1 CONICET. Cátedra de Estratigrafía y Geología Histórica, Escuela de Geología, Universidad Nacional de Córdoba. Av. Vélez Sarfield 299, 5000 Córdoba, Argentina. E–mail: jbenedetto@arnet.com.ar

2 CONICET. Cátedra de Paleontología, Escuela de Biología, Universidad Nacional de Córdoba. Av. Vélez Sarfield 299, 5000 Córdoba, Argentina. E–mail: tsanchez@com.uncor.edu

Key words: Brachiopods. Bivalves. Ordovician radiation. Gondwana. Argentina.

During the Ordovician, the western and northwestern margins of Gondwana were flooded by epicontinental seas which penetrated profoundly into the continent. The resulting Ordovician ‘epeiric’ seas were filled by thick successions of siliciclastic sediments, and fringed by magmatic arcs and associated volcano–sedimentary back–arc basins (e.g., Puna basin). Unlike other peri–Gondwanan successions, marine deposits in northwestern Argentina and Bolivia display a nearly continuous stratigraphic record across the Cambrian–Ordovician boundary. Strata ranging in age from Late Cambrian to Mid Ordovician contain rich faunas consisting of trilobites and brachiopods, together with less common molluscs (rostroconchs, bivalves, gastropods and nautiloids). These successions form an excellent basis for studying the earliest stages of the Ordovician biodiversification on the temperate to cold water platforms of western Gondwana. This paper provides a discussion on the diversification patterns of Gondwanan articulate brachiopods and bivalves compared with global trends. Brachiopod and bivalve diversification on Gondwanan platforms, at the level of higher taxa, was essentially different from that on the low–latitude carbonate platforms (Babin, 1993; Benedetto and Sánchez, 1996; Sánchez and Waisfeld, 1995, Waisfeld et al., 1999; Benedetto, 2001). Such ‘megatrends’ of diversification at the beginning of the Ordovician radiation were responsible for the Gondwanan faunal signature, which persisted throughout the entire Ordovician and, at a lesser degree, until the Silurian–Devonian.

The initial radiation of brachiopods occurred on the Early Cambrian peri–Equatorial platforms, but since the late Atdabanian and Botomian brachiopods became widespread in both carbonate and mixed clastic–carbonatic facies (Ushatinskaya, 1996). By the Late Cambrian, after an important extinction event involving the kutorginids, protorthids and bohemiellids, low–latitude shelves were inhabited mainly by pentamerids, anomalorthids (= alimbellids), eoorthids, billingsellids and finkelnburgiids. All these groups crossed the Cambro–Ordovician boundary without significant changes in diversity, with exception the pentamerids, which experienced an important radiation during the Tremadoc and Arenig. In analyzing the Ordovician brachiopod radiation on the North American craton, Patzkowsky (1995) recognized a first phase of diversification accomplished during the Tremadoc–Arenig (Ibexian), characterized by a dominance of syntrophidine pentamerids in near–shore settings and orthoids in shelfal environments (the latter include orthids, orthidiellids, hesperonomiids and finkelnburgiids). This initial radiation included the earliest plectambonitoids (taffiids), which rapidly diversified in offshore settings during the Mid– LateOrdovician. Unlike the ‘Tropical Realm’ (Laurentia, Siberia, Australia, Kazakhstan), the Tremadoc clastic successions of Perunica, Baltica, Ibero–Armorica, Avalonia and NW Africa were typically inhabited by low–diversity, nearshore lingulate communities (Mergl, 1999), including only a few rhynchonelliform genera such as Poramborthis (Poramborthidae), Nanorthis (Nanorthidae), Protambonites (Tritoechiidae), Apheoorthina, Robertorthis, Jivinella (Eoorthidae). Evidence from the Central Andean basin of Argentina and Bolivia, as well as from other peri–Gondwanan sites, demonstrates that the rhynchonelliform radiation pattern in Gondwana was essentially different to that of Laurentia: (1) syntrophidine pentamerids are lacking, the only representatives of this order being rare camerelloids, such as the ubiquitous Camerella; (2) the Orthida radiation is the ‘hallmark’ of Gondwanan rhynchonelliform faunas. A number of the ‘true’ plectorthoideans (the bohemiellids and eoorthids probably do not belong to the main plectorthoid lineage) originated on Gondwanan shelves. A species of Kvania recovered from Late Cambrian beds of northwestern Argentina (Kvania sp.1, Benedetto, 2002) is likely to be ancestral to plectorthoid groups such as the plectorthids, giraldiellids, euorthisinids and nanorthids (the latter should be removed from the Orthoidea and transferred to the Plectorthoidea). All of them are well represented in the Tremadoc of the Central Andean basin; it is probable that the recently discovered plectorthid Lipanorthis may originate also from a Kvania–like ancestor (Benedetto and Carrasco, 2002); (3) dalmanellidines appear earlier (Arenig) on Gondwanan shelves and peri–Gondwanan continental fragments than in other paleocontinents, e.g., Dalmanella elementaria Williams (Dalmanellidae) in Avalonia; Tarfaya marocana Havlícek and probably Incorthis (Heterorthidae?) in Morocco, Bolivia and NW Argentina; Salopia? lipanenensis Benedetto (Linoporellidae) in NW Argentina. By the Caradoc, heterorthids and draboviids become highly characteristic for the Mediterranean Province, to which the Central Andean assemblages have been referred.

