The Ordovician chitinozoan biodiversification and its leading factors

Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 5 – 19 www.elsevier.com/locate/palaeo

The Ordovician chitinozoan biodiversification and its leading factors Aïcha Achab a , Florentin Paris b,⁎ a

Institut national de la recherche scientifique-INRS-ETE, 490, rue de la Couronne, Québec (Québec), Canada G1K 9A9 b Géosciences-Rennes, UMR 6118 du CNRS, Université de Rennes 1, 35042 Rennes-cedex, France Received 1 June 2005; accepted 2 February 2006

Abstract Regional and global dataset now available for almost all of the Ordovician fossil groups have led to a search for the causes controlling the major biodiversification events that occurred during the Ordovician period. A review of the physico-chemical state of the Ordovician world is presented, with an emphasis on the regional or global changes contemporaneous with the origination and extinction events that could have influenced the chitinozoan diversity. This study focuses on the chitinozoans because these enigmatic organic-walled microfossils are an important component of the Ordovician palaeoplankton and provide one of the best documented dataset. The intrinsic factors that initiated the Ordovician biodiversification of this group are not discussed because the chitinozoans are regarded as reproduction stages of cryptic “chitinozoan animals”, whose biological characteristics are speculative, at best. This study has two main goals: i) the evaluation of the impact of regional and global physico-chemical events on chitinozoan diversity, ii) the comparison of the biodiversification patterns of the chitinozoans with other selected benthic and pelagic Ordovician fossil groups. The chitinozoan diversification was progressive and showed similar patterns in Laurentia, Baltica and northern Gondwana from the Tremadocian to the late Darriwilian, when the group reached its acme in Baltica and northern Gondwana. From the Middle– Upper Ordovician boundary onward, the biodiversification pattern documented in Laurentia diverged drastically from the two other regions. In the Late Ordovician, the contribution of the Laurentian chitinozoans to the global curve was high, suggesting major faunal inputs from the two other regions, which significantly lowered the endemic character prevalent in the Early–Middle Ordovician. In the Late Ordovician, large anti-clockwise oceanic currents developed as the result of a thermohaline circulation. This marine circulation was likely to have been driven by a global cooling concomitant with a major palaeogeographic reorganisation of the southern hemisphere. These oceanic/climatic changes intervened in the breakdown of the existing chitinozoan endemism. Globally, the chitinozoan biodiversity was not much higher in the mid Late Ordovician than in the late Darriwilian diversity peak. The most obvious feature is the progressive decrease in diversity during the Late Ordovician, long before the Hirnantian glaciation. The influences of cosmic and volcanic parameters are excluded as they appear to have had too low impact on the Late Ordovician decrease of chitinozoan diversity. Correlation is noticed between some diversification events and sea-level changes, at least on a regional scale. More globally, however, a climatic control is favoured. A durable greenhouse environment gave an efficient support to the diversification. Conversely, the onset of an icehouse environment in the early Late Ordovician onward, culminating with the Hirnantian glaciation, is interpreted as a limiting factor for the chitinozoan diversification. The reasons behind the onset of this icehouse episode are not fully understood, but they appear to be linked to changes in the

⁎ Corresponding author. Tel.: +33 0 2 23 23 69 89; fax: +33 0 2 23 23 61 00. E-mail addresses: [email protected] (A. Achab), [email protected] (F. Paris). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.02.030

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palaeogeography (e.g., progressive closure of the Iapetus, northward drift of Avalonia and its docking with Baltica) and in the carbon cycle (e.g., high organic carbon burial with a lowering of the pCO2). © 2006 Elsevier B.V. All rights reserved. Keywords: Chitinozoans; Biodiversity; Global changes; Ordovician

1. Introduction Recently, under the auspices of IGCP Project n° 410, an attempt was made to evaluate the biodiversification patterns of most of the Ordovician fossil groups. Biodiversification curves have been established at species, generic or family levels, either globally, or on a regional scale (Webby et al., 2004). In the present paper we will focus on the biodiversification of the chitinozoans. The Ordovician time-slices (T.S.) used for the IGCP 410 project have been updated in order to take into account the new subdivisions of the Ordovician System adopted by the International Subcommission of Ordovician Stratigraphy (I.S.O.S.), although not yet ratified by the I.U.G.S. These changes mainly concern on the Upper Ordovician, with the Hirnantian becoming the seventh stage, and the base of the sixth Stage coinciding with the FAD of the graptolite Diplacanthograptus caudatus. As a consequence, the numbering of the timeslices of Webby et al. (2004) is modified here by substituting Roman numbers for Stage 6 (i.e., T.S. VIa– d = T.S. 5c–d plus T.S. 6a–b of Webby et al., 2004) and the Hirnantian (T.S. VII = T.S. 6c of Webby et al., 2004). Chitinozoans are organic-walled microfossils widely distributed in most Ordovician to Devonian marine sediments. Their minimal dependence to facies and their rapid evolution have allowed for the establishment of high-resolution regional biozonations. Although their biological affinities are unknown, these microfossils are regarded as representing eggs of soft-bodied metazoans (Paris and Nõlvak, 1999, and references therein). Alternatively, “resistant stages” (e.g., cyst) of unknown organzisms may be another explanation. Whatever they may have been, chitinozoans can be used to document the pattern of diversification of their parent producers (i.e., the “chitinozoan animals” sensu Paris and Nõlvak, 1999). Several parameters of chitinozoan Ordovician biodiversification (e.g., normalized standing diversity, turnovers, origination and extinction patterns) have been documented (Paris et al., 2004). Diversity curves have been drawn for the most widely investigated palaeogeographical domains, i.e. Laurentia, Baltica, Avalonia, south China and western, northern and eastern Gondwana. The present study is a second step in depicting the

