Early Ordovician (Arenig) faunal assemblages from western Argentina: biodiversification trends in different geodynamic and palaeogeographic settings

Palaeogeography, Palaeoclimatology, Palaeoecology 196 (2003) 343^373 www.elsevier.com/locate/palaeo

Early Ordovician (Arenig) faunal assemblages from western Argentina: biodiversi¢cation trends in di¡erent geodynamic and palaeogeographic settings Beatriz G. Waisfeld a; , Teresa M. Sa¤nchez a , Juan Luis Benedetto b , Marcelo G. Carrera a a

CONICET, Ca¤tedra de Estratigraf|¤a y Geolog|¤a Histo¤rica, Facultad de Ciencias Exactas, F|¤sicas y Naturales, Universidad Nacional de Co¤rdoba, Av. Velez Sars¢eld 299, 5000 Co¤rdoba, Argentina b CONICET, Ca¤tedra de Estratigraf|¤a y Geolog|¤a Histo¤rica, Facultad de Ciencias Exactas, F|¤sicas y Naturales, Universidad Nacional de Co¤rdoba, Av. Velez Sars¢eld 1611, Pabello¤n Geolog|¤a, Ciudad Universitaria, X5016GCA Co¤rdoba, Argentina Received 6 May 2002; accepted 2 May 2003

Abstract A survey of early Ordovician faunal assemblages from different geodynamic and palaeogeographic settings from the west of Argentina has been carried out. The distribution and dominance of four fossil groups (rhynchonelliform brachiopods, trilobites, sponges, and bivalves) are analysed and compared in three distinct basins: a passive-margin carbonate platform (Precordillera), a volcanic-arc island platform located at intermediate latitude (Famatina) and a siliciclastic pericratonic epeiric platform placed at intermediate^high latitude (Cordillera Oriental). Scales of analysis range from the level of the patterns exhibited by individual clades to the level of the assemblages along the onshore^ offshore gradient. Overall taxonomic diversity (at genus and family levels), alpha or within habitat diversity, and ecospace utilisation were evaluated and contrasted among bathymetric zones and among different basins. On this basis a mosaic of five distinctive type assemblages is characterised: (1) a demosponge^brachiopod type assemblage (Precordillera), (2) a brachiopod^bivalve type assemblage (Famatina), (3) a brachiopod^trilobite type assemblage (Famatina), (4) a trilobite^brachiopod type assemblage (Cordillera Oriental), and (5) a trilobite type assemblage (Cordillera Oriental). Possible large-scale controls in the configuration of these type assemblages are assessed. The structure and distribution of the type assemblages from the west of Argentina are interpreted to be largely controlled by environmental dynamics at each geodynamic setting coupled with the latitudinal position of the basins. Both largescale factors regulate a number of regional parameters such as sedimentary regime, volcanic activity, oceanic circulation, temperature, etc. In particular temperature appears as a critical control over the food supply and primary productivity as well as seasonality of the resources. According to the evidence analysed in the Argentine basins, the latter variables might have played a key role in the diversification of suspension-feeding organisms that, in turn, promoted the development of the Palaeozoic evolutionary faunas and the Ordovician radiation. Our case study from the west of Argentina further represents a small-scale model of the broad spectrum of the possible regional conditions that promoted the differential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.

* Corresponding author. E-mail address: [email protected] (B.G. Waisfeld).

0031-0182 / 03 / $ ^ see front matter > 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00464-4

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> 2003 Elsevier Science B.V. All rights reserved. Keywords: Early Ordovician; faunal assemblages; biodiversi¢cation; biogeography; western Argentina

1. Introduction Global-scale diversity trends have been documented by several authors (e.g. Sepkoski and Sheehan, 1983; Miller, 2000; Benton, 1995). However, to appreciate the underlying processes responsible for these trends, local or regional geologic and environmental contexts need to be understood. A causal relationship between an increase of Ordovician diversity and tectonic activity was suggested by Miller and Mao (1995) who found that diversity was higher in foreland basins than in carbonate platforms. Miller (1997a,b, 2000) emphasised the e¡ects of local environmental, geographic and ecologic factors in long-term diversity trends, and suggested that the Ordovician radiation is an aggregation of patterns and processes unique to particular regions or scales. Correlating biotic changes to physical transitions at a variety of scales permits a more comprehensive picture of global marine faunal patterns. Benedetto (2001a) analysed the palaeogeographic distribution of early Ordovician rhynchonelliform brachiopods on a global scale and demonstrated that assemblages from low-latitude carbonate platforms are di¡erent from those of cold-water clastic shelves in terms of life strategy and ecospace utilisation. The former are brachiopod-rich assemblages that include a variety of morphotypes with adaptations for diverse habitats along the bathymetric pro¢le. The second assemblage includes only low-diversity, morphologically conservative orthid assemblages usually con¢ned to shallow-water deposits. Distributional patterns of Ordovician rhynchonelliform assemblages throughout west Argentina basins allow three main biofacies belts to be di¡erentiated, the ‘orthid biofacies’ developed in autochthonous siliciclastic rocks, the ‘pentamerid^clitambonitid biofacies’ typical of volcanic-related sequences and the ‘plectambonitoidean biofacies’ inhabiting the Precordilleran carbonate platform (Benedetto, 2001a,b).

The present study describes patterns of diversity exhibited by rhynchonelliform brachiopods, trilobites, sponges, and bivalves throughout the Arenig in three present-day neighbouring basins from west Argentina: the Precordillera, the Famatina and the Cordillera Oriental basins (Fig. 1). These basins have di¡erent geodynamic histories and palaeogeographic signatures, thus we have the opportunity of a straight comparison of biotas not only inhabiting regions originally located at di¡erent palaeolatitudes but also from contrasting depositional settings such as passivemargin carbonate platforms, volcanic-arc related active margins, and clastic, cold-water pericratonic epeiric seas. Contrasting ecological and diversity patterns among these settings have been preliminarily analysed by Waisfeld et al. (2001a). Di¡erences are examined at various scales: (1) at the level of the dominant clades including patterns of ecospace utilisation, (2) at the level of assemblages within particular habitats along an onshore^o¡shore gradient, and (3) at the level of broad basin-scale type assemblages, analyzing the possible large-scale patterns that ultimately rule their composition and distribution. Our approach

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Fig. 1. Location map of the studied basins.

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is based on the hypothesis that the Ordovician faunal radiation was accomplished at di¡erent rates and following di¡erent clade-level diversity trends according to the palaeogeographic position of continents and the geodynamic context of each basin which, in turn, regulate a number of temperature-related physico-chemical and biotic parameters (Webby, 1992; Sa¤nchez and Waisfeld, 1995; Waisfeld et al., 1999; Sa¤nchez et al., 2002; Benedetto, 2001a; Miller, 1997a,b; 2000, among others).

2. Geologic framework 2.1. Precordillera basin The Argentine Precordillera is a high-level thrust and fold belt including a 2500-m-thick sequence of Cambrian and lower Ordovician limestones. In the Tremadoc^Arenig transition, the occurrence of widespread muddy fossiliferous carbonates associated with sponge-microbial reefmounds (Carrera, 1991; Can‹as and Carrera, 1993) marks the base of the San Juan Formation. This unit contains the fauna analysed herein. After this event, subtidal lithotopes prevailed during deposition of the San Juan Formation (Fig. 2). Skeletal wackestones and packstones intercalated with storm-related intraclastic grainstones are the most conspicuous lithologies in the Arenig. The middle ramp deposits accreted in the transgressive sequence. The maximum £ooding surface occurs at the middle part of the formation in the Oepikodus evae zone (Can‹as, 1999). Glauconitic levels, condensed skeletal concentrations and encrusting organisms associated with hard substrates occur in this interval. Storm deposits are less abundant but their occurrence indicates a middle ramp setting. Middle Arenig fossiliferous wackestones and grainstones are capped by stromatoporoid patch reefs and stromatoporoid^lithistid^algal reef-mounds (Can‹as and Keller, 1993; Keller and Flu«gel, 1996). This second reef-mound horizon appears after the regressive stage in the next transgressive sequence. The addition of stromatoporoid-like organisms among framebuilders is an important feature of the upper reef-mounds.

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The earliest conodonts recorded from the formation demonstrate that the lower San Juan Formation is uppermost Tremadoc (P. deltifer zone; Keller et al., 1994; Albanesi et al., 1998) while the diachronous upper levels record conodont and graptolite faunas of middle Arenig age in the north whereas in southern regions conodonts suggest an early Llanvirn age (Eoplacognatus suecicus zone) (Lenhert, 1995; Albanesi et al., 1998). Carbonate sedimentation culminated in the early Llanvirn with nodular wackestones and packstones containing diverse open platform faunas. Sedimentological and palaeoecological analyses of these faunas suggest a regional slope to the north (Can‹as, 1995, 1999; Sa¤nchez et al., 1996). In northern sections middle to late Arenig aged rocks are represented by the lower member of the Gualcamayo Formation (Baltoniodus navis and Microzarkodina parva zones; Albanesi et al., 1998). This unit, about 30 m thick, is composed of an alternation of bioclastic, micrograded wackestones and ¢nely laminated, pyritic mudstones and shales. This succession has been interpreted as a deep ramp environment, transitional to an intrashelf basinal setting (Astini, 1995; Can‹as, 1995). 2.2. Famatina basin The Famatina basin is characterised by a thick succession of sedimentary and volcano^sedimentary rocks included in the Suri and Molles Formations constituting the Famatina Group. These units are exposed in two di¡erent areas: the northern part of the Famatina Range (Chaschuil area) and the central region (Cachiyuyo and Saladillo Rivers), the latter containing thicker and more continuous successions (Fig. 2). In the Chaschuil area detailed stratigraphic and palaeoenvironmental studies suggest that the Suri Formation represents a regressive sequence that records the subsequent development of slope apron, shelf and fan-delta deposits (Ma¤ngano and Buatois, 1995, 1997). The fossiliferous part of this unit is represented by the Loma del Kilo¤metro Member that, according to Ma¤ngano and Buatois (1997), records volcanic arc-related sedimentation on a relatively high gradient, narrow

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Fig. 2. Generalised stratigraphic columns of Precordillera, Famatina and Cordillera Oriental, and distribution of the assemblages recognised in each basin. Abbreviations: eT, early Tremadoc; ns, nearshore; p.i.p., proximal inner platform; r-m, reef-mound.

and geographically restricted shelf. A shallowingup trend is recognised, varying from relatively deep-shelf deposits at the base to inner-shelf to lower shoreface environments with increasing in£uence of storm and wave action toward the top. Episodic sedimentation prevailed over background processes, storm action and mass £ow of mostly volcaniclastic detritus characterise this shallow-water setting. The upper part of Loma del Kilo¤metro Member records the return to deeper-water conditions.

