Relationships between brachiopod fauna (Lochkovian–Frasnian) from northwest Gondwana (Amazonas Basin) and environmental changes during the Devonian

Abstract

The Devonian marked a significant phase in the evolutionary history of the Brachiopoda. During this period, the Phylum achieved its peak diversity (Emsian) and suffered one of its greatest declines (Frasnian–Famennian), ranking second only to the mass extinction event at the Late Permian period. In Brazil, the Amazonas Basin has a rich record of Devonian brachiopods distributed in four stratigraphic units: i) Manacapuru (Lochkovian), ii) Maecuru (early Eifelian), iii) Ererê (late Eifelian), and iv) Barreirinha (early Frasnian) formations. This article aims to survey the occurrences of brachiopods during the Devonian in the Amazonas Basin and to discuss the possible factors that influenced their diversity. Environmental changes may explain the variation of brachiopod diversity throughout the Devonian in the Amazonas Basin. We noted that the diversity variation of Devonian brachiopods in the Amazonas Basin unfolded in three distinct stages. The peak of brachiopod diversity occurred in the early Eifelian (Maecuru Formation), when the Amazonas Basin was between the subtropical latitudes 30°S and 60°S, under shallow marine and cold conditions, justified by the absence of carbonates, evaporites, and reefs in the region (Stage 1). The first diversity decline is recorded in the Ererê Formation (late Eifelian), attributed to a warmer climate and deeper water than in the Maecuru Formation (Stage 2). During the Frasnian, there was a second diversity decline of brachiopods in the Amazonas Basin (Barreirinha Formation). A major global transgression occurred in the Late Devonian. In this period, the Amazonas basin experienced the deepest marine conditions in its history. The brachiopods of the Barreirinha Formation occur in layers of black shale (offshore), attributed to an anoxic or dysoxic environment of high stress, which explains the low diversity of brachiopods in this unit (Stage 3).