CONICET | Buscador de Institutos y Recursos Humanos

This study aims to elucidate diversity trends during the Middle Devonian in northwestern Argentina through the characterization of palynological events. A dataset of spore genera counts from two boreholes (San Antonio x-1, Santa Victoria x-1), ranging in age from the Eifelian to Givetian, was analyzed. Several measures of diversity were compiled and contrasted with the composition of the floras. The Simpson, Shannon-Wiener and standing diversity indices together with Pielou and Hurlburt?s evenness measures were calculated. The standing diversity reaches its maximum during the Eifelian and rapidly decreases towards the Givetian. This tendency is also supported by the other computed parameters. The floras, during the Eifelian, are composed of apiculate, cavate and boldly sculptured spores represented mainly by the genera Apiculiretusispora, Grandispora and Verrucosisporites respectively. The decline in the evenness, towards the top of the sections, is shown when the Verrucosisporites genus outnumbers in the assemblages. The domination of these specimens is a pattern seen in other parts of Gondwana during the same time frame. The increment of this particular genus, associated with filicopsids, co-occurs with the inception of Geminospora lemurata and Biharisporites parviornatus. These species have a well-defined onset worldwide during the late Eifelian?early Givetian and are known to have affinities with archaeopteridaleans. Both, marine and continental elements, sustain a marginal depositional setting, with minor shoreline shifts for both assemblages during the Eifelian. By the early Givetian there is a significant increase of marine components. The diversity analysis and inferred palaeoenvironment support a change in the conditions, that show a stressed domain during the late Eifelian?early Givetian, which led to the predominance of few existing species and the establishment of new ones in the flora.