The age of the lower part of the Loma del Kilo¤metro Member was established by Vaccari and Waisfeld (1994) on the basis of a mid-Arenig trilobite fauna. In shell beds from the upper part of the member the mid-Arenig Baltoniodus navis zone was reported by Albanesi and Vaccari (1994). The stratigraphy and sedimentary history of the Suri and Molles Formations exposed in the central region of the Famatina Range were recently studied by Astini and Benedetto (1996) and Astini

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(1998, 1999). The overall succession, greater than 2000 m thick, re£ects deposition on a high gradient, mixed siliciclastic^volcaniclastic platform to volcanic apron. The succession in the Suri Formation displays a well-de¢ned shallowing-upward trend ranging from relatively deep oxygen-de¢cient waters at the base to shallow-marine facies, punctuated by increasingly more frequent and thicker subaqueous pyroclastic surges and volcanic breccias and subaqueous £ows toward the middle and upper parts. Storm- to wave-dominated sand bodies are more common toward the top. The Molles Formation was deposited in a tidally dominated environment, and a transition to continental environments at the top of the unit is suggested by Astini (1998). According to this author, the shallowing-upward trend and ¢nal emergence associated with increasing volcanic content is related to an active volcanism and rapid growth of a volcanic arc. The graptolite-rich lower part of the Suri Formation is considered lower^middle Arenig by Toro and Brussa (1997) (Baltograptus de£exus and Didymograptellus bi¢dus zones). Carbonaterich shell beds from the upper part of the Suri Formation and the base of the Los Molles Formation yielded a conodont association referable to the upper part of the Oepikodus evae zone (middle Arenig) (Albanesi and Astini, 2000). 2.3. Cordillera Oriental basin The early Ordovician siliciclastic sedimentary rocks of the Cordillera Oriental consist of approximately 4500 m of shallow-marine deposits, including the Santa Victoria Group, composed of the Santa Rosita Formation (latest Cambrian^ late Tremadoc) and the Acoite Formation (early^middle Arenig). The latter is overlain at certain localities by the Sepulturas Formation. Palaeoenvironmental analysis of the Acoite Formation, that crops out in the western side of the Cordillera Oriental, were provided by Astini and Waisfeld (1993) and Astini (1994). This 2300-mthick unit is interpreted as a wave-dominated deltaic system. The lower half of the Acoite Formation (early Arenig) is composed of black and grey graptolitic shales deposited in a dysaerobic outer-

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Fig. 3. Early Ordovician schematic palaeogeographical reconstruction of the Southern Iapetus. Abbreviations: A, Avalonia; B, Baltica; CAB, Central Andean Basin (includes Argentine Cordillera Oriental); F, Famatina volcanic arc; G, Gondwana; L, Laurentia; NFL, Newfoundland; P, Precordillera terrane; WP, western Puna. Modi¢ed from Benedetto (1998a).

shelf environment, below normal wave base. The upper part of the Acoite Formation is characterised by several upward thickening and coarsening cycles with abundant wave-generated structures (Fig. 2) deposited in an increasingly shallow shelf environment, indicative of an active shoreline progradation (Astini and Waisfeld, 1993). The uppermost part of the Acoite Formation corresponds to the informal ‘upper sandy member’ of Astini and Waisfeld (1993), interpreted as shoreface deposits strongly in£uenced by storm and wave activity. The general shallowing-up trend corresponds to the transgressive and highstand system tracts (lower and upper parts of the Acoite Formation, respectively). Lithofacial studies in coeval strata in the eastern side of the Cordillera Oriental are still preliminary. The Acoite Formation is interpreted to have been deposited in a distal innershelf setting, below the normal-storm wave base (Waisfeld et al., 1999). Coarsening-upward cycles and thick sandstone bodies developed in the

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uppermost part might be correlated with the shoreline progradation documented in western areas of the basin by Astini and Waisfeld (1993). The age of the Acoite Formation, according to the records of the T. phyllograptoides, T. akzharensis, B. de£exus and D. bi¢dus zones, is earliest Arenig to mid-Arenig (Toro, 1997).

3. Geodynamic setting and biogeographic signature The three analysed basins display distinct tectonic histories and di¡erent palaeogeographic positions with respect to the pre-Andean margin of Gondwana (Fig. 3). Hence, faunas from the Precordillera, Famatina and Cordillera Oriental show particular biogeographic signatures. The available data support the hypothesis of the Precordillera as an exotic terrane accreted to Gondwana margin (Benedetto et al., 1999 and references therein). The Precordillera is interpreted as a far-traveled microplate, rifted from the southern Appalachian margin in the Cambrian and accreted to the pre-Andean margin of Gondwana probably during the later Ordovician. The development of a more than 650-m-thick passive-margin carbonate succession strongly supports its low-latitude placement during the early Ordovician. Although no palaeomagnetic data are available for the Arenig, evidence from lower Cambrian strata suggests that the Precordillera was positioned at the same latitude as Laurentia. Gradual changes in faunal provincialism (Benedetto, 1993, 1998a; Astini et al., 1995; Benedetto et al., 1999) as well as marked variations in climatically sensitive sedimentary rocks (i.e. tropical Cambro^early Ordovician carbonates, temperate Caradoc^early Ashgill foramol carbonates, glacial diamictites in the Ashgill ; Astini, 1995) largely support a drift history of the Precordillera terrane through the southern Iapetus Ocean. The assemblages considered in this study mostly developed during the ‘isolation stage’ proposed by Benedetto et al. (1999) (Fig. 3). During this stage the carbonate ramp developed on a microcontinent almost completely isolated from large palaeocontinents such as Gondwana and Laurentia. Biogeographic a⁄nities of early Ordo-

vician rhynchonelliformeans account for a mixture of Toquima^Table Head, Baltic, Celtic, and endemic genera, but a recent statistical analysis of mid-Arenig rhynchonelliformeans shows stronger links with the Laurentian Toquima^Table Head province (Benedetto, 2003). Trilobite faunas, according to Vaccari (1995), contain Laurentian elements (Bathyurid province) associated with some forms present also in Australia and Baltica, as well as few endemic elements. Sponges from the Precordillera show Appalachian biogeographic af¢nities by the middle Arenig, but some endemic forms, including a new family, emphasise the geographic isolation of the fauna (Carrera and Rigby, 1999). Based on the geochemical signature of volcanic rocks, Mannheim (1993) interpreted the Famatina basin as a back-arc basin. Recent extensive analysis by Pankhurst et al. (1998) indicates that the Famatina magmas originated by crustal anatexis and are typical of ensialic marginal (or back-arc) basins in which extension was not enough to form oceanic crust, then they were £oored by slightly thinned continental crust. The rhynchonelliform faunas from Famatina recently described by Benedetto (2003) form a well-de¢ned cluster with the typical Welsh and Central Newfoundland Celtic assemblages (Benedetto, 1998a; Neuman, 1999), reinforcing the link between the Famatina volcanic arc and other mid-latitude Iapetus terranes. A subordinate number of taxa is shared with the ‘European’ Baltic and Mediterranean provinces. In contrast, trilobites account for peculiar mixed biogeographic a⁄nities of warmer eastern and cooler western Gondwanan regions as well as few Baltic elements (Vaccari, 1995; Waisfeld and Vaccari, 1996). This assemblage is consistent with the mixed biogeographic nature expected to be found at temperate latitudes according to Cocks and Fortey (1988). From a geodynamic perspective the Ordovician successions of the Cordillera Oriental were deposited in extended, relatively stable and west facing marine platforms developed between the border of the Guyana craton and the western Gondwana (‘proto-Paci¢c’ or ‘southern Iapetus’) active margin. This autochthonous Gondwana basin (Central Andean basin) is interpreted to have devel-

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oped in a back-arc position related to the emplacement of a magmatic arc farther to the west in the western Puna region and south of the Salar de Atacama in northern Chile (Bahlburg and Beitkreuz, 1991; Bahlburg and Herve¤, 1997). Biogeographic a⁄nities of rhynchonelliformeans indicate close links with northwestern Gondwana (Mediterranean province) (Benedetto, 1998b). The trilobite fauna is composed of a relatively large number of endemic forms as well as western Gondwana elements (Waisfeld, 1995).

