scale (Franeck and Liow O2 NA NA 2.00 0.01 2019) analyses, brachiopod faunas of this inter- O3 1.85 0.99 NA NA val on the Estonian shelf cluster into highly dis- O4 3.79 0.01 NA NA O5 5.03 0.01 1.36 0.59 tinctive time groups, while Livonian basin O6 6.31 0.01 4.19 0.01 brachiopod assemblages are not as distinct O7 5.68 0.01 1.80 0.03 (Hints et al. 2018). S1 2.94 0.01 NA NA S2 NA NA 1.76 0.95 Immigrations from Avalonia (increasingly S3 3.19 0.01 2.75 0.85 from the early Sandbian onward) and Lauren- S4 1.94 0.15 3.54 1.00 tia (increasingly from the late Sandbian S5 1.64 0.97 1.43 0.01 onward) also contributed to temporal turnover during O6 (Hints and Harper 2003; Hansen a Late Ordovician diversity peak and a diver- and Harper 2008; Rasmussen and Harper sity crisis across the Ordovician/Silurian 2011b; Rasmussen et al. 2012). They occurred boundary. These patterns are roughly consist- alongside changes to sediment-transporting ent with global and regional richness curves sea currents within the Baltic paleobasin (Kiipli for the GOBE (e.g., Trubovitz and Stigall et al. 2008, 2009) and reflect the regional “mid- 2016; Colmenar and Rasmussen 2017; Hints dle Caradoc facies and faunal turnover” (Mei- et al. 2018; Rasmussen et al. 2019) and the dla et al. 1999; Ainsaar et al. 2004). Temporal LOME (e.g., Sheehan 2001; Harper et al. 2013; turnover also makes a large contribution to Rasmussen 2014), including in the Baltic paleo- apparent diversity in time bin S3, which basin (e.g., Nestor et al. 1991; Rasmussen et al. includes two major regional and global biotic 2007, 2009; Kaljo et al. 2011; Hints et al. 2018). turnover events (Ireviken and Mulde events; The hierarchical partitioning schemes and mul- Calner 2008). tiple diversity indices used here allow us to dis- The amplitude and frequency of sea-level sect the roles of environmental heterogeneity change can influence evolutionary rates at glo- and temporal turnover in generating these bal and regional scales. Globally and region- regional patterns. ally, high origination and extinction rates Impact of Temporal Turnover on Ordovician– cluster at sequence boundaries in shallow- Silurian Diversity Patterns.—Temporal turnover water environments, and contrastingly at max- within time bins has a major impact on the imum flooding intervals in deeper-water envir- regional richness curve for the eastern Baltic onments (Holland 1995, 2020; Holland and paleobasin (Fig. 6A) (an effect also noted by, Patzkowsky 2002). Sea-level changes can act e.g., Kröger and Lintulaakso [2017]). The as a global diversification driver in marine eco- impact is largest in time bin O6 (late Sand- systems, because sea-level falls can isolate mar- bian–early Katian) where the total richness ine communities for long periods of time until strongly exceeds the richness estimates of the sea-level increases allow them to disperse (Sti- respective regional stage bins (Fig. 5). The prin- gall et al. 2017). In the Baltic paleobasin, cipal source of temporal substrate changes in repeated cycles of sea-level rise and fall during the Baltic paleobasin is sea-level change (e.g., the Middle Ordovician have also been sug- Lazauskiene et al. 2003; Harris et al. 2004), gested as a contributor to brachiopod diversifi- and bin O6 coincides with high faunal cation through the same process (Pedersen and

Rasmussen 2019). Because sea-level change habitat heterogeneity in shallow-marine car- may influence global and regional turnover bonate environments was an important rates, the completeness of the fossil record, regional diversity driver. High beta diversity and facies characteristics in parallel, the impact among carbonate-dominated lithostratigraphic of any one of these factors on temporal turnover units occurs whichever diversity index is used, cannot be isolated in this study. underlining that high beta diversity in Impacts of Facies Changes and Heterogeneity.