Invertebrados fósiles de la formación lipeón (silúrico superior) del área Bermejo-Los toldos, Noroeste Argentino

Fossil invertebrates from the Upper Silurian Lipeon Formation from the Bermejo-Los Toldos area, northwestern Argentina. Fossil she- lly fauna from the upper section of Lipeon Formation (Silurian - Silurian-Devonian transition) in the Bermejo river-Los Toldos river area, northern Sierras Subandinas, Argentina-Bolivia boundary are described. The fossil record mostly consists of numerous plant rests with good preservation with very abundant chips. Shelly fauna appears only in few levels and it is in- tegrated almost exclusively by molluks, where bivalves are dominant. Anomalodesmatids and nuculaceans came from Angosto de Alarache, while in Los Toldos river there is an interesting association integrated by Nuculites sp. cf. N. frigidus Bradshaw y McCartan and Andinodesma sp. cf. A. radicostata Rehfeld y Mehl. This association is similar to those Ludlovian described for deposits of Tarabuco Formation of the Bolivian Interandino. Other molluks are present and consist of Tentaculites of two morphotypes and a very poorly preservated bellerofontid. Trilobites are represented only by a cephalic rest of a phacopid cal- monid. Throughout all levels, a very low diversity of fossil shelly fauna persists. The alternating anaerobic conditions and slightly reducing seawater for clay shale deposits, with conditions of moderate to high energy for sandy deposits with anoma- lodesmatids and nuculaceans, in both cases with a defined proximity to the continent are shown.