4. Source of data and methods The faunal compilation presented in Figs. 4, 5 and 6, and Table 1 and synthesised in Figs. 7 and 8 was assembled from published records of Arenig aged rhynchonelliform brachiopods, trilobites, bivalves, and sponges from the three basins considered, as well as our own new unpublished information. This represents an updated version of the data presented in previous papers (Benedetto et al., 1999; Waisfeld et al., 1999; Sa¤nchez et al., 2002) in which literature sources for the taxonomic data reported up to that time can be found. Most recent systematic contributions come from Waisfeld (2001), Waisfeld et al. (2001b), Vaccari (2001), Benedetto (2001c), and Sa¤nchez (2001a,b). A complete compilation of taxonomic data from the Famatina basin has not been previously presented. It is mainly derived from Harrington and Leanza (1957), Acen‹olaza and Ra¤bano (1990), Vaccari and Waisfeld (1994), Vaccari (1995), Waisfeld and Vaccari (1996), Benedetto (1994, 2003), and Sa¤nchez (1997, 2001b). We restricted the analysis to rhynchonelliform brachiopods, trilobites, bivalves and sponges because these are the most abundant and best studied taxa in these basins. Other fossil groups, such as gastropods, nautiloids and linguliform brachiopods, are subordinate members in faunal assemblages, and since these groups are comparatively poorly known they were not included in the present study. No quantitative or abundance data are included in this study, instead, taxonomic diversity on familiar and generic levels is considered

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as a key to evaluate the dominance of particular clades. It should be noted that the data set is restricted to the Arenig. However, because of particular depositional histories, the preserved record of Arenig fossiliferous strata is di¡erent in each basin (Fig. 2). For example, in the volcaniclastic succession of the Famatina Range faunal records from the Suri and Los Molles Formations are con¢ned to the middle Arenig. No early or late Arenig fossiliferous rocks are hitherto known in the basin. In turn, in the Cordillera Oriental the analysed data are derived from the early and middle Arenig (Acoite Formation) because some relicts of probable later Arenig age (e.g. Alto del Co¤ndor Formation) are largely unfossiliferous. In contrast, in the Precordillera basin the Arenig record is fairly complete, including the limestones of the San Juan Formation as well as the mixed carbonate^clastic Gualcamayo Formation (lower member). These age di¡erences do not a¡ect the large-scale patterns documented herein. Temporal changes throughout the Arenig were analysed in detail elsewhere (Waisfeld et al., 1999; Sa¤nchez et al., 2002). No faunal transitions between major clades were recognised along this time span, instead, only faunal turnovers on familiar or generic level within particular clades were documented. Fossil occurrences were plotted in four broad environmental categories either for siliciclastic or for carbonate sedimentation (see Fig. 9). It should be noted that the topographic constraints in volcanic-arc related platforms strongly modify the shelf pro¢le (see Percival and Webby, 1996, ¢g. 2). They di¡er from passive-margin platforms in the higher bottom slope gradient and, hence, in the potential regional extent of each environmental zone; as well, di¡erences in the type and rate of the sedimentary input and frequency of episodic sedimentation should be expected. However, for the purposes of this study, an analogy is assumed between both types of sedimentary settings in terms of sandstone/shale ratios, water energy, storm events, etc. Zone 1. Nearshore ; subtidal and above fairweather wave base. Coarse to medium-grained sandstones with parallel lamination or cross bed-

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ding. Fossils occur in shoreface coquinas and in ¢ne-grained partings between sand bodies. Zone 2a. Proximal inner platform ; below fairweather wave base and above normal-storm wave base. Storm beds with parallel lamination and HCS, interbedded with bioturbated, fair-weather ¢ne-grained beds. Fossils occur in storm beds and interbedded ¢ne-grained sediments. Zone 2b. Distal inner platform; below normalstorm wave base and above maximum-storm wave base. Massive ¢ne-grained sediments, with occasional thin sandstone bodies (distal tempestites). Fossils occur in low densities in siltstones and shales, and in pavements. Occasional carbonate concretions, often hosting well-preserved fossils. Zone 3. Outer platform ; below maximumstorm wave base. Fine-grained sediments, frequently black shales. Scattered occurrence of benthic fauna, otherwise barren. A similar four-fold division was employed for the carbonate platform of the Precordillera basin. On the basis of sedimentologic criteria it is best characterised as a gently deepening ramp (Can‹as, 1995). It should be noted that the basin environment (Z3) in the Precordillera is not recorded with con¢dence during the Arenig. The earliest report of graptolitic black shales with scattered benthic fauna is referred to the earliest Llanvirn, hence it was not considered in the present analysis. Zone 1. Inner ramp; above fair-weather wave base. Carbonate sands, skeletal grainstones, oncolithic grainstones, and eventual reef-mounds and patch reefs Zone 2a. Middle ramp; below normal wave base and above storm wave base. Thick beds of skeletal wackestones and packstones sporadically interrupted by storm-derived intraclastic grainstones. Zone 2b. Distal ramp; below storm wave base, only a¡ected by exceptional storms. Nodular wackestones and mudstones interbedded with black shales. Zone 3. Basin margin (not recorded in the Arenig). We studied the structure and organisation of assemblages from the perspective of alpha (within habitat) diversity. Alpha diversity was examined

by assessing the genus richness of individual taxa at particular habitats. Generic diversity was used, instead of speci¢c diversity, because most genera are represented by single species. Comparison of variations in alpha diversity trends across the spectrum of platform habitats among di¡erent basins is shown in Fig. 7. Taxonomic diversity may or may not be closely related to ecological diversity (Schluter and Ricklefs, 1993). Thus, guild occupation of rhynchonelliform brachiopods, sponges, and bivalves has been analysed. According to Bambach (1983), the evaluation of guilds represents a method of categorising the ecologic complexity of communities. Guild analysis reveals both the nature and style of ecospace utilisation and the amount of ecospace utilised by the faunal assemblages. Di¡erentiation of guild patterns provides a useful tool to understand patterns of diversi¢cation on ecological grounds in the three analysed basins. It should be noted that in this contribution we did not attempt a community level analysis and the guild concept is just employed in order to assess possible di¡erences in ecospace utilisation during the Arenig among contrasting habitats. The composition, guild occupation, and distribution of assemblages through the bathymetric pro¢les of each basin are shown in Table 2. These assemblages are summarised in ¢ve basin-level ‘type assemblages’ (Fig. 10), each characterised on the basis of: (1) the array of higher taxa upon patterns of generic alpha diversity and familial diversity, (2) the taxonomic composition at familial or higher categories (depending on the clade considered), and (3) the ecospace utilisation (among rhynchonelliformeans, sponges, and bivalves).

5. Clade patterns 5.1. Rhynchonelliform brachiopods Rhynchonelliformeans are suspension-feeding, epifaunal dwelling organisms, with di¡erent morphologies that can be related to di¡erent life styles, and thus guilds. Recognition of rhynchonelliform life strategies, based on morphofunc-

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Fig. 4. Guilds recognised for rhynchonelliform brachiopods and the distribution of taxa in the studied basins. Abbreviations: P, Precordillera; F, Famatina; CO, Cordillera Oriental. Asterisks indicate endemic genera.

tional analysis, follows Bassett (1984), and Sa¤nchez and To¤¡olo (1996). However, a careful analysis concerning most rhynchonelliform genera listed in the present work is needed. In view of the new evidence on variation of life styles in Recent rhynchonelliform brachiopods (Richardson, 1997) recognition of guilds is preliminary. In general, some major taxa ^ for example orthids ^ can be considered as occupying a single guild. However, a more detailed analysis shows that within major taxonomic groups there are a number of di¡erences in shell outline, valve convexity, and other minor features that can be related to particular life strategies. On this basis, 11 rhynchonelliform life styles, which are correlated with guilds, have been tentatively recognised for the 36 rhyn-

chonelliform genera recorded in the three studied basins (Fig. 4). Three additional genera have not been included in any of these guilds because of di⁄culties in the recognition of several features. On the basis of some features in common, these guilds can be grouped as (following Bassett, 1984): (1) pedunculate, low attached, (2) semi-infaunal, and (3) liberosessile. Ambitopic forms (low-attached juveniles and resting adults) are viewed as a separate guild. The ¢rst major group (pedunculate, low attached) includes a variety of shell outlines, shell convexities, and ornamentation types, which may be di¡erentiated provisionally into seven guilds (Fig. 4). Due to the small size of shells some low-attached genera have been interpreted as living interstitially (G5).

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5.2. Bivalves

5.3. Sponges

Bivalves display a range of di¡erent life strategies according to their relationships with the substrate and the feeding type : infaunal or epifaunal, and detritivorous or suspensivorous. Also, they can be free living or bysally attached. Interpretation of the life strategies of the listed genera, based on morphofunctional analysis, follows Stanley (1970). On the basis of the possible life styles of the listed genera, several guilds can be recognised. A detailed analysis of life styles (e.g. rapid or slow burrowing) is beyond the scope of this paper, and the proposed guilds are tentative. Bivalve guilds include infaunal detritus-feeding, infaunal suspension-feeding, and epifaunal, byssate suspension-feeding types. Endobyssate types have not been recognised among the included taxa (Fig. 5). Infaunal detritus feeders (G2) are normal dwellers on soft substrates, supporting low-oxygen levels and turbidity. G3 (epibyssate genera) are related to ¢rm substrates.

Sponges are ¢lter-feeding organisms that show great plasticity in form even within speci¢c populations. Several experiments and studies have demonstrated the in£uence of environment, especially water currents, on the sponge form and structure (Trammer, 1983; Palumbi, 1984; Reitner and Keupp, 1992; Carrera, 1997, and references therein). However, no guild de¢nition has been used to di¡erentiate sponge morphology and internal structure in their palaeocologic analysis. The group as a whole has been used as a single guild in palaeoecological studies of reef communities (Watkins, 1993). Carrera (1997) recognised three main morphotypes in the early Ordovician of the Argentine Precordillera, whereas Cech and Carrera (2002) include demosponges in three guilds in a palaeoecological survey of the Arenig limestones. Six sponge guilds have been recognised here for the Arenig communities of the Precordillera (Fig. 6). Demosponges include three guilds di¡erentiated according to form and internal structures (central cavity and canals). The cylindrical erect guild (G1) comprises those sponges with deep and large central cavity. Water £ow is induced to enter the sponge body in non-turbulent waters. These sponges required hard substrates for attachment. The opposite end member is represented by the discoid guild (G2), which includes those discoid, domical or laminated sponges with central cavity reduced or absent. These sponges are adapted to turbulent waters; their inhalant and exhalant streams can be e⁄ciently separated only owing to a continuous movement of the surrounding water. These sponges are unable to live in environments with moderate to high sedimentation rates. The hexactinellid spicules and associated roottufts composed by giant monaxons are included in a single guild (G4). These sponges are adapted to life in quiet waters and soft substrates. Calcareans are represented by a single laminated to conical heteractinid genus (guild G5). The open skeletal structure and composition of heteractinids suggest that they inhabit quiet water environments in shallow carbonate settings (Rigby, 1983).

Fig. 5. Guilds recognised for bivalves and the distribution of taxa in the studied basins. Abbreviations: P, Precordillera; F, Famatina; CO, Cordillera Oriental. Asterisks indicate endemic genera.

