Chitinozoans and acritarchs from the Ordovician peri-Gondwana volcanic arc of the Famatina System, northwestern Argentina

The Early Ordovician Suri and Molles formations of the Famatina Group in northwestern Argentina contain distinct assemblages of acritarchs and chitinozoans. The lower Suri Formation yields peri-Gondwana organic microfossils. The acritarch microflora consists of typical cold-water taxa such as Arbusculidium filamentousum, Striatotheca, and Vavrdovella in association with temperate water taxa such as Ampullula and Rhopaliophora. The formation also yields two distinct Gondwanan chitinozoan assemblages, one characterized by Conochitina decipiens and the other by Eremochitina brevis. Both groups of organic-walled microfossils undergo important changes in composition in the succeeding Molles Formation. Baltisphaeridium and Rhopaliophora with subordinate species of Peteinosphaeridium are dominant in acritarch assemblage, while small, smooth, or weakly ornamented species of Conochitina characterize the chitinozoan populations. These changes are considered to be related to paleogeographic and paleoenvironmental conditions.