Biodiversification patterns in the Early Ordovician of western Argentina.

WAISFELD, B.G., T.M. SÁNCHEZ AND M.G. CARRERA

PALAIOS

SEPM

Lugar: Lawrence, Kansas; Año: 1999 p. 198 - 214

0883-1351

Biodiversification patterns in two Argentine Early Ordovician basins with different geodynamic histories are presented. We address trilobites and articulate brachiopods from the Santa Rosita (Uppermost Cambrian-upper Tremadoc) and the Acoite (lower-middle Arenig) formations in the Central Andean basin, and trilobites, articulate brachiopods, and sponges from the San Juan Formation (upper Tremadoc-Llanvirn) and the lower member of the Gualcamayo Formation (upper Arenig) in the Precordillera basin. Faunal turnover mediated by immigration and/or local radiation events are recorded in both basins. In the Central Andean basin, the Tremadoc Kainella and Ceratopyge faunas are dominated by olenid trilobites, displaying a worldwide distribution and high levels of immigration. The subsequent early Arenig Thysanopyge fauna records a local radiation event, affecting primarily a single family (Asaphidae). The succeeding Famatinolithus fauna records the immigration of taxa associated with radiation within several trilobite families. Faunal turnover in the Precordillera basin in the middle and late Arenig is mainly related to the immigration of taxa. Trilobite diversity is low during late Tremadoc-middle Arenig; however, a local deepening in the northern part of the basin resulted in a significant diversity increase in the late Arenig, mediated by the immigration of more cosmopolitan genera. Sponge diversity in the late Tremadoc and late Arenig is related to the development of reef mounds. The highest diversity occurred in the middle Arenig, and was the result of the origination of new taxa (among the Streptosolenidae and Nexospongiidae) and the invasion of hexactinellid sponges. The middle Arenig was a critical time in the early history of brachiopods in both basins as they show a significant expansion, which appears to be strongly controlled environmentally. Radiation and immigration episodes among the three investigated groups occur at different times, depending upon their particular biotic characters, and are strongly overprinted by the local tectonic basinal histories. In the time span investigated, trilobite richness in the siliciclastic successions of the Central Andean basin is remarkable and shelf assemblages largely retain a Cambrian aspect. Trilobite and brachiopod diversity and abundance fluctuate according to the lithologies and types of shell beds considered. In the Precordillera basin, brachiopod and sponge diversity is considerable, and shelf assemblages in these carbonate successions display a general Paleozoic aspect. Trilobite diversity and relative abundance in these assemblages do not decrease significantly as it should be expected. The timing of brachiopod and trilobite diversification exhibits a slight decoupling from published global genus-level curves. Processes such as radiation, extinction, and immigration of taxa in local communities are overprinted strongly by geodynamic histories and paleogeographic location. The latter are ultimately responsible for the decoupling of basinal patterns from global trends expected for the Ordovician radiation or the development of evolutionary faunas. A new understanding of processes of the Ordovician radiation arise from the study of diversification patterns and community structure in different paleogeographic settings. Hence, the analysis of local phenomena in their provincial context greatly contributes to the understanding of biases from the global-scale diversification and evolutionary models.