CONICET | Buscador de Institutos y Recursos Humanos

Abstract. Two bryozoan taxa occurring in the 13 Hirnatian (Upper Ordovician) deposits in 14 western Argentina document a first postglacial community associated with a mid- to high15 latitude brachiopod assemblage, known as the typical Hirnantia fauna, in the Argentine 16 Precordillera. Helopora fragilis (Hall, 1852) and an indeterminate phyloporinid occur 17 within a thin but conspicuous shell bed that overlies diamictitic deposits from the lower 18 member of the Don Braulio Formation. The abundance of well preserved specimens of 19 bryozoans together with the dominance of suspension feeders suggests a mid-shelf setting 20 (offshore transition) with an intermediate to low sedimentation rate, low turbidity, and 21 nutrient-rich conditions. Hirnantian bryozoan assemblages identified from tropical and 22 subtropical regions are rather rare, and this assemblage represents a first high-latitude 23 Hirnantian record. The low diversity of bryozoans may be related to high-latitude location 24 of Argentina during the Late Ordovician. Helopora occurs mainly in Laurentia with several 25 species occurring in the Katian and with a few Hirnatian occurrences in Gondwana. This 26 broad distribution may imply that this genus had a broad environmental tolerance. It is one 27 of the genera that successfully crossed the Ordovician/Silurian boundary. However, 28 although this genus shows a post-extinction diversification and a more widespread 29 distribution in Laurentia (with numerous species recorded in the Silurian and Devonian of 30 Canada, Russia and China), it became extinct in Gondwana. We hypothesize that 31 environmental conditions after the Ordovician/Silurian boundary may have prevented the 32 settlement of bryozoans and caused the extinction of Helopora in the Argentine 33 Precordillera.