Ordovician chitinozoan diversification. Its main goal is to establish definite ties between chitinozoan diversification patterns and regional or global extrinsic factors, such as palaeogeography, oceanic circulation, sea water composition, sea-level fluctuation, climate changes, volcanic or tectonic activities and cosmic events (e.g., bolide impact, gamma-ray bursts). A second aspect of our investigations is concerned with the comparison of the pattern of chitinozoan diversification with those of other selected benthic (bryozoans, brachiopods) and pelagic (graptolites) or necto-benthic (trilobites) components of the Ordovician fauna. This comparison may provide new ideas for addressing the problems of chitinozoan ecology and their biological affinity. In our study, three palaeobiogeographical palaeoplates i.e., Laurentia, Baltica and northern Gondwana are considered (Fig. 1). The chitinozoan diversity on these palaeoplates (Fig. 2) can be regarded as representative of the global behaviour of the group because the data derived from them represents more than two-thirds of the Ordovician chitinozoan database and documents assemblages occurring at low, intermediate and high latitudes. 2. Main physico-chemical parameters of the Ordovician world Among the cosmic parameters, no data are available on the impact of important gamma-ray bursts, recently evoked by some authors (Melott et al., 2004) for the Late Ordovician mass extinction, as a driving factor for the fluctuations in chitinozoan diversity. On the other hand, chitinozoans were the first group that recovered after the Late Ordovician bolide impacts on a shallow marine platform in Baltoscandia (Grahn et al., 1996). No significant changes in the chitinozoan fauna were noticed and thus, impact of extraterrestrial bodies of moderate size had no determining effect on the diversity of the group. 2.1. Ordovician palaeogeography The Ordovician was a period of important plate dispersion. The land masses were mostly concentrated in the southern hemisphere with the exception of Laurentia, which straddled the equator, while the vast Panthalassic

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Ocean occupied the northern hemisphere (Fig. 1). The dispersion and the contrasted palaeolatitudinal location of the Ordovician palaeoplates, microplates and terranes resulted in the generation of a high degree of endemicity among the marine biota. The occupation of new ecological niches, the temporary geographical isolation of faunas and the contrasting climatic belts of the Late Ordovician, largely contributed to the development of one of the highest Palaeozoic faunal diversities. Laurentia straddled the equator during the entire Ordovician period, while Baltica was moving northward from an intermediate latitude in the earliest Ordovician to a lower latitude in the Late Ordovician. This movement was the result of the opening of the Rheic Ocean, which was concomitant with the closure of the Iapetus Ocean (Fig. 1). Avalonia, which had left northern Gondwana in Late Cambrian–earliest Ordovician times (Prigmore et al., 1997), drifted more rapidly than Baltica and docked with it by Late Ordovician times. In the meantime, the Iapetus Ocean was in a closure phase and the passive eastern margin of Laurentia changed to a convergent setting marked by the collision of a series of microterranes and volcanic islands during the Late Ordovician (Mac Niocaill et al., 1997). Gondwana, after an initial southward drift, moved progressively northward during the Ordovician (Paris and Robardet, 1990). Consequently, the northern Gondwana regions (e.g., present day North Africa, the Arabian Peninsula and southern and parts of central Europe) changed from a polar or circumpolar location in the Early Ordovician to a less high latitudinal position in the latest Ordovician (Fig. 1). A high degree of faunal endemicity and different biodiversification patterns should be expected from the remote location of these three palaeogeographical provinces occupying different climatic belts. The climatic contrast increased during the Late Ordovician when an icehouse environment progressively replaced the existing greenhouse conditions. It was susceptible to intensify the faunal provincialism. However, other factors (e.g., oceanic currents, convergence of plates) had an opposite effect, favouring the weakening of this provincialism by faunal mixing. 2.2. Ordovician oceans 2.2.1. Ordovician transgressions and sea level The Ordovician was characterised by high sea levels, perhaps the highest in the Phanerozoic (Nielsen, 2004). Because of the peneplaned profile of many cratonic areas, global Ordovician transgressions generated extensive epireic seas (Barnes, 2004a), which created new ecological niches favourable to faunal diversification.