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Table 1 Distribution of trilobite taxa Precord. 1 Metagnostidae G. (Geragnostus) Dividuoagnostus Illaenidae Illaenus Platillaenus Nanillaenus Leiostegiidae Annamitella Leiostegium Cheiruridae Macrogrammus Pliomeridae Benedettia Pliomera Gogella Pliomeridius Rossaspis Bathyuridae Uromystrum Peltabellia Arancibia Telephinidae Carolinites Oopsites Opipeuter Telephina Asaphidae Opsimasaphus? Merlinia Araiocaris B. (Basiliella) Suriaspis M. (Ekeraspis) Megalaspidella Kayseraspis Thysanopyge Asaphidae n.g.1

Famatina

2a 2b 1

2a 2b 3

C. Oriental

Precord. Famatina

1

1 2a 2b 1 2a 2b 3 1 2a 2b 3

x

2a 2b 3 x

x x x

x x

x

x

x

x

x x x

x x x x

x x

x

x x

x

x x x x

x

x x

x

x

x x x x

x

x

x x

x x x x x

x

The stromatoporoid guild (G6) includes the genus Zondarella (Keller and Flu«gel, 1996), these primitive stromatoporoid-like organisms are the dominant members of the upper reef-mound. Early Ordovician sponges have a disparate distribution. Demosponges and calcareous sponges mainly inhabited carbonate platforms while hexactinellid and monaxonid sponges thrived along

Zuninaspis Australopyge Hoekaspis Niobides Branisaspis Ogyginus Sanbernardaspis Asaphidae n.g. 2 Nileidae Nileus Illaenopsis Nileidae n.g. Raphiophoridae Ampyx Lonchodomas? Rhombampyx Raphiophoridae n.g. Shumardiidae Changchowilla Conophrys Olenidae Bienvillia Hypermecaspis Saltaspis Porter¢eldia Psilocara Calymenidae N. (Neseuretus) Salterocoryphe Colpocoryphe Pharostomina Dikelokephalidae Hungioides Trinucleidae Famatinolithus Orometopidae Araiopleura Trilobite families Trilobite genera

C. Oriental

x x

x x x x

x x x x x x x x x x x x

x x x x

x x

x x x

x x x x x

x x x

x

x x x x x x x x

x x x x x x x x x

x x

x x

3 6 10 3 10 6 2 6 9 8 3 4 8 11 3 14 10 3 8 15 24 5

the black-shale continental margins and exceptionally siliciclastic platforms (Carrera and Rigby, 1999, 2001, in press). Thin-walled hexactinellid species with root-tufts are found mainly in shales or quiet waters in siliciclastic and exceptionally in carbonate platforms at least during the Ordovician. The delicate feeding structures of demosponges and calcareous sponges and their require-

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tiation of guilds among many invertebrate fossil groups (Bambach, 1983); in contrast, there are severe constraints in applying this kind of analysis to trilobites. Many aspects of their life strategies, in particular the relationship to the morphology of many groups, are poorly known. Hence, guild structure among trilobites is di⁄cult to assess.

Fig. 6. Guilds recognised for sponges and the distribution of taxa in the studied basins. Abbreviations: P, Precordillera; F, Famatina; CO, Cordillera Oriental. Asterisks indicate endemic genera. Numbers of families and genera of G6 are tentative.

ments for attachment prevent their development in siliciclastic turbid environments with shifting substrates. 5.4. Trilobites There is a broad consensus about the di¡eren-

Fig. 7. Histograms showing the numbers of families and genera within environmental zones. Abbreviations: fam, families; gen, genera.

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Only recently Fortey and Owens (1999) presented a comprehensive review of several exoskeletal designs and their possible relationship with particular feeding strategies. Some feeding habits suggested by Fortey and Owens (1999) are recognised among the trilobites occurring in the studied basins. According to these authors, trilobites of the families Asaphidae and Nileidae display some morphological traits that can be linked to a predatory/scavenger habit. This feeding style could also have been present among representatives of the Calymenoidea, associated to the development of stalked eyes and a semi-infaunal life habit. A ¢lter-feeding strategy mediated by the development of a cephalic ¢lter chamber might have been present among the raphiophorids and trinucleids. However, recently Fortey (2001) accounts for equivocal conclusions about some of the groups potentially included in this feeding category. Particle feeders are represented by olenimorphs and shumardiids. Trilobites that might have achieved a pelagic or epipelagic life mode (possible plankton feeders) are represented, for example, by telephinids (cf. Fortey, 1975, and Fortey and Owens, 1999). Apart from these clades, the feeding habits for the morphological types present in the studied basins (e.g. pliomerids, leiostegiids, illaenids, dikelokephalids) cannot be determined. Thus, only alpha diversity at generic and familiar levels was evaluated (Table 1 ; Fig. 7).

6. Basin level patterns of distribution of faunal assemblages 6.1. Assemblages from the Precordillera basin The inner ramp (Z1) records the development of two low-diversity reef-mounds. The lower reefmound complex was built by lithistid sponges, the receptaculithid Calathium, and microbial benthic communities (Carrera, 1991; Can‹as and Carrera, 1993). Sponges are characterised by the taxonomic diversi¢cation of a single family, the Anthaspidellidae, while the other benthos is strongly reduced. The second reef-mound is mainly composed by microbes and stromatoporoid-like

355

organisms; sponges, rhynchonelliform brachiopods and trilobites are accessory components. In addition, the level bottom assemblages of the high-energy inner ramp contain low-diversity rhynchonelliform faunas, dominated by orthides, and subordinate trilobites (Fig. 7 ; Table 2). The middle ramp (Z2a) environment includes a high-diversity assemblage dominated by rhynchonelliform brachiopods and sponges. Taxonomic diversity of the rhynchonelliformeans is high: 11 families represented by 18 genera. Diversi¢cation of rhynchonelliformeans was achieved by developing 7 di¡erent guilds (Fig. 4), including the liberosessile guild (G11) with several genera, whereas the semi-infaunal guild (G9) is represented by a single genus. The dominance of attached forms (G1, G2, G4) re£ects a ¢rm substrate. As well, the availability of additional attachment sites might have contributed to the high diversity of low-attached rhynchonelliformeans. In this sense, direct evidence of small orthids living on the surface of the cylindrical sponges together with other epizoans, mainly bryozoans and stemmed echinoderms, has been reported by Carrera (2000). Sponges attain their highest levels of both generic and familial alpha diversity, with 10 genera and 5 families. Discoidal and encrusting sponges, belonging to the discoidal guild (G2), and isolated hexactinellid and calcareous spicules (G4, G5) occur mainly associated with ¢rm grounds in condensed glauconitic levels. Small cylindrical and vase-shaped sponges (G1) are also present in the associated skeletal-wackestone levels. Bivalves occupied only the epifaunal guild (G3). They are represented by two taxa, one of which, Ambonychiidae indet., is normally associated with carbonate, ¢rm substrates. Trilobites are relatively scarce in Z2. Vaccari (1995) suggested that this association is comparable to the Illaenid^ Cheirurid biofacies de¢ned by Fortey (1975) in shallow platforms at equatorial settings. The increase in alpha diversity and number of guilds of rhynchonelliform brachiopods and sponges follows a broadly similar pattern from Z1 to Z2a (Fig. 8). However, an opposing strong decrease of suspension-feeding genera and a strong increase of trilobites is veri¢ed from Z2a to Z2b. In the distal ramp (Z2b) the increase of

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trilobites includes the disappearance of some families of the Z2a assemblages as well as the appearance of several families not represented before. The trilobite fauna displays mixed features of the nileid (cf. Vaccari, 1995) and the olenid biofacies established by Fortey (1975). The noticeable change in faunal composition is related to a shift in environmental conditions restricting suspension-feeding types, associated with the development of softer substrates, lower oxygen levels, and cooler waters. Waisfeld et al. (1999) and Sa¤nchez et al. (2002) suggested that the increase of trilobites in this environment was facilitated by the sedimentary change from exclusively carbonate to mixed carbonate^siliciclastic sedimentation (deposition of mudstones, black shales, and bioclastic grainstones), the development of suitable substrates and the establishment of immigration paths associated with the deposition of distal ramp facies. At a broader scale, two distinct type assemblages are discriminated (Fig. 10): (1) a demosponge^ brachiopod type assemblage on the inner and middle ramp (Z1 and Z2a) including the anthaspidellid and the plectambonitoid biofacies, and (2) a trilobite type assemblage on the distal ramp (Z2b), yielding trilobites of both the nileid and olenid biofacies. The demosponge^brachiopod type assemblage is exclusive to the passivemargin, warm-water carbonate platform and shows high levels of ecospace utilisation mediated by the diversi¢cation of suspension-feeding types. 6.2. Assemblages from the Famatina basin

Fig. 8. Distribution of guilds and trilobite families across environmental zones.

The nearshore (Z1) yielded moderate diversity assemblages dominated by rinchonelliform brachiopods and bivalves with subordinate trilobites. (Fig. 9 ; Table 2). The taxonomic diversi¢cation of bivalves is remarkable, taking into account their scarcity and low diversity in other environmental zones. However, this diversi¢cation is taxonomic because only two guilds have been recognised. Most genera are suspension feeders, shallow infaunal dwellers (G1), and a single one is an epifaunal form (G3), suggesting low turbidity levels and ¢rm substrates. These conditions may have been tempo-

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Table 2 Composition and distribution of assemblages in the studied basins

Precordillera

Famatina

Nearshore (Z1)

Proximal inner platform (Z2a)

Distal inner platform (Z2b)

Low-diversity brachiopod assemblages B: G1 [3], G11 [2], 2 other guilds [1] Bi: absent S: G1 [2], G3 [1], G6 [1] T: leiostegids [2], pliomerids [1] Reef-mounds B: G1 [1], G11 [1] Bi: absent T: bathyurids [2] Moderate diversity brachiopod^bivalve assemblage B: G1 [2], 4 other guilds [1]

High-diversity brachiopod^ sponge assemblage

High-diversity trilobite assemblage

B: G1 [5], G2 [2], G8 [2], G11 [4], 3 other guilds [1] Bi: G3 [2] S: G1 [3], G2 [3], G3 [2], 2 other guilds [1] T: illaenids [2], telephinids [2], 4 other families [1]

B: G1 [1]

High-diversity brachiopod^ trilobite assemblage

Low-diversity trilobite assemblage (northern Famatina) B: absent

Low-diversity trilobite assemblage B: absent

Bi: absent S: absent T: telephinids [2], asaphids [1], raphiophorids [1]

Bi: absent S: absent T: olenids [2], agnostids [1]

Bi: G1 [5], G3 [1] S: absent T: asaphids [1], calymenids [1], raphiphorids [1]

