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1 December 2008 Silurian biotic crises in the northern Greater Caucasus (Russia): a comparison with the global record
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None of the “Big Five” mass extinctions occurred during the Silurian Period. However, a very good regional record from the northern part of the Greater Caucasus makes it possible to evaluate the strength of the Silurian events. During the mid-Paleozoic, the study region belonged to the Hun Superterrane, having been located in between the Prototethys-Rheic and Palaeotethys oceans. An analysis of the stratigraphic ranges of 99 graptolite and 35 conodont species, coupled with improvements in the regional stratigraphic framework, reveals a Llandovery peak in graptolite diversity and a subsequent decline towards the end of the Silurian. In contrast, conodonts radiated during the late Ludlow and remained quite diverse in the Pøidoli. The regional and global dynamics of both groups correspond quite well. Among a number of regional biotic crises, the most outstanding occurred at the Llandovery-Wenlock transition, the global analogue for which is the Ireviken Event. This might have been a true mass extinction. Although the only potential mass extinction (i.e., the Ireviken Event) evidently corresponded to regional and global abrupt sea-level fall, eustasy appears an unlikely cause of the observed diversity changes. An increase in carbonate sedimentation in the late Silurian explains the long-term trends in graptolite and conodont diversity changes. Bottom water changes and glaciations were among the probable triggers of the Ireviken Event. Extraterrestrial impact also may be hypothesized to explain this short-term event, whereas no large superplume activity is known from the Silurian.

Dmitry A. Ruban "Silurian biotic crises in the northern Greater Caucasus (Russia): a comparison with the global record," Paleontological Research 12(4), (1 December 2008).
Received: 3 December 2007; Accepted: 1 July 2008; Published: 1 December 2008

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