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Ordovician sea-level curves have been established for North America (Ross and Ross, 1992, 1995) and Baltica (Nestor and Einasto, 1997; Nielsen, 2004). The North American curve documented by Ross and Ross (1995, Fig. 1) shows three second-order sets with different types of third-order sequences. The first set corresponds to the Ibexian (i.e., Lower Ordovician time-slices T.S. 1a–2c in Webby et al., 2004) and records highstands of relatively long duration and lowstands of short duration. The succeeding second-order set corresponds to the Whiterockian (Middle Ordovician and lower part of the Upper Ordovician, i.e. T.S. 3a–5a in Webby et al., 2004) and represents a long interval of much lower sea level, terminating in an abrupt sea-level drop prior to the extensive Blackriverian marine transgression. According to Nielsen (2004, Fig. 10.2), the North American latest Whiterockian lowstand and the Blackriverian transgression correspond respectively in Baltoscandia to the Vollen Lowstand and to the Arnestad drowning, whereas the preceding Furudal Highstand reaches its maximum close to the base of T.S. 5a (i.e. close to the FAD of N. gracilis). The third Ordovician second-order set of Ross and Ross (1995) is characterised by abrupt and important sea-level fluctuations. Nielsen (2004, Fig. 10.3) considers that the high sea level inferred for the Mohawkian and the lower Cincinnatian matches the later part of the Late Llanvirn–Caradoc Highstand interval (Fig. 3). Two major sea-level falls respectively recorded in the terminal foliaceus graptolite Zone (early Stage 6, base of T.S. VIa= T.S. 5c of Webby et al., 2004) and at the top of the clingani graptolite zones (top of T.S. VIa) are also recognised in Baltoscandia i.e., Frognarkilen and Solvang lowstands events of Nielsen (2004). They have been interpreted as a possible effect of an early and short-lived development of an ice cap on Gondwana (Pope and Read, 1998; Nielsen, 2003; Bourahrouh et al., 2004; Saltzman and Young, 2005). Cincinnatian rocks indicate four transgressions and regressions of short duration (Ross and Ross, 1995) recognisable also in Baltoscandia. A low sea-level interval corresponding to the Hirnantian glacial maximum is recorded in the early Hirnantian (end of the Richmondian/base of the Gamachian, i.e. base of T.S. VII = T.S. 6c of Webby et al., 2004). Ross and Ross (1995) also indicated a short-lived sea-level rise in the Gamachian on Laurentia (i.e., in the late Hirnantian). Although tentative, correlations between the North American and Baltoscandian curves suggest global sea-level rises (e.g., evae, basal gracilis, linearis zones) and shallowing events (terminal foliaceus, late clingani, and basal complanatus zones). Moreover, the Late Ordovician patterns of the two sea-level curves suggest that changes were more rapid than during the Early and Middle Ordovician and that they might have been under a glacio-eustatic control.

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The Ordovician sea-level curve for northern Gondwana is not yet finalised. However, it is possible to identify several major cycles. The first cycle began with the Tremadocian transgression and ended with the mid Arenig regression. The second cycle spanned the Darriwilian. The third cycle started in T.S. 5b–c. These cycles were interrupted by drastic sea-level drops in late Stage 5 (post dalbyensis–pre tanvillensis chitinozoan biozones, i.e. upper T.S. 5b), early Stage 6 (fistulosa chitinozoan biozone, i.e. in T.S. VIb), and particularly in the lower Hirnantian (Paris et al., 1995). The late Hirnantian transgression is interpreted as the result of the main melting phase of the Gondwanan ice cap (Paris et al., 2000). 2.2.2. Oceanic circulation On a physico-chemical level, deep and surface oceanic circulations are responsible for the diffusion of the heat and the homogenisation of the chemical composition of the ocean. Concerning the marine fauna and flora, the surface Ordovician currents, depending on their orientation, may have acted as seaways or conversely, as barriers for the spreading of the pelagic elements. Consequently, the Ordovician oceanic circulations, although admittedly poorly known, probably had an important impact on the biodiversification and must be taken into consideration. The Ordovician period was characterised by an odd distribution of lands and oceans. In the Lower Ordovician, the northern hemisphere Panthalassic Ocean covered about half of the earth surface and the Iapetus Ocean, separating Laurentia from Baltica, was at its maximum extent. The latitudinally oriented Rheic Ocean separated Gondwana from Avalonia and Baltica, and the Tornquist “sea” separated Avalonia from Baltica (Fig. 1A). Because of the equatorial barriers resulting from the geographical distribution of large palaeoplates (Laurentia, Siberia and North China), the circulation within each ocean was more or less independent. Heat, and faunal exchanges, were thus of limited amplitude, favouring the development of endemic fauna. Moreover, because of the greenhouse conditions (Berner, 1994; Berner and Kothavala, 2001), the thermohaline oceanic circulation was not active. The Early Ordovician was thus characterised by stable, densitystratified oceans (Barnes, 2004a, and references therein). During the Middle and the Late Ordovician, the palaeoplate movements, e.g. the motion of Gondwana, the northward drift of Baltica and other microcontinents such as Avalonia, towards Laurentia continued. These move-

