New record of isograptid graptolites from the Puna region, northwestern Argentina: biostratigraphical and palaeobiogeographical insights

Abstract

Records of isograptid graptolites from South America remain scarce and poorly known from Ordovician deposits at the Central Andean Basin (northwestern Argentina, Bolivia and Peru), but are widely distributed and documented in numerous stratigraphical sections in the Precordillera Argentina. Taxonomic assignations and correlations in the Puna region, northwestern Argentina, have often been regarded with uncertainty due to their poor preservation, tectonic deformation, low diversity, and isolated outcrops of difficult access. Here we document, for the first time, the presence of Isograptus divergens in the Vega de Catua section and revise previous records from the Tafna section. The age and correlation of the graptolite faunas from the Central Andean Basin are refined, contributing to the understanding of the palaeobiogeographical affinities and palaeoecology of South American isograptids. These new records allow for a more precise dating of the studied deposits to the Middle Ordovician (late Dapingian and possibly early Darriwilian), bridging the gap between the ‘Isograptus victoriae’ and Levisograptus austrodentatus biostratigraphical units previously proposed for the region. It also enables a more accurate global correlation of the studied Ordovician deposits with the Australian succession in which Oncograptus upsilon and Cardiograptus morsus biozones are developing, supporting the widespread (pandemic) distribution of the Isograptus genus. Furthermore, it highlights the preference of species of the genus Isograptus for deep-water palaeoenvironments of the Puna region, in contrast to the more open one of the Cordillera Oriental.