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

WAISFELD, B.G., SÁNCHEZ, T.M. BENEDETTO, J.L., Y CARRERA, M.G.

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

Elsevier

Lugar: The Netherlands; Año: 2003 p. 343 - 373

0031-0182

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 (rhynchonelliformbrachiopods, 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 asiliciclastic 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 largescalefactors 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 primaryproductivity 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 conditionsthat promoted the differential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.