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ments were related to the closure of the Iapetus and the acceleration of the Rheic opening (see above). The existing sluggish oceanic circulation was disrupted or deeply modified (Barnes, 2004a). More active anticlockwise currents in the southern hemisphere gyre (Fig. 1C) favoured faunal inputs (e.g. incursion of Baltoscandian taxa in northern Gondwana, Henry et al., 1976). During the early Late Ordovician, the onset of a glaciation (Pope and Read, 1998; Bourahrouh et al., 2004; Nielsen, 2003; Saltzman and Young, 2005), following a long greenhouse period, generated a thermohaline circulation with drastic consequences on the climate and the nutriment resources. A major impact on the biodiversity should be expected from such a major global change. 2.2.3. Sea water chemistry Shields et al. (2003) have documented a constant decrease of the 87Sr/86Sr ratio during the Early and Middle Ordovician. A marked excursion occurred at the Darriwilian–Caradoc boundary (Fig. 3). This important shift is considered to represent the decreasing influence of craton derived strontium or, conversely, a more important contribution of unradiogenic strontium from juvenile submarine volcanism and hydrothermal activities (Shields and Veizer, 2004, and references therein). The timing of the excursion correlates with the early Late Ordovician (T.S. 5a–b, i.e. gracilis transgression and Keila Drowning Event, sensu Nielsen, 2004), which thus could be the result of an acceleration of the opening of the Rheic Ocean (i.e., with a high MORB production). The neodymium isotopes are tracers of water masses and sediment provenance. They suggest that during the Early and Middle Ordovician the Laurentian seaways were isotopically distinct, although this distinction disappeared by the Late Ordovician when the global ocean displayed a more juvenile volcanic signature (Shields and Veizer, 2004). Other chemical signatures of the Ordovician oceans are provided by the δ13C, and by the δ18O. Carbon and oxygen isotopic ratios provide information about the level of biological activity, upwelling, salinity and temperature. The δ18O excursion, which is documented globally in the early Hirnantian, can be confidently related to the stocking of fresh water in the Gondwanan ice cap (Lécuyer and Allemand, 1999; Brenchley, 2004). The various δ13C excursions recorded during the Middle and Late Ordovician (Ainsaar et al., 1999, 2004, and references therein) have a less obvious relationship with

Fig. 1. Schematic reconstruction of the Ordovician palaeogeography: A) Early Ordovician (around 475 Ma), B) Middle Ordovician (around 465 Ma), and C) latest Ordovician (444 Ma, during the Hirnantian glaciation). Interrupted lines indicate the oceanic ridges, triangles give the schematic position of subduction zones, white arrows symbolize possible oceanic currents.

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a glaciation. However, Kump et al. (1999) suggested that the enhanced organic carbon production and its burial resulted in a drawdown of the pCO2 to levels susceptible to initiate ice build-up (Pope and Steffen, 2003; Herrmann et al., 2004). Saltzman and Young (2005) interpreted the Eureka Quartzite, which followed on the Chatfieldian δ13C excursion, in terms of an eustatic drop.

This event (late clingani graptolite Zone) was contemporaneous to other important sea-level drops reported in Baltica (Nielsen, 2004) in T.S. VIa (= T.S. 5c sensu Webby et al., 2004) and in northern Gondwana in the post dalbyensis–pre tanvillensis chitinozoan biozones. Such changes in the carbon cycle could have influenced the diversification of the chitinozoans through a

Fig. 2. Normalized diversity curves (BTD: Balanced total Diversity sensu Paris et al., 2004) for the Ordovician chitinozoans from Laurentia, Baltica and northern Gondwana regions, and global curve (respectively from Achab, Nõlvak, Paris, in Paris et al., 2004). The dashed lines represent the available samples per time-slice. The Roman numbers, i.e. VI and VII, correspond respectively to the sixth Ordovician Stage and to the Hirnantian.

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Fig. 3. Main physico-chemical parameters and events of the Ordovician world compared with the global diversity curve of the chitinozoan species. The arrow beneath the column with the 87Sr/86Sr curve indicates the lowering trend. White and black arrows symbolize respectively regressive (L) and transgressive (H) trends/events in the column of sea-level changes (HS = Highstand Interval, LS = Lowstand Interval sensu Nielsen, 2004). In the shelf facies column, “G” corresponds to north Gondwana, “B” to Baltica, and “L” to Laurentia. The light grey colour symbolizes the terrigenous deposits (shale, siltstone, sand) and dark grey indicates carbonate dominated sedimentation. (limestone, marl, dolomite). The size of the black triangles in the δ13C column is roughly representative of the importance of the excursion.

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profound modification of the primary production of the phytoplankton, which was likely part of the trophic chain of the “chitinozoan animals”. 2.3. Ordovician climates Regional climates are controlled by global conditions, the latitudinal location, and the elevation of the area. Most of the Palaeozoic, from the Cambrian to the Carboniferous, has been interpreted as a greenhouse period (Berner, 1994; Berner and Kothavala, 2001), interrupted by short-lived ice ages in the Hirnantian (Brenchley, 2004), the Late Devonian and in the Early Carboniferous. In the Ordovician palaeogeographic context, Laurentia was in a warm equatorial position, whereas Baltica moved from a temperate latitude to a warmer subtropical position in the Late Ordovician, as documented by the sediments and fauna (Nestor and Einasto, 1997). The northern Gondwana regions remained at high latitudes and underwent a cold climate during the onset of the Late Ordovician glaciation, especially during the Hirnantian glacial maximum. The rapid build-up of a huge ice cap on Gondwana, the subsequent lowering of tropical temperatures, and the drop of sea level, are reflected by the short-lived positive Late Ordovician δ18O excursion (Brenchley et al., 1994; Finney et al., 1999; Shields and Veizer, 2004, and references therein). The duration of the Late Ordovician glaciation is still a matter of debate. New glacial evidence prior to the wellknown Hirnantian glaciation on Gondwana is progressively emerging both from isotopic, sedimentological and sea-level data (Pope and Read, 1998; Hamoumi, 1999; Nielsen, 2003, Bourahrouh et al., 2004; Saltzman and Young, 2005). Therefore, the dramatic climate change in the Hirnantian should be regarded as part of a longer process initiated in the early Late Ordovician. The shift from slow sea-level changes during the Early and Middle Ordovician to a pattern of rapid changes during the Late Ordovician is likely to have been registered in the biodiversity pattern of the Late Ordovician faunas. Indeed, such event should have deeply modified a number of environmental parameters. It created (drowning events) or suppressed (regressive events) numerous marine ecological niches in response to rapid eustatic sea-level variations (see Nielsen, 2004) prior to the Hirnantian glacial climax. 2.4. Ordovician sedimentation Sedimentary conditions were different on the shelves of the three selected Ordovician palaeoplates. On Lau-

