An endemic conodont fauna of Darriwilian (Middle Ordovician) age from the Santa Gertrudis Formation, southwestern Gondwanan margin and its paleobiogeographic relationships

Abstract

A large collection of conodonts (over 4200 elements) from the Santa Gertrudis Formation of the Cordillera Oriental, NW Argentina, is analyzed. The mostly endemic fauna of this unit and the lack of index species require complex taxonomic and biostratigraphic interpretations. We determined new species apparatuses by reviewing previous literature with a different taxonomic approach: Erismodus saltaensis n. sp., Erraticodon aldridgei n. sp., Gallinatodus elegantissimus n. gen. et sp., Pyramidens cactus n. gen. et sp., P. spinatus n. gen. et sp., Zentagnathus gertrudisae n. sp. Although the lack of key conodont species for intercontinental correlation precludes biostratigraphic assignment to a particular biozone, the overlapping ranges of recorded species restrict the age range to the upper Lenodus variabilis, Yangtzeplacognathus crassus and the lower Eoplacognathus pseudoplanus zones, of middle Darriwilian age (Dw2, Middle Ordovician). The depositional environment of the Santa Gertrudis Formation is primarily a shallow inner platform influenced by waves but not by storms; its paleogeographic location in mid-high latitudes exerts important control over the conodont biofacies, similar to the Arabian Darriwilian faunas of the Gondwanan margin. A narrow marine seaway along the suture zone connecting East and West Gondwana is proposed to explain the evolution of lineages of shallow cold-water conodont faunas from the Middle Ordovician onwards.