The volcanic–arcs developed along the peri–Gondwanan subduction zone (‘Puna–Famatina–Avalonia arc’, Benedetto, 1998b) hosted more diverse brachiopod assemblages including several endemic taxa, allowing Neuman (1972) to recognize a Celtic brachiopod province. In western Argentina, the Famatina back–arc basin contains a typical Arenig Celtic fauna with addition of a few Gondwanan taxa such as Incorthis, Prantlina?, and Hesperonomia orientalis (Benedetto, 2003). Their occurrence supports faunal exchange between the epicontinental Gondwanan shelves (Central Andean basin) and the peripheral marginal seas (Puna and Famatina arc–related basins). Like other Celtic assemblages, most of Famatinian genera are older than congeneric species from low–latitude settings. An interesting aspect is that, as far as we aware, the Mid Arenig Famatina fauna is among the earliest known Celtic assemblages, and consequently may have played a key role in the early radiation of Ordovician brachiopods.

The Cambrian record of bivalves is very poor and the hitherto known genera (Fordilla, Pojetaia, Tuarangia and probably others, see discussion in Cope, 2002 and references therein) do not show clear affinities with any Early Ordovician group, the fordilloid dentition being neither paleotaxodont nor actinodont (Carter et al., 2000). Bivalves that appear in the Tremadoc and Early Arenig successions of NW Argentina are among the earliest known (Harrington, 1938), and some of them could represent the basal stem of such clades as anomalodesmatan and cycloconchoideans.

The bivalves Intihuarella Sánchez and Ucumaris Sánchez (Family Intihuarellidae), recently discovered in the Late Tremadoc (P. deltifer Zone) Floresta Formation (Sánchez and Vaccari, in press), probably may represent the earliest and simplest stage in the evolution of the Superfamily Cycloconchoidea. Intihuarella is close to the ancestry of the Family Cycloconchidae (Sánchez, in press). The earliest known cycloconchids Fortowensia Cope and Carminodonta Cope come from the early Arenig of South Wales (Cope, 2002 and references therein). By the Mid Ordovician, cycloconchids rapidly diversified throughout Gondwana and peri–Gondwanan areas, but only Cycloconcha Miller can be traced to Laurentia. The rapid diversification of the cycloconchids on Gondwanan shelves led to the appearance of peculiar taxa such as Famatinadonta (Mid Arenig, Famatina basin), whose features preclude any direct phylogenetic relationship to the intihuarellids (Sánchez, 2001a). The Late Tremadoc Ucumaris Sánchez is the type genus of the Family Ucumariidae, which also includes the Early Arenig Ucumaropsis Sánchez. Both genera show radial pustulation covering the entire shell surface. Because this kind of ornament is a diagnostic feature of the Anomalodesmata, ucumariids are considered as being the earliest members of this group. The Welsh genus Arenigomya Cope (1996) is slightly younger (Early Arenig) than Ucumaris and is approximately coeval with Ucumaropsis; it also displays the typical anomalodesmatan radial pustulation. By the Late Ordovician, anomalodesmatan taxa expanded into Laurentia, Siberia, and Kazakhstan (Cope, 2002). Catamarcaia Sánchez and Babin (1993) from the Middle Arenig of the Famatina basin, the earliest hitherto known arcoid, is a phylogenetically significant genus which is likely to be ancestral to the arcoids (Sánchez, 1995; Carter et al., 2000). No other arcoids have been recorded from the remainder of the Ordovician. Ambonychiids diversified mostly on the low–latitude carbonate platforms of Laurentia, Baltica, and East Gondwana (Australia). The earliest representative of this group was discovered in Early Arenig limestones of the Laurentian–derived Precordillera terrane (Sánchez, 2001b), whereas the Mid Arenig Notonychia Sánchez (2001a), from the Famatina basin, and Cleionychia from South Wales (Cope, 2002) are the oldest Gondwanan ambonychiids.

The clastic platforms of Gondwana were not only the site of a rapid diversification of bivalves and orthide brachiopods but also the site of origination of several clades, most of which dispersed later to low latitude areas. In the vast northwestern basin of Argentina the earliest known plectorthoideans of ‘modern’ type appear in the Late Cambrian–earliest Tremadocian. They probably are close to stem groups of the nanorthids, euorthisinids, plectorthids and giraldiellids. The Gondwana radiation differs from the ‘Laurentian’ pattern proposed by Patzkowsky (1995) in that the first diversification phase (Late Cambrian–Tremadoc) seems to be unrelated phylogenetically to the clades that dominate the carbonate platforms during the Cambrian. The second phase began in the mid–Arenig in both siliciclastic and volcaniclastic successions. We believe that such island arcs marginal to Gondwana (Famatina and western Puna basins) played a key role in the early radiation of brachiopods and bivalves (Waisfeld et al., 2001; Waisfeld et al, in press; see also Harper and Mac Niocaill, 2002). Among brachiopods, it is remarkable not only the apparition of numerous endemic genera (and some families) but also the first apparition of platystrophiids, skenidiids, productorthids, glyptorthids and ahtiellinids. By the Late Arenig, most of these groups expanded to other intra–Iapetus islands. Among bivalves, the volcanic–arc related settings were the place of origin of the arcoids (Catamarcaiidae). The new discoveries of bivalves in the Early Ordovician of NW Argentina basin confirm the importance of the Gondwanan shelves in the earliest bivalve radiation already pointed out by Babin (1993), and help to clarify the earliest stages in the evolution of such bivalve clades as Cycloconchoidea and Anomalodesmata. Finally, we would emphasize the importance of the continuous Cambrian–Early Ordovician clastic successions of NW Argentina to better understanding the early phases of the Ordovician biotic radiation in the temperate to cold water oceanic realm.

This work was supported by grants from the Agencia Nacional de Promoción Científica y Tecnológica, PICT 5387, and CONICET, PIP 05/1316.