rentia, the Lower Ordovician (Ibexian) is represented by broad carbonate sequences (Fig. 3), the result of passive margin conditions. During the Whiterockian, the development of the Taconic foreland basin resulted in a change from very shallow carbonate deposits, through siliciclastic dominated deep-water shales and siltstones, and finally to shallow marine to non-marine sandstone and mudstone in the Late Ordovician. Shelf deposits on Baltica are mostly represented by bioclastic carbonates, and by carbonates with corals and stomatoporoids related to the northern drift of Baltica towards lower latitudes (Nestor and Einasto, 1997). In the early Early Ordovician (T.S. 1a–2b), however, sandy episodes occurred (e.g. “Obolus” and “glauconite sand”; see Nõlvak, 1997). In deeper environments, marls, siltstones and mudstones prevailed during the entire Ordovician (see Nestor and Einasto, 1997, and references therein). Terrigenous deposits were dominant in the Ordovician sedimentation on the northern Gondwana platform (Fig. 3). This was partly due to the high latitudinal location of the area during this period. A few calcareous sandy horizons, however, are reported locally in the Middle Ordovician. The only widespread occurrence of bioclastic limestone on northern Gondwana is in the Late Ordovician (Villas et al., 2002, and references therein). The significance of these carbonates, which could represent either cold water carbonates (Cherns et al., 2004) or a brief warming prior to the Late Ordovician glaciation (Fortey and Cocks, 2004), is a matter of debate. Nevertheless, they represent a drastic change in the environment as documented by the concomitant and spectacular increase in the diversity of the benthic fauna (Webby et al., 2004). However, the nature of the shelf sediments had probably only a limited impact on the chitinozoan diversity because alternating shale and limestone beds usually yield identical chitinozoan assemblages (Paris, 1981). 2.5. Ordovician tectonics and volcanism In addition to illustrating the drift history of various terranes, Fig. 1 shows the location of the main Ordovician rifting and subduction zones related to, respectively, the opening and to the closure history of the Rheic and the Iapetus oceans. The development of subduction zones and the collision of several terranes and volcanic arcs along the Laurentian margin were responsible for the Taconic orogeny. This is the major Ordovician tectonic event and it generated an important increase of the volcanic activity. K-bentonites are well known in Mohawkian

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sequences (Upper Ordovician, T.S. VIa) of Laurentia (Huff et al., 1992). The pyroclastic ashes belong to a felsic to calc-alkaline suite characteristic of volcanism along an active tectonic setting. In eastern Laurentia, they were ejected from the subduction zone related to the Taconic orogeny. Bentonites, possibly cogenetic with the Laurentian ones, are also widely distributed in Baltoscandia (Bergström et al., 1995; Huff et al., 1999). Abundant bentonites are also present in Middle Ordovician sequences of the Argentine Precordillera (Huff et al., 1995). The Ordovician succession of eastern Avalonia contains numerous and sometimes very thick volcanic sequences, both from basaltic and acid suites, in the British Isles (Fortey et al., 2000, and references therein) and in Belgium (André et al., 1986). The cratonic part of the northern Gondwana regions (e.g. Sahara, Arabian Peninsula) shows little evidence of Ordovician volcanic activity. On the southern side of the Rheic Ocean, however, several episodes of short-lived basaltic and/or rhyolitic volcanism are reported (NW Spain, Brittany, Bohemia). However, when compared to the Laurentian and other volcanic provinces, they were of a too small magnitude to have had a significant impact on the global climate or the oceanic composition, and thus most likely had no influence at all on the chitinozoan diversity. Barnes (2004b) postulated an Ordovician superplume event, he also noted that there was little chance of preservation for massive basalt production (e.g., giant oceanic dyke swarms, oceanic plateaus and flood basalts) because of the total subduction of Palaeozoic oceanic floors. Indirect evidence of such gigantic volcanic activity, however, is possibly given by the important strontium isotope excursion from the late Mid Ordovician onwards (Shields et al., 2003). Alternatively, the substantial decrease of the 86Sr/87Sr during the Late Ordovician might be related to an acceleration of the Rheic spreading rates (Shields and Veizer, 2004). 3. Ordovician faunal diversity The biodiversification patterns of most Ordovician fossil groups have recently been investigated (Webby et al., 2004). The diversification patterns, at the specific (or generic) level, of selected groups regarded as representative of the Ordovician benthos, nectos and pelagos (brachiopods, bryozoans, trilobites and graptolites) are compared to the patterns of chitinozoan diversification (Fig. 4). Information on the environmental behaviour of the “chitinozoan animals” is expected from such comparisons.

