Graptolite and conodont faunas of Upper Ordovician (Sandbian) successions of the Argentine Precordillera: Biostratigraphic implications

Graptolites and conodonts are frequent fossils in Sandbian (Late Ordovician) outcrops of the Argentine Precordillera, where they present continuous and abundant records that enable detailed biostratigraphic studies. This work deals with the integration of previously available information on early Sandbian graptolites and conodonts, together with new paleontological data from the Las Aguaditas Creek (middle and upper members of the Las Aguaditas Formation), Las Chacritas River (Las Aguaditas Formation), and Cerro La Chilca (Los Azules Formation) sections from the Central Precordillera of the San Juan Province. The presence of the key graptolite Nemagraptus gracilis and the associated graptolite taxa allow for the documentation of the N. gracilis Zone. Specimens of N. gracilis and key conodont species, such as Pygodus anserinus coexist, along with a rich conodont fauna including Baltoniodus variabilis, whose range straddles the boundary between the Pygodus anserinus and the Amorphognathus tvaerensis zones. Detailed biostratigraphic ranges of the rich graptolite and conodont associations documented in the three study areas are provided. These records and an accurate taxonomic review allow a thorough description of the recognized zones. The use of conodont and graptolite zones from different areas of the Precordillera enables the verification of a diachronous contact, representing a hiatus that spans the middle to upper Darriwilian. Moreover, a brief taxonomic description of the identified graptolite species is provided.