The Late Devonian mass extinction

No less than 11 global extinction events were identified in the Givetian through Famennian stages (Walliser 1996) but only the lower and upper Kellwasser events (Frasnian-Famennian boundary) and the Hangenberg event (uppermost Famennian) amount to a magnitude that deserve the name mass extinction (Hallam and Wignall 1997). Yet the stress imposed by the many smaller events, especially the earlier Thaganic event in the uppermost Givetian, was probably crucial for the overall extinction patterns in maintaining a high level of environmental stress throughout the Late Devonian.

At least 70% and perhaps as many as 82% of the marine species became extinct during this time period (McGhee 1996, 2001; O Figure 16.8). Among the groups that were most severely hit were the reef builders (Fagerstrom 1994; Hallam and Wignall 1997; Copper and Scotese 2003), but other benthic organisms, especially tropical families, also suffered heavy losses during both major crises. The toll was no less severe in planktonic and nektonic groups, and the armored agnathans and the placoderms went completely extinct (Hallam and Wignall 1997).

Quite important for the elucidation of the possible causes of the Late Devonian mass extinctions is that their selectivity differed between the different events. The Taghanic event affected mainly benthic taxa from low-latitude, shallow-water environments (Hallam and Wignall 1997). The Kellwasser events also affected mainly warm-water species as well as planktonic and pelagic groups. During the Hangenberg crisis, it was the planktonic and nektonic groups that were most severely hit while the benthic groups showed a better survival than at the Frasnian-Famennian boundary.

The extinction patterns in the Late Devonian were highly complex and a result of several mechanisms spread over a time period of more than 10 Ma, with the most severe perturbations concentrated at the Frasnian-Famennian boundary and in the latest Famennian (Sandberg et al. 2002). Global cooling of the oceans (Copper 1986) was certainly one of the main causes of the extinctions (McGhee 1996,2001), although the Gondwana glaciation only started in the Late Famennian (Caputo 1985; Algeo and Scheckler 1998). Yet beside the cooling, frequent sea-level changes including both eustatic rises associated with spreading anoxic waters and o

O Figure 16.8

Extinction patterns during the Late Devonian mass extinction. Modified after Hallam & Wignall 1997

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