Cordillera Oriental

Low-diversity trilobite^brachiopod assemblage B: G1 [5], G2 [1], G3 [1] Bi: G2 [1] S: absent T: asaphids [3], 5 other families [1]

B: G1 [3], G2 [3], G5 [5], G9 [2], G10 [2], 3 other guilds [1] Bi: G1 [3], G2 [1] S: absent T: asaphids [2], pliomerids [3], raphiophorids [2], 7 other families [1]

Outer platform (Z3)

Bi: absent S: absent T: olenids [2], 9 other families [1]

B: G1 [4], G2 [1]

Moderate-diversity trilobite assemblage (central Famatina) B: absent Bi: absent S: absent T: asaphids [2], pliomerids [2], telephinids [2], raphiophorids [2], 2 other families [1] Moderate-diversity trilobite Low-diversity assemblage trilobite assemblage B: G1 [1] B: absent

Bi: G1 [1], G2 [1] S: absent T: asaphids [4], pliomerids [2], raphiophorids [2], calymenids [2], 5 other families [1]

Bi: G1 [1], G2 [1] S: absent T: asaphids [12], olenids [5], raphiophorids [2], 5 other families [1]

High-diversity trilobite assemblage

Bi: absent S: absent T: olenids [3], 2 other families [1]

Abbreviations: B, brachiopods; Bi, bivalves; S, sponges; T, trilobites. Numbers in brackets represent the number of genera. For guild abbreviations, see Figs. 4, 5 and 6. See discussion in text.

rary taking into account the fact that bivalves are restricted to thin horizons. Ma¤ngano and Buatois (1995, 1997) pointed out that this shallow-water setting is stressful and unstable under the in£uence of volcanic activity and episodic sedimenta-

tion and suggested their strong control on benthic communities. However, faunal diversity in the nearshore of the Famatina basin does not appear to be signi¢cantly low, being broadly similar to the diversity values recorded in environmental

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zone Z1 from the other basins (Fig. 9 ; Table 2). Vaccari et al. (1993) compared this assemblage to the Neseuretus biofacies (cf. Fortey and Owens, 1978) emphasising that it is more diverse and differs in composition from the Neseuretus biofacies found in south Wales and elsewhere. On the proximal inner platform (Z2a) high-diversity assemblages composed of rhynchonelliform brachiopods, trilobites, and bivalves are recorded, either in relatively restricted, muddy substrates that alternate with storm and volcaniclastic beds or in storm shell beds. Rinchonelliformeans attain similar levels of alpha diversity to those of equivalent habitats in the Precordillera basin. This diversi¢cation is also remarkable on ecological grounds, because eight di¡erent guilds can be recognised (Fig. 4 ; Table 2). Rhynchonelliform guilds include two semi-infaunal types (G9 and G10) whereas the liberosessile guild (G 11), occupied by strophomenids, is absent. Benedetto (2001a) suggested that increase of strophomenids could be mainly related to warm waters, but the evidence available (see above) suggests that water temperature in Z2a was largely temperate. Then, the control on the diversi¢cation of strophomenids might be best explained by the coincidence of both temperature and sedimentary regimes (carbonate vs. clastic). Also a variety of trilobites families is recorded (Table 1). Signi¢cant di¡erences in morphological types might imply that trilobites also developed a variety of life modes and strategies in this setting (Hungioides-pliomerid biofacies). Bivalves are minor components in these assemblages and are represented by two guilds, G1 and G2 (Fig. 5). Reduction of genera in G1 (infaunal suspension feeders) and the appearance of deposit-feeding (G2) types could be related to an increase of turbidity from Z1 to Z2a. The development of this high-diversity assemblage is coincident with the progradation from a mud-dominated platform (Z2b) to a sandy one, with frequent storm-induced processes and an increase of volcanic activity (cf. Astini, 1998). In the distal part of the inner platform (Z2b) assemblages are entirely composed of trilobites (Figs. 2 and 9; Table 2). In the northern part of the Famatina basin low-diversity assemblages

yield only four trilobite taxa, two of which are epipelagic (telephinids) and hence independent of bottom conditions. Vaccari et al. (1993) referred this association to the Raphiophorid biofacies de¢ned by Fortey and Owens (1978) for middle platforms at high latitudes. The scarcity of benthic forms and the absence of rhynchonelliform brachiopods and bivalves point to a stressful environment possibly linked to high rates of sedimentation and harsh bottom conditions (unstable, soft substrata?). This pattern is consistent with data from ichnofabrics, which, according to Ma¤ngano and Buatois (1995, 1997), re£ect simple benthic ecosystems and under-exploited infaunal ecospace. In contrast, the distal inner shelf (Z2b) in central Famatina yielded a more diverse assemblage, also composed exclusively of trilobites, represented by benthic forms. Low-diversity assemblages of the outer platform (Z3) are also exclusively composed of trilobites. Composition is restricted to olenids and a single agnostid and is comparable to the Olenid biofacies (cf. Fortey and Owens, 1978). It is important to note that there is a slight decrease in diversity of families between the Z2a and Z2b zones, whereas an abrupt change in diversity and composition occurs between Z2b and Z3. The development of this assemblage might be linked to a restricted, oxygen-depleted environment, in a deeper-water zone. The above assemblages are organised into three distinct type assemblages (Fig. 10): (1) a brachiopod^bivalve type assemblage in nearshore settings (Z1), (2) a brachiopod^trilobite type assemblage on the proximal inner platform (Z2a), including the pentamerid^clitambonitoid and the Hungioides-pliomerid biofacies, and (3) trilobite assemblages on the distal inner shelf (Z2b and Z3), embracing also the Hungioides-pliomerid biofacies (Z2b), and the Olenid biofacies (Z3). These assemblages £ourished on a volcanic arc platform, at intermediate latitudes. Peculiar aspects include the lack of sponges, the taxonomic diversi¢cation of bivalves in assemblage 1, and the high levels of taxonomic and ecologic diversi¢cation achieved among rhynchonelliform brachiopods in assemblage 2.

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6.3. Assemblages from the Cordillera Oriental basin In the Cordillera Oriental trilobites are by far the most abundant and diverse group in every environmental zone. In the nearshore setting (Z1) moderately diverse assemblages dominated by trilobites and rhynchonelliform brachiopods are present (Fig. 9; Table 2). Taxonomic diversity of trilobites is relatively low in comparison with that in deeper environmental zones (Fig. 8). Waisfeld (1995) suggested that the assemblages from this setting were intermediate between the Raphiophorid and the Neseuretus biofacies established by Fortey and Owens (1978) for the Arenig of Wales. In the proximal part of the inner platform (Z2a) high-diversity trilobite-dominated assemblages occur. These assemblages might be included in the Famatinolithus fauna (cf. Waisfeld et al., 1999). It contains, apart from the trinucleid Famatinolithus, taxa belonging to several families (Tables 1 and 2). This assemblage is comparable to the Raphiophorid biofacies of Fortey and Owens (1978) (Waisfeld, 1995). The development of the Famatinolithus fauna was favoured by the shoreline progradation in the upper part of the Acoite Formation. Shallow shelf environments brought more oxygenated waters and provided new ecologic settings that might account for changes in community organisation and complexity. It is important to note that assemblages from nearshore (Z1) also might be included in the Famatinolithus fauna, although comparatively this trilobite fauna is greatly impoverished. The decrease in the diversity of trilobites in this setting is interpreted to be the result of the relatively high-energy conditions in shallow water. In the Z1 rhynchonelliformeans are mainly represented by orthids, occupying low-attached guilds (G1 dominant, G2, G3 subordinate). According to Benedetto (2001a), orthids were tolerant to cold water in siliciclastic shelves. A slight decrease is evident from Z1 to Z2, where only two low-attached guilds (G1 and G2) are recorded. The highest generic diversity of trilobites is attained in the distal part of the inner platform (Z2b). Comparatively, familial diversity is low in

359

this environment, and records a substantial diversi¢cation of a single family (asaphids). Assemblages in this zone are characterised by a particular suite of endemic asaphids, and largely correspond to the Thysanopyge fauna (Waisfeld et al., 1999, and references therein). The Thysanopyge fauna developed in distal inner-shelf settings, periodically associated with dysaerobic bottom conditions. Rhynchonelliformeans are restricted to a single guild (G1) represented by Nanorthis. Patterns of diversi¢cation of rhynchonelliform brachiopods and trilobites through zones Z1 to Z2b are clearly di¡erent (Fig. 8). Although orthids are tolerant to cold waters, their decrease from Z1 to Z2a and Z2b might be the result of dysaerobic bottom conditions in distal shelf settings (Z2b). Bivalves are recorded in zones Z1, Z2a, and Z2b, and are represented by two guilds: infaunal suspension feeders (G1) and infaunal detritus feeders (G2). Development of deposit-feeding types indicates a substrate control on bivalve assemblage composition and also low water exchange where sedimentation rates are high. On the outer platform (Z3) graptolite-rich black shale facies are mostly devoid of benthic fauna, except on the most proximal part of the outer shelf (Fig. 2 ; Table 2). Low-diversity assemblages are exclusively composed of trilobites (olenids and asaphids), and restricted to discrete bedding planes within thick barren shales. Their occurrence was interpreted by Waisfeld (2001) as the result of episodic oxygenation of the bottom water probably due to extraordinary storm-induced events (cf. Wignall, 1994). On the basis of taxonomic composition Waisfeld (2001) suggested that these assemblages display mixed attributes of the Raphiophorid and the Olenid biofacies de¢ned by Fortey and Owens (1978). On a broad scale, two distinct assemblage types are recognised (Fig. 10): (1) trilobite^rhynchonelliform assemblages in nearshore settings (Z1), including the orthid biofacies and an impoverished Famatinolithus fauna, and (2) trilobite assemblages on the proximal and distal inner platform and outer platform (Z2a, Z2b and Z3), involving the Famatinolithus fauna, the Thysanopyge fauna and an olenid-like biofacies from proximal to distal settings. Both type assemblages developed on

