Morphometric reduction in Australocoelia palmata associated with the Devonian marine transgression

During the Devonian, the Gondwana occupied areas close to the South Pole and hosted many epicontinental seas, with a benthic fauna dominated by brachiopods. Due to the limited preservation of chronostratigraphic markers (e.g. conodonts and graptolites) on this paleocontinent, the study of ichnology and palynology has helped to track Devonian events in the South American and African continents. In the Paraná Basin, there are records of several transgressive and regressive cycles that occurred throughout the Devonian. Thus, this study considers changes in the size and shape of the brachiopod (Australocoelia palmata Morris and Sharpe, 1846) in the Lower-Middle Devonian Chapada Group (Early Pragian-Eifelian) in the state of Mato Grosso, Brazil as a proxy for paleoenvironmental change. Morphometric analysis (length, width and length x width) shows that fossils collected in transgressive strata (Lower Unit 2, Chapada Group) had the most diverse sizes. However, fossils from a regressive context (Upper 2 Unit, Chapada Group) tend to be 25% smaller in length and width. This case of dwarfism in Australocoelia palmata may be related to changes in the composition of marine waters, resulting from an anoxic moment, driven by sea-level fluctuations. •Transgressive and regressive cycles impacted the brachiopod fauna of the Paraná Basin.•Anoxic conditions in the Devonian of the Paraná basin hampered the growth of brachiopods.•The morphometric reduction of brachiopods indicates paleoecological stresses during the Emsian.