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3.1. Diversity of selected benthic fauna Two groups of benthic fauna, bryozoans and Rhynchonelliformean brachiopods, have been selected to illustrate the diversification pattern of mostly fixed fauna requiring a low food supply through a period of time with changing physico-chemical characters. The diversification curves of the two groups have been documented globally. The bryozoans display a progressive but moderate diversification at the species level (Taylor and Ernst, 2004), from their origination in the late Tremadocian to the early Late Ordovician (T.S. 5a of Webby et al., 2004). A steep increase generating a first diversity peak occurred during the early Late Ordovician (T.S. 5b–a), followed by a dramatic drop (loss of 1/3 of the species) in T.S. VIb (6c in Webby et al., 2004). A brief recovery (T.S. VIc = 6a in Webby et al., 2004) preceded the Hirnantian mass extinction (Fig. 4). A fairly different pattern is registered in the Rhynchonelliformean brachiopods (Harper et al., 2004). Their global diversity at generic level increased slowly from the early Tremadocian to the early Middle Ordovician (T.S. 3a), and then increased rapidly in T.S. 3b (Fig. 4). The diversification continued through the Darriwilian to early Stage 6 (T.S. VIa), with only a brief pause in the latest Darriwilian (T.S. 4c). It is noteworthy that this stasis in the diversification in these brachiopods precisely coincides with the peak of diversity of the chitinozoans, graptolites and trilobites of Baltica (see below). The Rhynchonelliformean brachiopods, like most other Ordovician groups, registered a dramatic drop of diversity (Fig. 4) during the Hirnantian glaciation (Harper et al., 2004). 3.2. Diversity of the pelagic and the “free” necto-benthic fauna The graptolites (Graptoloidea) have been selected to illustrate the behaviour of pelagic fauna, as they are often associated with chitinozoans and therefore believed to be ecologically close to the “chitinozoan animals”. Cooper et al. (2004) have provided excellently documented diversity curves (normalized diversity) for Avalonia, Baltica and Australasia. The limited information available on the Ordovician chitinozoans from Australasia (see Winchester-Seeto in Paris et al., 2004) prevents any comparison with the graptolite curve. The Avalonia and Baltica graptolite curves are very different from the Australasian curve, even if they all display a marked increase in the late Early Ordovician (T.S. 2b). The peak of diversity observed in T.S. 2b in Australasia may be related to the low latitudinal position of the area (Cooper et al., 2004). The

14 A. Achab, F. Paris / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 5–19 Fig. 4. Normalized diversity curves of selected Ordovician fauna. Bryozoan global species curve from Taylor and Ernst (2004, Fig. 16.1), Rhynchonelliformean brachiopod global generic curve from Harper et al. (2004, Fig. 17.1), trilobite regional species curves for Anglo-Welsh regions and for Baltica from Adrain et al. (2004, Figs. 24.9, and 24.10), and regional curves for the graptolite species of Avalonia and Baltica from Cooper et al. (2004, Figs. 27.3A and 27.4A, modified). The global chitinozoan curve is from Paris et al. (2004, Fig. 28.6).

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Baltica graptolite curve registers a maximum of diversity in the late Darriwilian (4c) in harmony with the chitinozoans. However, the missing Late Ordovician part of the Baltic graptolite curve does not allow further comparisons. The graptolite curve for Avalonia is more complete but shows a different pattern as it registers a maximum diversity in the early Darriwilian when the graptolites from Baltica display a reduced diversity. The reason of this peculiar pattern is unclear but it may be in part related to a cumulative effect resulting from the setting of new oceanic seaways (see Fig. 1) generating faunal inputs of various palaeogeographical origins (e.g. northern Gondwana, Laurentia). Trilobites are a well-known fossil group, present in the Ordovician in a large range of ecological niches (from pelagic forms to scavengers). Several welldocumented regional diversity curves are available (Adrain et al., 2004). We have selected the Avalonia and Baltica trilobite curves as they are palaeogeographically the closest to the area investigated for chitinozoans. The Avalonian trilobite diversification is very regular when compared to the diversification pattern of the trilobites from Baltica (Fig. 4). The late Darriwilian peak (T.S. 4c), corresponding to an increase of one-third of the mean Ordovician diversity, is not observed in Avalonia. Only the Ashgill diversification (T.S. VIc–d) is interpreted as reflecting the extreme heterogeneity of the environment during this time interval (Owens and McCormick in Adrain et al., 2004). The docking of Avalonia with Baltica and Laurentia began at this time (Cocks and Torsvik, 2004, and references therein), but a mixing, with addition of the species from these palaeobiogeographical units, seems to have been unlikely, as the steep increase of the Avalonian trilobite diversity has no counterpart in Baltica (Fig. 4). 3.3. Patterns of chitinozoan diversity and their possible causes The Ordovician chitinozoan biodiversity curves for Laurentia, Baltica and northern Gondwana (Fig. 2) have been established on the basis of a dataset of more than 8000 productive samples. The diversity curves and their related parameters (normalized diversity at species level, origination, extinction rates and turnover ratios) have been discussed in detail (Paris et al., 2004). This paper will focus only on the most important events shown by the diversity curves and considered to be of global significance. 3.3.1. Lower and Middle Ordovician The diversity curves illustrated on Fig. 2 clearly show that during the Early and Middle Ordovician, the

