PALAEOZOIC/Devonian 197 Figure Marine biodiversity during the Devonian, where bio diversity is measured as the number of genera of marine organ isms that are well preserved in the fossil record (Data from Sepkoski (1996).) The factors controlling the Devonian pattern of biodiversity gain and loss are complex, and not all factors are agreed upon to the present day In general, the biodiversity gain seen in the Lochkovian was a recovery of biodiversity lost in extinctions that occurred in the Late Silurian The Pragian saw the evolution of many new taxa, pushing Devonian marine biodiversity to levels higher than any that had existed previously in the Palaeozoic, but many of these new taxa were endemic, however, and having restricted geographic distributions and confined to local regions Global sea-level began to rise in the Emsian and continued to rise through the Frasnian (perhaps due to the initiation of continental rifting in the Ukraine region of Europe), gradually breaking down barriers between marine regions and facilitating species migration With the progress of rising sealevels and increased species migration in the oceans, endemic local faunas successively became extinct and were replaced by more cosmopolitan migrants By the Early Frasnian, biodiversity in the Devonian seas had dropped to levels comparable to those seen at the beginning of the Devonian (Figure 3), but these seas were now populated by cosmopolitan species with wide geographic distributions The gradual decline in marine biodiversity from the Emsian through to the Givetian was mirrored in the gradual increase in extinction intensity within marine faunas during this same interval (Figure 4) Extinction intensity reached a peak in the Late Givetian, with the elimination of most of the previously existing endemic faunas, and then fell to levels comparable with those seen in the Lochkovian and Pragian and within the Figure Extinction intensities during the Devonian, where intensity is measured as the percent of marine genera that go extinct per time interval Positions of the Late Devonian (F/F) and end Devonian (D/C) extinction peaks are marked with arrows (Data from Sepkoski (1996).) cosmopolitan faunas of the Frasnian (Figure 4) Very abruptly, extinction intensity within these same cosmopolitan faunas jumped to a level seen only five times in Earth history, and precipitated an equally abrupt loss of marine biodiversity (Figure 3) This is the Late Devonian mass extinction, also termed the Frasnian/Famennian (F/F) mass extinction or the Kellwasser bioevent, and was the first of the Late Devonian biodiversity crises The second Late Devonian biodiversity crisis occurred at the very end of the Devonian, and is variously termed the end-Devonian extinction, the Devonian/Carboniferous (D/C) extinction, or the Hangenberg bioevent The extinction intensity of the end-Devonian extinction was only slightly more severe than that seen in the Late Givetian (Figure 4), but was markedly different in that it was an abrupt jump in extinction, rather than the culmination of a long trend of increasing extinction magnitudes (Figure 4) Causes of the Late Devonian Biodiversity Crises? The End-Devonian Biodiversity Crisis It is generally agreed that the end-Devonian biodiversity crisis, or Hangenberg bioevent, was triggered by global cooling and associated continental glaciation In the very latest Famennian, just before the end of the Devonian, extensive glacial ice-caps formed in regions