Paleoenvironmental stress in the Paraná Basin: Brachiopoda (Orbiculoidea) as proxies for Devonian biotic crises

Abstract

During the Devonian many sea-level fluctuations, anoxia, and biotic crises impacted marine faunas on a global and local scale. In southwestern Gondwana, these events can be identified by geochemical signatures, changes in the ichnofabric, and in the fossil assemblage. This study investigates the morphometric responses of brachiopods (Orbiculoidea; O. baini, O. bodenbenderi, and O. excentrica) to transgressive-regressive cycles and anoxic events (Basal Zlíchov, Choteč, Daleje and Kačák) through the Devonian of Paraná Basin. The Pragian-Emsian transgression, good paleoenvironmental conditions allowed brachiopods to reach a wide range of sizes, with large specimens (e.g.: dorsal valves of O. baini: 32.7 × 28.0 mm to 10.7 × 10.7 mm). Associated with the Basal Emsian Zlíchov Event, no significant size changes were detected during this transgressive event. However, the subsequent Choteč Event (Emsian-Eifelian) triggered an irregular pattern of growth lines, suggesting changes in the oxygenation and geochemistry of the basin waters. The peak of environmental stress occurred during the Eifelian-Givetian transgression (Kačák Event), leading to widespread anoxia and the “Lilliput Effect”. O. baini dorsal valves shrank by ∼90 % (15.7 × 15.1 mm to 5.2 × 5.9 mm), while O. excentrica exhibited similar dwarfing. Regional variations in growth patterns between northern and southern basins suggest localized paleoenvironmental stress and tropical water influx from the Parnaíba Basin. These findings highlight the differential impact of Devonian crises on brachiopods from the Paraná Basin, such as stable growth during transgressions, subtle ecophenotypic changes (pattern of growth lines) under moderate stress, and extreme size reduction during anoxia.