— carbonate-dominated lithologies affects com- Alongside temporal turnover, the development mon and rare genera similarly (Fig. 10). By con- and stabilization of an extensive regional car- trast, beta diversity in siliciclastic-dominated bonate platform in the Baltic paleobasin also environments is strongly influenced by rare enhanced regional diversity, through generat- taxa, so it declines considerably when the ing environmental differentiation at multiple Simpson diversity is used (Fig. 11A,C). spatial scales. The increased abundance of skeletal macro- From the late Sandbian–early Katian (O6) organisms (e.g., Põlma 1982; Põlma et al. time bin throughout the Late Ordovician, 1988), including metazoans and macroalgae faunal difference between siliciclastic- and (Kröger et al. 2020), would have contributed carbonate-dominated substrates had a large to the platform formation and facies differenti- impact on total regional diversity (Fig. 9), and ation, both by acting as a source of carbonate this differentiation is likely to partly reflect clasts and micrite to build a regional carbonate broadscale habitat differentiation between plat- shelf and by producing fine-scale habitat com- form and basin (Fig. 7). The richness curves of plexity in shoals, reefs, and algal meadows. the two lithologic categories run largely paral- An example of such a highly complex facies lel during the Lower–Middle Ordovician and mosaic is the Pirgu Stage carbonates of Estonia most of the Silurian but are divergent during and Lithuania (Hints et al. 2005). Additionally, the late Sandbian–Aeronian age (Fig. 8). the flat-topped platform geometry amplified Relatively low brachiopod diversity in basinal the spatiotemporal differentiation of sediments environments is a consistent feature of the via sea-level fluctuations, such as widespread diversity curves, and so we interpret it as a karst (Calner et al. 2010) and erosion (Kiipli predominantly biological signal despite inter- and Kiipli 2020). vals of carbonate dissolution in deep-marine The development of a carbonate platform in environments (Kiipli and Kiipli 2006). The dis- the Baltic paleobasin was not an isolated crepancy between carbonate-hosted and event. During the Middle Ordovician, rising siliciclastic-hosted brachiopod diversity can be sea levels generated extensive shallow-marine best explained as a consequence of the dichot- carbonate shelves worldwide (Miller et al. omy between shallow- and deep-water bra- 2005). The complex facies mosaic of Baltica is chiopod faunas during the Late Ordovician not unique but shows high similarities with (Hints and Harper 2003; Harper and Hints coeval tropical carbonate platforms of Lauren- 2016; see also “Results” section), and addition- tia and South China (e.g., Taylor and Sendino ally as an effect of high temporal turnover in 2010; Wang et al. 2012; Kröger et al. 2017a; Jin shallow-water siliciclastics during O6. et al. 2018; McLaughlin et al. 2019), which is While differentiation between shelf and evidence for the global scale of the specific con- basin environments generated broadscale ditions of the Late Ordovician–Silurian tropics. faunal differences (see also Hints and Harper The general characteristics of marine sub- 2003; Kaljo et al. 2011; Harper and Hints 2016; strates are likely to exert control over long-term, Hints et al. 2018), a large proportion of regional global evolutionary trends. The expansion of diversification resulted from faunal differenti- carbonate substrate area may have enhanced ation between lithostratigraphic units operat- diversity (Munnecke et al. 2010), perhaps ing at smaller spatial scales. This is more because specific carbonate environments may pronounced in carbonate-dominated litholo- have evolutionary impacts. Reefs act as sources gies than siliciclastic-dominated ones (compare of new taxa over the Phanerozoic (Kiessling Figs. 10C and 11C), hinting that the higher et al. 2010), and an Ordovician expansion of

