Late Cambrian agnostoid trilobites from the Famatina Range . Significado bioestratigrafico y paleoambiental

Guillermo Bodenbender (1916) firstly collected fossils ("Agnostus", "Obolus") from the Filo Azul Member of the Volcancito Formation at Rio Volcancito (Famatina Range, La Rioja Province), and tentatively assigned the unit to the "Upper Cambrian or Lower Ordovician". Recent studies have demonstrated that the Cambrian-Ordovician transition is well represented at Rio Volcancito. The lower part of the section is composed of dark-coloured marls and shales with Late Cambrian trilobites (e.g., agnostoids and olenids), whereas the upper part is mainly composed of green shales containing the Lower Tremadocian Jujuyaspis keideli and Rhabdinopora. The agnostoids Lotagnostus (Lotagnostus) sp., Micragnostus vilonii Harrington and Leanza, Pseudorhaptagnostus (Machairagnostus) tmetus Harrington and Leanza, P. (Machairagnostus) corrugatus (Suarez-Soruco), Gymnagnostusperinflatus (Harrington and Leanza) and G. bolivianus (Hoek) have proved to have great biostratigraphic value as guide fossils for the uppermost Cambrian. They are especially diverse in beds deposited under low-oxygen conditions, a fact that could denote a benthic or nekto-benthic mode of life in a nutrient rich environment.