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chitinozoan radiation was gradual and the chitinozoan diversity remained moderate until the early Darriwilian (less than 30 species per time-slice). This radiation was partly driven by intrinsic factors, as suggested by the great number of morphological innovations that appeared during this period (Paris et al., 1999). These intrinsic factors will not be considered here as the biological significance and the true affinities of the chitinozoans have not been definitively demonstrated. Extrinsic factors, e.g., stable greenhouse climate without thermohaline oceanic circulation, and the low rhythm of sea-level variations (see Ross and Ross, 1992; Nielsen, 2004) also contributed to the development of this gradual radiation (Fig. 3). The three curves display the same biodiversification pattern indicating that the main driving factors were global, acting in the three remote areas and favouring the development of specific chitinozoan communities. This is illustrated by the global curve, which registers values very close to the sum of the three sets of regional data (Fig. 2) during the Early and Middle Ordovician. An important peak in diversity is observed at the end of the Darriwilian (T.S. 4c) in Laurentia, Baltica and northern Gondwana (Fig. 2). This peak corresponds with the highest diversity recorded in Baltica and northern Gondwana. The total diversity observed on the global curve at this time level represents approximately the sum of the species reported in each palaeoplate. This confirms that the chitinozoan populations of each palaeoplate were distinct and that the chitinozoan geographical distribution was still marked by a strong latitudinal provincialism (Achab, 1988; Paris, 1993). As greenhouse environments persisted until the early late Darriwilian (Fig. 3), this provincialism is likely to have been related to the remote and different latitudinal location of the palaeoplates and to sluggish oceanic currents (Barnes, 2004a), not active enough for the mixing of the pelagic forms (Fig. 1). The significantly higher chitinozoan production recorded in northern Gondwana during the Early and Middle Ordovician suggests that the cooler high latitude environments were the most favourable for the proliferation of the “chitinozoan animals”. Alternatively, a lower predation pressure might be invoked as an additional explanation for this higher abundance. When comparing the diversity curves with the major sea-level “lowstand” and “highstand” Intervals defined by Nielsen (2004) for Baltica and correlated to the sealevel fluctuations documented by Ross and Ross for North America (1992, 1995), the first slight increase recorded by the global chitinozoan diversity curve during the Stage 2 can be correlated to the “Middle Arenig

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Highstand” of Nielsen (2004). The Laurentian curve shows a decrease in diversity during the lower part (T.S. 3a–4b) of Whiterockian series. This interval is interpreted as a lowstand interval prior to the “Late Llanvirn– Caradoc Highstand Interval” beginning in T.S. 4c and marked by a peak in the diversity. Nevertheless, these sea-level changes that may have had a large impact on benthic fauna (drowning episodes providing new ecological niches and regressions removing them) seem to be of less importance for pelagic forms. It is worth mentioning that the main diversity peaks for chitinozoans and graptolites from north Gondwana/ Avalonia and Baltica (Cooper et al., 2004) occur during T.S. 4c, at the end of a lowstand episode (Fig. 3) if one accepts the sea-level interpretation given by Nielsen (2004). Conversely, benthic fauna (e.g., Rhynchonelliformean brachiopods) show a stability (Fig. 4) of their diversity during this interval (Harper et al., 2004). 3.3.2. Upper Ordovician An important change occurred at the Middle–Upper Ordovician boundary. During the Late Ordovician the three regional diversity curves evolved distinctly, and the global chitinozoan diversity was at its maximum (Fig. 2). The highest diversity is observed at low latitudes (Laurentia), whereas the lowest values are registered at high latitudes (northern Gondwana), while Baltica exhibited intermediate values (Fig. 2). These modifications in the diversification pattern of the chitinozoans coincided with a major change in ocean water chemistry, expressed by a drop in the 87Sr/86Sr ratios. It is also contemporaneous with one of the largest Ordovician transgression (gracilis transgressive event) and with a long highstand period (Ross and Ross, 1992; Nielsen, 2004). The Late Ordovician Highstand Interval (Fig. 3), which is contemporaneous to a significant global increase of the diversity of the bryozoans (Taylor and Ernst, 2004) and of the brachiopods (Harper et al., 2004), is marked by a decrease in diversity of the Baltic and Avalonian trilobites (Adrain et al., 2004 and graptolites (Cooper et al., 2004). Northern Gondwana and Baltica chitinozoans display a pattern similar to the pelagic and nectic forms (Paris et al., 2004). Thus, the chitinozoans, or, better, their producers (“chitinozoan animals”) were not likely to have been sea bottom animals. In Laurentia, the late Whiterockian Lowstand interval is reflected by a lowered chitinozoan diversity during T. S. 5a–5b. This decrease is also recorded in northern Gondwana and in the global diversity curves and it is followed by the greatest diversity increase in the early Late Ordovician (T.M. VIa). A similar pattern is observed in the echinoderms during the Mohawkian