carbonate hardgrounds has been suggested as processes generated the conditions for the a diversification driver in hard substrate taxa development of tropical carbonate platforms (e.g., Taylor and Wilson 2003; but see also and facies mosaics in the Baltic paleobasin as Franeck and Liow 2020). In the Paleozoic, the well as in Laurentia and South China (see earl- global balance between the extent of carbonate ier discussion). Hence, as well as releasing and siliciclastic substrates may also influence environmental constraints, global cooling dur- the global balance between extinction and ing the Ordovician combined with other origination (Foote 2006), and the substrate regional-scale environmental changes to affinities of benthic clades (Miller and Con- enhance biodiversity through intensified bio- nolly 2001). While regional in scale, our logically mediated carbonate production and results support the hypothesis that the the development of heterogeneous carbonate expansion of shallow-marine carbonate environments. environments in the Ordovician enhanced Regional Response to the LOME.—A global regional diversity by enhancing substrate signature of the Ordovician–Silurian transition heterogeneity in the forms of both differenti- is a general shift from highly differentiated to ation between shelf and basin and finer-scale cosmopolitan faunas, including in brachiopods differentiation within carbonate (e.g., Sheehan and Coorough 1990; Darroch environments. and Wagner 2015). The end-Ordovician extinc- Impacts of Broadscale Environmental Changes: tion led to a global decline in provincialism, Climate and Paleogeography.—Simultaneous with genera lost during the extinction replaced diversification in both siliciclastic- and by those dispersing from elsewhere (Sheehan carbonate-dominated environments suggests 1975; Krug and Patzkowsky 2007; Finnegan broadscale, generalized diversification driver et al. 2016; Congreve et al. 2019; Penny and Krö- (s) affecting both types of lithologies. One of ger 2019; Rasmussen et al. 2019). Sheehan these drivers was repeated sea-level oscilla- (1975), discussing North America, and Ras- tions, which promoted allopatric speciation in mussen and Harper (2011b), discussing the glo- brachiopods (Lam et al. 2018; Stigall 2018; bal record, both suggest that the LOME led to Pedersen and Rasmussen 2019); the long-term, the loss of shallow-marine specialist brachio- global climatic cooling trend was another pod genera, which were replaced by assem- (Rasmussen et al. 2019). blages of presumably more eurytopic genera At a global scale, Ordovician climatic cooling with broader geographic and depth probably released environmental controls on distributions. the growth of skeletal organisms, allowing This is also evident in the eastern Baltic increased diversification (e.g., see Trotter et al. paleobasin. Notably, mean alpha diversity 2008; Kröger 2017; Edwards 2019; Rasmussen (i.e., diversity within lithostratigraphic units) et al. 2019). The shift of Baltica toward the tro- is largely unaffected by the extinction, and pics brought regional warming but kept the instead the regional diversity decline princi- Baltic paleobasin within this optimal tempera- pally consists of a reduction in faunal differen- ture range (Cocks and Torsvik 2005; Dronov tiation between lithostratigraphic units and and Rozhnov 2007). carbonate- and siliciclastic-dominated litholo- The development of a carbonate platform in gies after the Hirnantian, alongside a decline the Baltic paleobasin and the impacts on in richness at platform localities (Figs. 6A and regional diversity were ultimately a result of 9). In this context, the particular role of Baltica’s Baltica’s plate-tectonic drift toward the tropics. late Katian–Hirnantian deeper-water brachio- But time-specific climatic constraints within the pods, which became important during Silurian tropical realm, namely the intensity of ocean recovery in shallow-water habitats and on currents and levels of global SST, oxygenation, Laurentia, needs to be emphasized (Rasmussen and pH, controlled the details of how this plat- et al. 2010, 2012; Rasmussen and Harper 2011a,b; form accommodated, which organisms pro- Harper et al. 2013). This study corroborates duced the carbonate sediments, and the the finding of these previous works that characteristics of their facies mosaic. These the evolutionary postextinction shift in

