Lower Ordovician stratigraphy and trilobite faunas from the southern Famatina Range, La Rioja, Argentina

The Upper Member of the Volcancito Formation (Lower Ordovician) crops out at the Portezuelo de La Alumbrera Creek (southern part of the Famatina Range, La Rioja Province, north-western Argentina). The 160-m-thick section is mainly composed of outer shelf thin-bedded fine-grained sandstone/siltstone couplets, massive silty sandstones, and massive/laminated clayey siltstones (Facies Assemblage 1). Middle shelf massive/laminated clayey siltstones and hummocky cross-stratified calcareous sandstones and mudstones (Facies Assemblage 2) are restricted to the lower and the uppermost part of the section. Trilobites from the upper part of the succession include pelagic Corrugatagnostus sp., Prospectatrix sp., and Sagavia durandi sp. nov., and benthic Illaenopsis sp. nov., cf. Homalopteon sp., Hapalopleura longirachis sp. nov., Hapalopleura aff. longicornis Harrington and Leanza, Proteuloma sp. nov., Shumardia sp., Bienvillia sp., and Leptobenthos roartei gen. et sp. nov. Cyclopygids are the dominant element of the faunal assemblages. Precise international correlation is difficult because most of the taxa studied have not been described elsewhere, but generic occurrences and specific affinities suggest a latest Tremadoc age, Sedimentological and faunal analyses indicate low-oxygen-water bottom conditions. The distinctive morphology of Leptobenthos roartei supports a chemoautotrophic symbiotic lifestyle for this species. Occurrences of Corrugatagnostus, Prospectatrix and Illaenopsis provide new evidence for a largely peri-Gondwanan distribution of these genera.