The Frasnian-Famennian biotic crisis: How many (if any) bolide impacts?

The prime causation of the mid-late Devonian mass extinction near the Frasn ian-Famennian (F-F) boundary remains uncertain. Nevertheless, geochemical e vidence has been presented recently as decisive evidence of a giant bolide impact occurring precisely at the F-F boundary, which promoted the global m ortality episode. Palaeobiological data, however, imply a gradual global ch ange, which is otherwise seen as a record of either multiple extraterrestri al catastrophes or of impact-triggered Earth-bound mechanisms. Sedimentolog ical (mega-tsunami), physical (craters, microtektites), and geochemical rec ords remain either elusive in many aspects, or incompatible with the predic ted impact crisis pattern. Biotic succession across the F-F horizon is stil l poorly known, especially in continental domains, to evidence a synchronou s ("bedding-plane") killing event at the close of the crisis. Instead, the commonly documented stepwise loss of biomass and an unproved distinctive id ead zonei are hard to explain simply as sampling artifacts. The assumed mas s mortality precisely at the F-F boundary may be limited mainly to the pela gic realm. The underestimated role of early Variscan tectonism and associat ed volcanic-hydrothermal processes, resulting in thermal and nutrient pulse s, as possible prime controls of the F-F crisis is suggested, as well as re semblances to the superplume-conditioned eventful mid-Cretaceous interval, exemplified in the Cenomanian-Turonian mass extinction. Additional shocks, generated by minor cometary strikes, are not excluded but may have affected some F-F biotas or areas.