Marine ostracod faunas through the Late Devonian extinction events. Part II: the Hangenberg event

This work is the second part of a two-part study evaluating the changes in ostracod communities throughout the two Late Devonian biological crises (Kellwasser and Hangenberg events). The Hangenberg event was the most intense extinction of the Famennian, Late Devonian. It is sometimes considered as equivalent to the Kellwasser event at the Frasnian-Famennian transition, which is one of the Big Five mass extinctions of the Phanerozoic. In order to discuss the effects of the Hangenberg event on marine ostracod faunas, all well-dated data from the literature have been gathered. We here show that this event mostly affected the ostracod diversity at low taxonomic levels (species and genera). Nearshore ostracods were more affected than others but Thuringian deep-sea faunas were also greatly touched. When considering the geographic pattern of the event, ostracods from the Southern Variscan margin (Montagne Noire, France) appear to have been less affected than others, thus possibly representing a refuge zone.

The faunal recovery was certainly constrained by changes in palaeoenvironmental settings, in particular those associated with the transgression occurring during the early Tournaisian, lowermost Carboniferous. It may have increased the habitat size and availability in neritic environments, allowing an important diversification of ostracods from the Bairdiidae, Bairdiocyprididae and Paraparchitidae families in the event aftermath, as well as a greater connectivity between nearshore areas leading to a global reduction in endemism. It may also have triggered the migration of bathyal taxa upslope, as recorded in post-event assemblages on the Southern Variscan margin. The effects on pelagic taxa are more variable across localities and environments but they are often among the only taxa composing event assemblages, particularly in deep-sea settings of the Southeastern Laurussian margin.

At a global scale, nearshore ostracod diversity was slightly less affected by the Hangenberg event than by the Kellwasser one, but the former has more significantly influenced deep benthic faunas. Finally, the Kellwasser event triggered a more intense extinction within ostracod faunas while the Hangenberg one seems responsible for a more important change in faunal composition, particularly within nearshore environments.