(Sprinkle and Guensburg, 2003). Concerning the chitinozoans, however, the total diversity observed is not significantly higher than in Laurentia (Fig. 2). This indicates that the Laurentian assemblages shared most of the species represented in Baltica and northern Gondwana regions, but that they also had their own specific taxa not represented in these two other regions. This can be explained by the existence of active oceanic currents transporting the pelagic forms from higher latitudes to lower ones (Fig. 1C), thus softening the previous provincialism. It also indicates that the warm climate of Laurentia was favourable to the development of a more diversified chitinozoan fauna. The highest diversity depicted on the global diversity curve corresponds roughly to the “upper Darriwilian–Caradoc Highstand Interval” of Nielsen (2004) while the Late Ordovician declining diversity coincides with the “Ashgill Lowstand Interval”. In Laurentia a small peak in diversity recorded at the top of the Stage 6 (T.S. VId) coincides with the Ashgillian drowning events recognised by Nielsen (2004). At that time, as it was the case in T.S. VIa, the global diversity was not significantly higher than in Laurentia. The high diversity recorded during the Late Ordovician in Laurentia, in addition to its warmer environment, can be related to the Taconic Orogeny and to the development of the foreland basin. The low diversity attained, both regionally and globally (Fig. 2), during the Hirnantian (T.M. VII), reflects the dramatic climatic and oceanic circulation changes engendered by the glaciation climax on Gondwana. 4. Conclusions As for most Ordovician faunas, the chitinozoans had a progressive and rather regular increase of diversity through the Early and Middle Ordovician. This biodiversification pattern is interpreted as the combined result of intrinsic biological parameters (e.g., expressed by the high tempo of the morphological innovations), extrinsic physico-chemical factors resulting from a long and stable greenhouse period (fairly low latitudinal climatic contrast, sluggish currents) and from the palaeogeographical context (extreme dispersion of the main continental plates). The drastic changes recorded close to the Middle–Late Ordovician boundary include a significant lowering of the Late Ordovician chitinozoan diversity on Baltica and northern Gondwana, whereas, in Laurentia, this diversity is at its maximum because the regional diversity was incremented by species input from Baltica and northern Gondwana. These changes reflected a drastic weakening of the older provincialism and are most likely due to the development of an active

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thermohaline oceanic circulation contrasting with the previous inactive currents. We consider this modification of oceanic circulation to be largely related to the major changes that occurred in the configurations of the landmasses, as Avalonia approached and docked with Baltica and the Iapetus Ocean dramatically narrowed. New cells of oceanic circulations were generated, allowing a more effective mixing of the pelagic elements. It also allowed for the setting of active upwelling processes that increased the phytoplankton production, the burial of which might have induced a lowering of the pCO2, at term leading to the replacement of greenhouse by icehouse conditions. Thus, the Hirnantian mass extinction was the last act of a cooling episode that began several million years before climaxing in the early Hirnantian. The cooling was not constant, and warmer episodes occurred during late Stage 6 (Ashgill pro parte). The peculiar behaviour of the Laurentian chitinozoans reflected the higher “weight” of regional factors, such as the development of foreland basins and the increase in terrigenous influx connected to the Taconic Orogeny. The equatorial position of Laurentia acted as a natural buffer to the effects of the global cooling. Acknowledgements The authors thank John Riva (Canada) for checking the English and improving the text. They are grateful to the colleagues who provided data for the chitinozoan global diversity curve, and peculiarly to Jaak Nõlvak (Tallinn). The manuscript benefited from reviews by Jacques Verniers (Belgium) and Garry L. Mullins (U.K.). This is a contribution to IGCP Project n° 503 “Ordovician Palaeogeography and Palaeoclimate”. References Achab, A., 1988. Mise en évidence d'un provincialisme chez les chitinozoaires ordoviciens. Canadian Journal of Earth Sciences 25, 635–638. Adrain, J.M., Edgecombe, G.D., Fortey, R.A., Hammer, O., Laurie, J.R., McCormick, J.T., Owen, A.W., Waisfeld, B.G., Webby, B.D., Westrop, S.R., Zhou, Z.-Y., 2004. Trilobites. In: Webby, B.D., Paris, F., Droser, M., Percival, I. (Eds.), The Great Ordovician Diversification Event. Columbia University Press, New York, pp. 231–254. Ainsaar, L., Meidla, T., Martma, T., 1999. Evidence for a wide-spread carbon isotopic event associated with late Middle Ordovician sedimentation and faunal changes in Estonia. Geological Magazine 136, 49–62. Ainsaar, L., Meidla, T., Tinn, O., 2004. Middle and Upper Ordovician stable isotope stratigraphy across the facies belts in the East Baltic. In: Hints, O., Aismaar, L. (Eds.), WOGOGOB-2004, 8th Meeting of the Working Group on the Ordovician Geology of Baltoscandia, Abstracts, pp. 11–12.

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