Abstract

Limiting factors affecting faunal distribution in the Ordovician rocks of the Precordillera basin, western Argentina, are analyzed and tentatively established. Eight biotic intervals ranging from the Tremadoc to the Ashgill are defined based upon the distribution in time and space of articulate brachiopod, trilobite, sponge, and bryozoan genera. High magnitudes of turnover are recognized on the basis of vertical ranges of taxa; this observation also is supported by low indices of carryover and holdover. Times of high faunal change define boundaries between the biotic intervals. Abiotic constraints largely prevail over ecological factors in controlling most substantial shifts in faunal composition and distribution at the interval boundaries. Taxonomic composition through different intervals is consistent with a change in paleobiogeographic affinities throughout the Ordovician, from markedly Laurentian affinities in the lowermost Ordovician to increasing Gondwanic affinities towards the Middle and Late Ordovician. At a smaller scale, the development of successive intervals is overprinted strongly by changes in facies and tectonic-induced setting. The composition and structure of assemblages were modified dramatically in the late Arenig-early Llanvirn with the changeover from sponge and brachiopod-dominated assemblages to brachiopod and trilobite-dominated assemblages with abundant bryozoans. This change occurs in consort with a shift from carbonate facies to mixed carbonate-siliciclastic and exclusively siliciclastic facies, and is considered to be the result of sea-level rise and the development of subsiding depocenters in the northern and southern parts of the Precordillera basin. On paleoecological grounds, this episode has allowed the biotic intervals to be grouped in two distinct stages. The lower stage includes intervals 1 to 3, whereas the upper stage includes intervals 6 to 8; intervals 4 and 5 represent the diachronous arrangement of both types of assemblages due to the gradual southward flooding of the basin. Abiotic factors acted at different temporal and spatial scales. Hence, their effect on faunal composition is explained best by distinguishing hierarchical levels of incidence. On this basis, three major hierarchical levels of factors are proposed. The general context in which faunas developed is provided by the shifting position of the Precordillera terrane throughout the Ordovician; accordingly, a first-order hierarchical level is represented by paleogeographic position. The second-order level is represented by the combined effects of rapid sea-level rise and basin deepening, with a high paleoecological impact on the fauna. The third-order level includes single factors producing minor changes in otherwise stable community structures (e.g., minor sea-level changes, fluctuations in water temperature, different facies types).

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