SILURIAN EARLY DEVONIAN OSTRACODES FROM SOUTH-AMERICA (ARGENTINA, BOLIVIA) - PRELIMINARY INVESTIGATIONS

Ostracodes are described from the ''Cerro del Fuerte Section'' and nei ghboring localities, all in the Precordillera de San Juan (northern Ar gentina, San Juan Province), and from sparse faunas in Bolivia (Chuqui saca, Tarabuco Province). Additional material comes from the collectio n of Thomas (1905). This preliminary study gives the first detailed de scription of ostracode assemblages in the Upper Silurian and Lower Dev onian of the South American continent. Records of other abundant lower Paleozoic (Lower Ordovician to Lower Devonian) ostracodes in Argentin a, Bolivia, Brazil, Peru, and Venezuela are also reviewed. Twelve diff erent species, belonging to Palaeocopa (Beyrichiacea and Primitiopsace a) and to Binodicopa and Podocopa (Healdiacea, Thlipsuracea, Cypridace a) are described and a new genus is established (Australobollia n. gen .). Ostracode distribution at Cerro del Fuerte attests to the presence of the Siluro-Devonian boundary within the upper Los Espejos Formatio n and confirms recent stratigraphical attributions mainly based on con odonts and brachiopods (Benedetto et al., 1992). The beyrichiacean Hem siella indicates a late Ludlow to Pridolian age for the lower part of upper Los Espejos Formation. The non-palaeocope ''Thlipsurella-Phanasy mmetria-Ranapeltis'' assemblage found in the uppermost part of the for mation indicates an Early Devonian (Lochkovian) age. The present study reveals the existence of faunal links at genus (Silurian) and probabl y species (Early Devonian) level between South America (Argentina, Bol ivia), Laurentia, Avalonia-Baltica, and northern Gondwana. For example , close affinities (e.g., Ranapeltis sp. aff. rowlandi and Thlipsurell a sp. aff. ellipsoclefta) exist between the uppermost Los Espejos Form ation (Argentina) and contemporaneous horizons in the North American M id-Continent (e.g., Haragan Formation, Oklahoma; Shriver Formation, Pe nnsylvania). Two types of ostracode assemblages, ''beyrichiacean-domin ated'' and ''thlipsuracean-bairdiacean-cypridacean dominated,'' are re cognized in the Late Silurian-Early Devonian of Argentina and discusse d relative to other assemblages known in Laurentia, Avalonia-Baltica, and northern Gondwana. Their paleoecological significance in relation to marine bathymetry is addressed.