Marine ostracod faunas through the Late Devonian extinction events. Part I: The Kellwasser event

The Kellwasser event is one of Big Five mass extinctions of the Phanerozoic. It marks the boundary between the Frasnian and the Famennian, the two stages of the Upper Devonian. In order to discuss its effects on marine ostracod faunas, all well-dated occurrences from the literature have been gathered. We estimate that, at a global scale, the Kellwasser event has been responsible for the disappearance of 80% of ostracod species. However, the effect on ostracods was geographically heterogeneous, with values varying from 61% to 91%, probably related to local parameters. Our analyses show that this event mostly affected the ostracod diversity at low taxonomic levels (species and genera). Both nearshore and deep (Thuringian) benthic ostracods were affected. This event also influenced pelagic taxa, mostly represented by Entomozoidae, but they were still less affected than benthic ones at the global scale. Together with ostracods of the genus Bairdia (Bairdiidae, Podocopida), Entomozoidae are among the most diversified taxa represented in event assemblages, suggesting that they were more tolerant to oxygen depletion as it was proposed, at least for Bairdia, for the Permian-Triassic event. We show that the faunal recovery has mostly been characterized by diversification among cosmopolitan (particularly Bairdiidae and Bairdiocyprididae) and Thuringian taxa (particularly Bythocytheridae and, to a lesser extent Healdiidae, Tricorninidae and Rectonariidae). A global increase in endemism is observed through the Kellwasser event, likely related to the diversification of deep-water Thuringian taxa, leading to an increase in differences of taxonomic composition between the palaeoenvironmental settings. The widely reported sea-level fall at this time interval could also have participated in the increase in endemism by reducing the connexion between nearshore areas. This work is the first part of a two-part study focusing on the changes in ostracod community throughout the two Late Devonian biological crises (Kellwasser and Hangenberg events).