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pericratonic siliciclastic platforms, at intermediate to high latitudes. Assemblages show low levels of ecospace utilisation among rhynchonelliform brachiopods, and sponges are absent. Diversi¢cation of the trilobites follows two di¡erent patterns : a remarkable diversi¢cation of trilobites belonging to several families in shallow and oxygenated waters and a radiation of endemic asaphid trilobites in oxygen-restricted environments in distal settings. Bivalves are subordinate members of both assemblages. 6.4. Comparative analysis across bathymetric zones Nearshore settings of the three basins host relatively low-diversity assemblages sharing broadly similar levels of alpha diversity, which normally is lower than in the next, inner shelf zone Z2a. However, similar bathymetric positions do not imply similarities in faunal content. Reef-mounds and sponges £ourished in the Precordillera basin, where the most complete array of feeding types and life strategies is represented. Diversi¢cation of the bivalves and the rhynchonelliform brachiopods is remarkable in the Famatina basin. In the Cordillera Oriental basin the taxonomic diversity of rhynchonelliform brachiopods and trilobites is similar, however, the ecospace utilisation among the rhynchonelliformeans is the lowest of the three basins. The guild group ‘pedunculate, low attached’ is present in the three basins, but the number of individual guilds di¡ers: four guilds in the Famatina, three in the Cordillera Oriental, and only two in the Precordillera (Fig. 4). Proximal inner platforms display the highest peaks in taxonomic diversity in the three basins. In the Precordillera basin rhynchonelliform brachiopods and sponges attain the highest levels of taxonomic and ecologic diversity (four groups of brachiopod guilds and six sponge guilds are

361

present; Fig. 4) whereas trilobites and bivalves are minor components of assemblages. Although not listed in this contribution, bryozoan, gastropods, echinoderms, and nautiloids are also present. Trophic chains in the Z2a of the Precordillera are by far the most complete of the three basins. Increase of the rhynchonelliform brachiopods and trilobites is notable in the Z2a of Famatina basin. Although the brachiopod liberosessile guild is lacking, it is important to note that the low-attached interstitial guild (G5) only occurs in this basin. Assemblages are completed with gastropods and some isolated bryozoans. In contrast, in the Cordillera Oriental trilobites are by far the most abundant and diverse group. In addition to bivalves and rhynchonelliform brachiopods, subordinate gastropods, linguliform brachiopods, and ostracods are recorded in these assemblages. In this basin, the alpha diversity of Z2a is lower than those of the next zone (22 genera in Z2a, 27 genera in Z2b). The distal inner shelf is dominated by trilobites in the three basins. Main di¡erences relate to the relative richness and taxonomic composition of trilobite families. The diversi¢cation of asaphids into 12 genera (half of the total number of genera) is the most notable feature in the Cordillera Oriental. Nautiloids are also frequently recorded in these assemblages. The outer platform (only recorded in the Arenig in Famatina and Cordillera Oriental) is depleted in terms of other faunas except for trilobites. In both basins trilobites display low diversity and are mainly represented by olenids. Taxonomic composition and dominance patterns are broadly similar, re£ecting comparable environmental parameters (oxygen levels, substrate type). In sum, both extremes of the bathymetric pro¢le (Z1 and Z3) show broadly similar trends displaying low diversity and low levels of ecospace utilisation. In contrast, in the inner shelf (Z2a and

Fig. 9. Distribution of the assemblages across the environmental zones. (A) Precordillera. (B) Famatina. (C) Cordillera Oriental. Histograms show diversity of di¡erent groups within each habitat. Abbreviations: g, genera; f, families; sl, sea level; fwb, fairweather wave base; nswb, normal-storm wave base; mswb, maximum-storm wave base. Lithologic keys in shelf pro¢les as in Fig. 2; volcanic and volcaniclastic intercalations in Z1 and Z2a from Famatina are indicated with V.

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Z2b) diversity is higher and a wider spectrum of guilds has been recognised. Thus, the investigation of diversi¢cation patterns on ecological grounds should be better performed across the proximal inner shelf and distal inner shelf zones.

7. Possible large-scale controlling factors Patterns of taxonomic and ecologic diversity from the low-latitude Precordillera carbonate platform and the cold-water siliciclastic Cordillera Oriental platform are very di¡erent and can be considered as end-member ‘type assemblages’. The volcano^sedimentary faunal assemblages from the arc-related Famatina basin share some features with both these regions. At the largest scale, the palaeogeographic position of continents controls the climatic conditions prevailing in the depositional areas or basins, and regulates the faunal exchange with other regions. In a previous work (Waisfeld et al., 1999), this was considered the ¢rst-order factor determining the striking di¡erences between the early Ordovician assemblages from the Cordillera Oriental and the Precordillera basins of Argentina. The present quantitative analysis of taxonomic diversity and ecospace occupation of di¡erent clades in basins located at di¡erent palaeolatitudes supports this conclusion, and adds new evidence to identify the possible large-scale factors determining the type assemblage pro¢le in each basin. Although it is evident that the water temperature gradient played a major role in controlling the ecologic and taxonomic diversity of marine ecosystems, emerging questions include, ¢rstly, the extent of the relative importance of factors directly or indirectly dependent on the palaeogeographic placement such as food availability, seasonality and global surface circulation, and, secondly, in how far the distribution of clades was in£uenced by the geodynamic context. Pearson and Rosenberg (1987) argued that food availability is the most fundamental variable underlying the structure of marine benthic communities and that gradients in food supply induced by variations in latitude, depth and water movement might have been responsible for the

evolution of major community types. Allmon and Ross (2000) also emphasise the role of nutrient conditions in organising ecological relationships and a¡ecting evolutionary processes. Another factor controlling taxonomic and ecologic diversity might be associated with seasonality. According to Valentine (1983), this accounts for differences in benthic assemblages from dissimilar climatic zones. He suggested that seasonality in trophic resources along with other environmental variables might give rise to particular patterns in species dominance and faunal composition. For example, primary food sources are continuous in the tropics and become increasingly seasonal or episodic as latitude increases (see also Pearson and Rosenberg, 1987). Valentine (1983) further pointed out that changes in the higher taxonomic composition of benthic communities during the Phanerozoic leading to the development of the three evolutionary faunas de¢ned by Sepkoski (1981) might have been related to changes in seasonal strategies of development. One of the e¡ects of global oceanic circulation is the development of upwelling zones adjacent to continental margins, which are associated with high primary productivity and an unusual accumulation of organic matter leading to oxygen-de¢cient bottom waters. Zezina (2001) stated that most rhynchonelliform species cannot live in such hypertrophic conditions resulting in ‘faunistical gaps’ along the upwelling regions. However, its e¡ect on the distributional patterns of early Palaeozoic rhynchonelliformeans and other benthic invertebrates is di⁄cult to assess. In this sense, food strategies and ecological requirements changed through time and, additionally, the oceanic palaeocurrent patterns within the Iapetus remain largely hypothetical. The e¡ects of food availability and seasonality upon ancient communities in general and the studied assemblages of western Argentina in particular are di⁄cult to document. As stated above, we assume, on the basis of sedimentological, petrological, palaeontological, and palaeomagnetic evidence, that the three analysed basins were located at di¡erent latitudes in the Arenig. Consequently, it is reasonable to expect a gradient in food supply and seasonality from the low-latitude

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Precordillera to the mid-latitude Famatina and, in turn, to the higher-latitude Cordillera Oriental. The development of the demosponge^brachiopod type assemblage in the carbonate platform seems to be tightly linked with the low-latitudinal placement of the Precordillera which ensured a thermal regime of uniform non-seasonal conditions, a steady food supply with high primary productivity that, in turn, favoured mainly heterotrophs, suspension-feeding organisms. Physical factors promoting the development of these assemblages might include a relatively high environmental stability, low sedimentation rates, low substrate mobility, and high oxygen levels. At the other extreme of the latitudinal gradient, the trilobite-dominated type assemblages of the Cordillera Oriental largely £ourished on the inner and outer platforms. Only a brachiopod^trilobite type assemblage developed in proximal settings. Coarser substrates, more oxygenated waters, higher turbulence distributing particulate food favoured suspension feeders in the nearshore and likely promoted the increase of rhynchonelliformeans. In the Cordillera Oriental, however, rhynchonelliform brachiopods never became important either in richness or in ecological diversity, so that this diversi¢cation did not imply a signi¢cant increase in ecospace utilisation. Such low-diversity orthidine-dominated assemblages are distinctive of shallow-water, high-latitude siliciclastic shelves of the world during the early Ordovician (Benedetto, 2001a). According to the intermediate to high latitudinal position of the Cordillera Oriental, it might be argued that, comparatively, lower availability of food resources and also a probable moderate to high seasonality in these resources might have existed (cf. Valentine, 1983). As suspension-feeding types largely depend upon plankton populations (Levinton, 1974, Allmon, 1988) £uctuation in primary productivity expected at this latitude might have constrained the development of this trophic type. Pearson and Rosenberg (1987) (see also Fauchald and Jumars, 1979) argued that in nutrientpoor environments, or in environments with resources below tolerable levels, sessile types are reduced since mobility is necessary to get su⁄-

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cient food. In contrast, shallow-water regions with turbulence or strong wave action keeping food particles in suspension favours sessile, suspension-feeding organisms (Pearson and Rosenberg, 1987). Hence, variation in nutrient availability in the onshore^o¡shore pro¢le might explain patterns of distribution of these organisms. In summary, the development of the trilobitedominated type assemblage in the Cordillera Oriental is tightly linked to the environmental conditions imposed by the siliciclastic regime such as high rates of sediment input, turbidity, and soft substrates, favouring diversi¢cation of trilobites and reducing suspension-feeding types. Additionally, moderate to high seasonality and low and/or episodic food supply expected at high latitudes might also be a potential factor underlying the particular structure of this type assemblage. Patterns of the type assemblages from Famatina may have resulted from the peculiarities of the volcanic environment. Benedetto (2001a) recognised the pentamerid^clitambonitid biofacies as characteristic of volcanic-arc related successions. Moreover, radiations of endemic forms are recognised among rhynchonelliform brachiopods and bivalves in these assemblages (Benedetto, 2003; Sa¤nchez, 1997, 2001a) (Figs. 4 and 5). High endemism of rhynchonelliformeans in volcanic-arc islands was also noted by Neuman (1984) and Harper et al. (1996). It is important to remark that several studies have noted a correlation between faunal diversity and volcaniclastic substrates (see Lockley, 1990). In this sense, Palaeogene brachiopod faunas from New Zealand associated to calcareous tu¡s are two or three times more diverse than those associated to mudstones or sandstones, and only are comparable with peak diversity levels in limestones (Lee, 1986; Lockley, 1990). With respect to the bivalves, Babin (1993) pointed out that the Ordovician diversi¢cation initially took place on the siliciclastic, temperate to cold-water Gondwanan shelves. However, alpha diversity of bivalves in the nearshore of Famatina basin is higher than in any of these settings (cf. diversity in the Cordillera Oriental, a typical Gondwanan basin; Fig. 9) and also the three guilds recognised among the bivalves are