brachiopods had simultaneous paleogeo- modulated by sea level, niche breadth, and graphic and paleoecological dimensions, with environmental stressors. Although the Baltic shifting habitats among clades. paleobasin retained its heterogeneous car- Globally, the biotic recovery from the LOME bonate shelf environment through the Silur- was protracted, and Silurian global richness ian, an increase in the dominance of never returned to Ordovician levels (Rasmus- brachiopods with broad environmental and sen et al. 2019). This global pattern is also geographic distributions may have reduced reﬂected in the Baltic paleobasin (Figs. 5, 6A). the impact of habitat heterogeneity during As noted in previous studies (e.g., Harper the Silurian recovery from the LOME. et al. 2015; Harper and Hints 2016; Hints et al. 2018), the regional faunal homogenization Acknowledgments was long-lived, and while regional diversity recovered during the early Silurian, it never We thank L. Hints and L. Ainsaar for con- returned to Ordovician values. In the Silurian, structive discussion during development of while the carbonate platform persisted in the this study, U. Toom for help with locating rele- Baltic paleobasin, faunal differentiation vant literature, S. Scholze for data entry into the between lithostratigraphic units had a persist- Paleobiology Database over the course of this ently reduced impact on regional diversity, study, and D. Matthews for assistance with with generally more similar brachiopod assem- compiling lithologic data. This paper is part blages occupying siliciclastic and carbonate of the project “Ecological Engineering as a Bio- environments. The Silurian decline in faunal diversity Driver in Deep Time,” funded by the differentiation between lithologies, combined Academy of Finland, and is a contribution to with the rise of more cosmopolitan brachiopod the IGCP program 653 “The Onset of the assemblages, suggests that habitat heterogen- Great Ordovician Biodiversiﬁcation Event.” eity became a less important driver of regional O.H. acknowledges support from the Estonian diversity than it had been in the Ordovician. Research Council grant PRG836.

Conclusions Data Availability Statement 1. The development of a carbonate shelf in the Data and R scripts for this study are available eastern Baltic paleobasin during the Middle at: A. Penny, O. Hints, and B. Kröger, 2020, Ordovician was one of several important Data and code for: Carbonate shelf develop- drivers of regional diversification, and this ment and early Paleozoic benthic diversity in link is largely a result of high faunal hetero- Baltica: a hierarchical diversity partitioning geneity (beta diversity) between lithostrati- approach using brachiopod data, Dryad data- graphic units in carbonate-dominated set, https://doi.org/10.5061/dryad.1g1jwstv0. environments. Coeval carbonate development occurred at other localities worldwide, and our study examines the processes that Literature Cited may have driven regional diversity in these Aaloe, A., and H. Nestor. 1977. Biohermal facies in the Juuru Stage (Lower Llandoverian) in Northwest Estonia. Pp. 71–88 in environments. D. Kaljo, ed. Facies and fauna of the Baltic Silurian. Academy of 2. Temporal changes in paleoenvironment Sciences of the Estonian S.S.R. Institute of Geology, Tallinn. (principally due to sea-level change) and [In Russian.] Ainsaar, L., T. Meidla, and T. Martma. 2004. The Middle Caradoc faunal composition (due to migration, ori- facies and faunal turnover in the Late Ordovician Baltoscandian gination, and extinction) can have a major palaeobasin. Palaeogeography, Palaeoclimatology, Palaeoecol- impact on regional diversity curves, but ogy 210:119–133. fi Alroy, J. 2014. Accurate and precise estimates of origination and this impact can be quanti ed using hierarch- extinction rates. Paleobiology 40, 374–397. ical diversity partitioning. Bauert, H., O. Hints, T. Meidla, and P. Männik. 2014. Fourth Annual 3. Substrate heterogeneity emerges as a con- Meeting of the IGCP 591, Estonia 10–19 June 2014. Abstracts and fi Field Guide. P. 202. sistent regional diversi cation driver over Brocklehurst, N., M. O. Day, J. Fröbisch. 2018. Accounting for differ- the Ordovician, though its effects are ences in species frequency distributions when calculating beta

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