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present. In this sense, bivalves appear to follow a similar diversi¢cation trend as the rhynchonelliform brachiopods. In contrast, the number of endemic forms among the trilobites is very low. A similar contrasting trend between the patterns of diversi¢cation exhibited by trilobites and rhynchonelliform brachiopods was noted in other volcanic settings by Webby (1992), and Adrain and Fortey (1997). The e¡ects of volcanism on the marine biota are still poorly understood. Endemism among some fossil groups in volcanic settings is one of the patterns more widely accepted (e.g. Neuman, 1976; Bruton and Harper, 1985; Percival and Webby, 1996). Vermeij (1995) argued that the effect of the submarine volcanism is critical in promoting large-scale evolutionary innovations and diversi¢cations of organisms, and that phenomena such as rising temperature, transgressions, enhancement of nutrient supplies, and carbon dioxide are primary extrinsic controls on the marine biota. Additionally, the e¡ect of volcanically induced heating broadens the warm-weather zones, a¡ecting particularly the mid- and high latitudes. Vermeij (1995) pointed out that a rise in temperature brings about an increase in primary productivity that, in turn, favours certain kinds of speciation and also stimulates diversi¢cation. In this sense, Allmon and Ross (2000) provided a theoretical framework on how nutrient supply may directly a¡ect the process of speciation, either during revolutions or at ‘normal’ times. Two main factors may potentially in£uence the observed patterns in the type assemblages from Famatina. First, a volcanically-induced warming e¡ect and a consequent increase in nutrient supply. This e¡ect may have been responsible for the radiation of endemic forms among bivalves and rhynchonelliform brachiopods. Concerning trilobites, the warming e¡ect is only evident in the taxonomic composition of the assemblages and thus in their biogeographic signatures. Rising temperature may have altered the expected latitudinal gradient between Famatina and the other two basins, bringing about conditions more likely to occur at lower latitudes. The second factor may be linked to a wider range of habitats due to heterogeneity of volcanic

processes that, in turn, provided a wider availability of sites for larval settlement and also may have favoured allopatric speciation. In this sense, habitat partitioning associated, for example, with orogenically active settings is thought to be related to an increase in the possibilities of speciation (Cracraft, 1985; Miller and Mao, 1995; Miller, 1997a,b). Development of a variety of sites suitable for larval settlement is a necessary condition for the establishment of such high-diversity assemblages. Thus, the origination of endemic taxa coupled with immigration from other islands or continents signi¢cantly contributed to enhanced diversity.

8. Discussion and implications Some broader scale implications arise from the distinctive patterns of composition and distribution of the ¢ve type assemblages recognised in western Argentina. 8.1. Controls on the structure of the faunal assemblages A complex array of factors is thought to be responsible for the mosaic of type assemblages recognised in the three investigated basins. These factors may be explored at di¡erent levels, those a¡ecting the dominance of particular clades or the megastructure (in the sense of Pearson and Rosenberg, 1987) of the type assemblages and those in£uencing the macrostructure or the taxonomic composition within the higher taxa (Figs. 10 and 11). Composition at the level of clades and their dominance within the type assemblages is interpreted to be the result of two large-scale controls: the geodynamic setting of the basin and its palaeogeographic location (Fig. 11). The sedimentary regime (siliciclastic vs. carbonate) is one of the main factors that is controlled by the geodynamic setting. For example, the occurrence of demosponges largely depends on the environmental conditions associated with the carbonate sedimentation. In contrast, the occurrence of bivalves is favoured by siliciclastic regimes (cf. Babin, 1993;

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Fig. 10. Characterisation of the ¢ve large-scale type assemblages from western Argentina. Note that the type assemblage dominated by trilobites is present in distal settings of the three basins. Also note that the pentamerid^clitambonitid biofacies and the Hungioides-pliomerid biofacies extend across two adjacent type assemblages in Famatina. Similarly, the nearshore trilobite^brachiopod assemblage includes the Famatinolithus fauna also extended in the contiguous type assemblage.

Miller, 1997a,b). Volcanic activity is another factor strongly dependent on the geodynamic context. According to the evidence presented above, this factor promoted diversi¢cation among rhynchonelliform brachiopods and, to a lesser extent, among bivalves. The e¡ect of the palaeogeographic location in controlling several biotic processes has been addressed by Waisfeld et al. (1999, 2001a) and Sa¤nchez et al. (2002). In the case of the type assemblages de¢ned herein, latitude and temperaturerelated variables are critical factors a¡ecting the megastructure (e.g. occurrence of demosponges). It should be noted that latitude together with distance from shore (depth) and volcanic activity are the main modifying factors of the temperature. Water temperature largely varies across latitudinal gradients, however in the volcanic settings, volcanically-induced warming modi¢es the normal pattern. Position in the shelf pro¢le also regulates temperature which is higher in shallower areas. Temperature is a critical parameter not only because several biological functions are largely dependent on it but also because temperature controls food supply and primary productivity. As was discussed above, latitude further controls stability of the food resources and pri-

mary productivity. For example, high levels of nutrient availability and low stability at low latitudes (e.g. Precordillera) promoted type assemblages dominated by demosponges and rhynchonelliform brachiopods. On the other hand, low levels of food supply and low stability at high latitudes (e.g. Cordillera Oriental) appeared to strongly constrain these groups in contrast to trilobites. Only at shallow depths of water in the latter setting a greater availability of food driven by water turbulence and higher temperatures favoured a slightly more complex megastructure with the diversi¢cation of rhynchonelliform brachiopods. At the second level of analysis the macrostructure of the type assemblages or the taxonomic composition within clades is the result of speci¢c responses to particular factors. The latter are mainly related to local environmental variables at the basin level (e.g. hydraulic regime, substrate type, turbidity, oxygen levels) and are re£ected in the distinct composition of biofacies (Fig. 10). The distribution of guilds also emphasises the role of the substrate, for example, the brachiopod liberosessile guild (G11) and the bivalve epifaunal guild (G3) only developed in the carbonate ramp of Precordillera. The other major control on the

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Fig. 11. Summary of the suggested controls on the structure of the type assemblages. See discussion in text.

taxonomic composition is largely dependent on climatic conditions (temperature) at di¡erent palaeogeographic sites. Faunal exchange with other areas and patterns of oceanic circulation are also dependent on the palaeogeographic location and, ultimately, in£uence the taxonomic composition within biofacies, contributing to the de¢nition of faunal provinces. 8.2. Geodynamic settings and provincialism Although it has long been known that there is a correlation between latitude and provincialism, the in£uence of the geodynamic setting in the origin of biogeographic units has not been clearly established. The relationship between geodynamic context and faunal assemblages was partially addressed by Miller and Mao (1995) in assessing the relationship between orogenic activity and diversi¢cation of taxa during the Ordovician radiation. Some questions arise from the present study. It may be asked, ¢rstly, to what extent the ‘type assemblages’ de¢ned in the Argentine basins can be recognised in similar palaeoclimatic and geodynamic settings elsewhere, and, secondly, whether there is any kind of relationship between the type assemblages, geodynamic frameworks and provincialism ? The ¢rst question can be addressed only in a broad sense because the faunal data from most basins are not robust or detailed enough to make comparisons at a similar scale, using the same parameters employed in the present work. However, early Ordovician assemblages from low-latitude, passive-margin carbonate platforms do not di¡er substantially from the

demosponge^brachiopod type assemblage de¢ned in the Precordillera. Its rhynchonelliform faunas, for instance, are closely comparable, at genus level, with those from the warm-water ‘American Realm’ (Williams, 1973) or ‘Low-latitude Realm’ (Neuman and Harper, 1992). Moreover, the periequatorial palaeocontinents (i.e. the Laurentia, Kazakhstan, Siberia, and Australasian plates) display a rather similar proportion of higher taxa (orthides, clitambonitidines, pentamerids, plectambonitoids) (Benedetto, 2001a). Of them, the plectambonitoids become highly diversi¢ed, especially in quiet, moderately deep waters and markedly diminish toward higher latitudes. The dominance and distribution of demosponges in the Precordillera basin also follow a global pattern. Similar sponge^algal facies occur in early Ordovician rocks deposited around the margin of Laurentia from Newfoundland to the Great Basin (Alberstadt and Repetski, 1989), in South China (Carrera and Rigby, 1999), and Siberia (Webby, 1999). The global invasion of demosponges onto the shallow shelves, by the early Ordovician, could have been favoured by a global sea-level rise on epeiric seas carrying nutrient-rich waters and the development of suitable substrates such as reef-mounds or hard-grounds, associated with the beginning of the calcite sea times (Wood, 1999; Carrera and Rigby, in press). Finally, it should be noted that the Precordilleran carbonates contain trilobites of the warm-water Bathyurid province, even though they are associated with Baltic and Australian forms (Vaccari, 1995). The close correlation between subduction-related volcano^sedimentary successions and typical Celtic Realm rhynchonelliform brachiopods, to which the Famatinian faunas belong (Benedetto, 1994, 2003), has long been noted by Neuman (1976, and references therein) who suggested that the Celtic assemblages inhabited peri-insular settings within the Iapetus Ocean. More recently, Harper et al. (1996) considered that this realm developed seaward of Gondwana at mid- to high latitudes. Based on rhynchonelliformean evidence, Benedetto (1998a,b) proposed the geographic continuity of the Celtic province along the western margin of Gondwana (Fig. 3). This realm, whose recognition has been questioned by

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Cocks and McKerrow (1993) (see also Cocks, 2001), is characterised not only by a suite of endemic genera but also by an unusually high degree of endemism (like in other volcanic island settings, i.e. the late Ordovician New South Wales volcanic islands ; Webby, 1992). As well a number of genera appear for the ¢rst time within the realm and later migrate to other provinces (i.e. Toquima^Table Head). According to Harper and Mac Niocaill (2002), both marginal and intra-oceanic Iapetus terranes acted as sources for radiation on the continental platforms and as refuges for relict taxa. Evidence from Gondwanan marginal basins emphasises their importance not only as sites of generic origination but also of familial di¡erentiation (Benedetto and Sa¤nchez, in press). The proportion of brachiopod higher taxa (orders or suborders) among the typical Celtic localities is relatively similar (Benedetto, 2001a). Furthermore, they display a distinctive aggregate of families including representatives of hesperonomiids, skenidiids, rectotrophiids, gonambonitids, tritoechiids, grorudiids, and ahtiellinins, the latter subfamily including most of the strophomenides recorded in the realm. Famatinian bivalves, like the rhynchonelliform brachiopods, are characterised by a high level of endemism (Fig. 5) (Sa¤nchez, 1997, 2001a). Trilobites, to the contrary, exhibit low endemism and their taxonomic composition is similar to that of any shallow shelf located at a similar latitude, displaying a mixture of cold- and warm-water taxa. A similar contrasting trend between the patterns of diversi¢cation exhibited by rhynchonelliform brachiopods and trilobites was noted on the low-latitude late Ordovician island shelves of New South Wales by Webby (1992) and in the upper Arenig Tourmakeady Limestone (western Ireland) by Adrain and Fortey (1997). A survey of typical Celtic Realm assemblages shows that most of them are linked to volcano^ sedimentary successions (Neuman, 1976, 1984; Bruton and Harper, 1985), suggesting that the distinctiveness of this biogeographic entity results from the interplay of palaeogeographic constraints with a particular set of ecological^environmental factors prevailing in volcanic settings (see above). In other words, the origin of the Cel-

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tic Realm, and probably its disappearance by the mid^late Llanvirn, was directly or indirectly linked to the development of long, perhaps discontinuous, volcanic arcs (i.e. the Penobscott arc; Van Staal et al., 1998) roughly parallel to latitudinal belts, within the temperate zone. ‘Celtic’ rhynchonelliformeans are also present in far-removed volcanic settings such as northeastern China (Harper et al., 1996), but their palaeogeographic connection with the Iapetus volcanic islands is still poorly understood. Baltic faunas, like the Celtic ones, developed within the temperate climatic belt, but in a very di¡erent geodynamic framework: a tectonically stable epicontinental basin with a very low sedimentation rate (Dronov and Holmer, 1999). The distinctive faunal pattern of Baltica (i.e. radiation of gonambonitoid and lycophorid brachiopods ; cf. Egerquist, 1999; Harper and Hinds, 2001 ; endemic asaphid trilobites) in comparison with that of the volcanic islands suggests that basins developed at comparable latitudes and in relatively close geographic proximity are not necessarily inhabited by similar faunal assemblages, the ensemble of physical factors linked to their geodynamic histories being the third controlling factor. 8.3. Implication for the model of evolutionary faunas The distribution of individual clades has shown a di¡erent array in each of the investigated regions. In a broad sense, in clastic cold-water settings (Cordillera Oriental) trilobites largely dominate the assemblages in all bathymetric belts. This clade is restricted to most distal settings at low latitudes (Precordillera), and to the inner and outer platform in temperate waters, volcanic arc-related settings (Famatina). In contrast, rhynchonelliform brachiopods dominate the assemblages in warm-water, middle ramp carbonate environments, achieving also a high variety of adaptive strategies. The group is highly diverse in equivalent habitats in volcaniclastic settings and forms almost one-guild assemblages on the nearshore of clastic shelves. Bivalves are scarce elements in most associations, but are important members of the assemblages in nearshore, volcaniclastic set-

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tings. Demosponges are present only in the shallow waters of the carbonate ramp. At a broader scale, the distinct environmental distribution of particular clades parallels the taxonomic diversi¢cation that took place during the Ordovician. Sepkoski and Sheehan (1983) indicated that representatives of the three evolutionary faunas segregated in di¡erent portions of the shelf during the early part of the Palaeozoic. Sepkoski and Miller (1985) further documented the strong spatio^temporal patterns in the development of the evolutionary faunas. However, for the lower part of the Ordovician the picture presented by these authors remains far from clear. For the nearshore setting Sepkoski and Sheehan (1983) presented contrasting results: the cluster analysis grouped brachiopod-dominated communities and the factor analysis assembled mollusc-dominated communities. Hence, they concluded that the radiations ¢rst emplaced mixed brachiopod^mollusc communities in the nearshore during the earliest Ordovician and that lower Ordovician communities appear to be intermediate in composition between typical shelf communities of the Palaeozoic fauna and the more modern-looking nearshore communities of the later Ordovician. Sepkoski and Miller (1985) presented a Q-mode factor analysis of Palaeozoic community data. According to their results, lower Ordovician communities from most shelf environments still load highly on the Cambrian fauna (Factor 1). These authors also found a gradation of several nearshore communities (composed of gastropods, nautiloids, orthids, and trilobites) with moderate loadings on Factor 3 (Modern Fauna) and moderate to low loadings on Factor 1 indicating the partial shift to mollusc-rich communities. Two end-members are recognised among the type assemblages from western Argentina : those inhabiting the warm-water carbonate ramp (Precordillera) and those from the clastic, temperateto cold-water shelf (Cordillera Oriental). The former includes the demosponge^brachiopod type assemblage, dominated by suspension feeders that largely ¢t the Palaeozoic fauna, an onshore^o¡shore pattern match predicted by Sepkoski (1981), Sepkoski and Sheehan (1983), and

Sepkoski and Miller (1985). The trilobite-dominated type assemblage of the distal ramp in the Precordillera is of Cambrian type. However, unlike the other Cambrian-like trilobite assemblages from Famatina and Cordillera Oriental, this one hosts a certain amount of members of the Whiterock trilobite fauna de¢ned by Adrain et al. (1998). In the Precordillera basin the Whiterock fauna diversi¢ed diachronously since the mid^late Arenig to the early Llanvirn tracking the progressive development of distal ramp settings. In the temperate to cold-water shelf of the Cordillera Oriental the trilobite type assemblage largely corresponds to the Cambrian Fauna (Sa¤nchez and Waisfeld, 1995; Waisfeld and Sa¤nchez, 1996; Waisfeld et al., 1999, 2001a), however, segregation across the shelf does not ¢t predictions. Proximal inner-shelf and nearshore type assemblages from Famatina display intermediate conditions and cannot be readily ascribed either to the Cambrian or to the Palaeozoic evolutionary faunas. Faunal assemblages from volcanic-arc islands have not been previously analysed from the perspective of the model of the evolutionary faunas. The single exception is the study of the onshore^ o¡shore record of island biotas in a late Ordovician low-latitude setting in central New South Wales by Webby (1992). Comparing the patterns exhibited by the island biota with the low latitude North American Platform, Webby (1992) found strong departures from the predicted patterns. A similar comparison might be drawn between faunal pro¢les from the Famatina island-arc type assemblages and those from the Cordillera Oriental cratonic shelf. The brachiopod^bivalve type assemblage is readily distinguished from the trilobite^brachiopod type assemblage of the Cordillera Oriental. Sepkoski and Sheehan (1983) and Jablonski et al. (1983) reported a cluster of communities distinguished by the dominance of bivalves in the late Ordovician in an onshore setting as one of the earliest emplacements of the Modern fauna. However, the type assemblage from Famatina suggests that indeed the Modern fauna might have initiated its expansion even earlier in the Ordovician. It should be noted that the communities resembling the ‘Modern fauna’ of Sepkoski and Miller (1985) (with gastropods and

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nautiloids) di¡er from the assemblages of Famatina and those reported by Jablonski et al. (1983). The signi¢cant participation of bivalves in the latter two allow considering them as better approximations to the Modern fauna. The brachiopod^trilobite type assemblage in the proximal inner shelf of Famatina with a variety of guilds occupied by rhynchonelliformeans di¡ers markedly from the trilobite-dominated assemblage from equivalent habitats in the Cordillera Oriental. A latitudinal gradient is not enough to explain such di¡erences with the assemblages from the Cordillera Oriental. Instead, these di¡erences could be best explained on the basis of the particular geodynamic context in which the type assemblages developed. The structure of this type assemblage and the patterns of diversi¢cation of rhynchonelliform brachiopods resemble the trends exhibited in equivalent habitats, but at regions placed at lower latitudes (e.g. Precordillera or North American Platform). This pattern is thought be the result of the environmental dynamics adjacent to volcanic settings, including suitable volcaniclastic substrates favouring rhynchonelliform brachiopods and an increase in nutrient supply mediated by volcanically-induced warming in an otherwise intermediate latitude region. Finally, Vermeij (1995) suggested that increased submarine volcanism and directly related e¡ects (warming, increased levels of atmospheric CO2 , increased global nutrient supply and energy levels) might have triggered the Ordovician radiation, enabling the diversi¢cation of organisms with relatively high metabolic rates. Miller (1997a,b) (see also Martin, 1996) further supported Vermeij’s view, however, suggested that these factors should have acted in concert with other agents. According to our study, the structure and distribution of the type assemblages from western Argentina reveal that the patterns of biotic change leading to the development of faunas of ‘Cambrian’, ‘Palaeozoic’ or ‘Modern’ aspects are mediated by the environmental dynamics at each geodynamic setting, coupled with the climatic conditions at di¡erent palaeogeographic locations. In the context of these large-scale factors, physical and biotic controls over the food supply played a key

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role in the diversi¢cation of suspension-feeding organisms leading to the expansion of the Palaeozoic evolutionary fauna and the Ordovician radiation. In this context, our case study from the west of Argentina further represents a small-scale model of the broad spectrum of the possible regional conditions that promoted the di¡erential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.

Acknowledgements The authors acknowledge ¢nancial support from CONICET (PID 05/1316) and ANPCyT^ FONCyT (PICT 99 No. 5387) through grants to T.M.S. and from Fundacio¤n Antorchas through a grant to B.G.W. Constructive reviews of David Harper and Mary Droser contributed to the improvement of the manuscript.

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