Kozlowskii Event: mass faunal extinction and global environmental perturbations recorded in the upper Silurian succession of the Prague Synform.

Ladislav Slavík, Petr Štorch

Upper Silurian sedimentary succession of the Prague Synform preserved particularly complete record of prominent global biotic crisis, known as Kozlowskii or Lau Event, as well as subsequent recovery of the marine biota. The most affected among pelagic faunas were planktonic graptolites. Almost simultaneous, selective extinction of several little phylogenetically related and greatly specialized taxa was recorded at the top of the kozlowskii Biozone. Also some common and widespread pelagic ostracods became extinct whereas pelagic orthocerid cephalopods and some nektonic crustaceans passed unaffected through the extinction interval. Conodont fauna suffered less extinction than suggested by earlier data from Gotland – the type area of the Lau Event. In the Prague Synform, sediments of the critical interval are dominated by rather long-ranging universal forms with considerable size-reduction of the specimens (Liliput Syndrom). Faunal extinction was apparently coincident with major fluctuations in sea-level indicated by local unconformities associated with debris-flows slumped from adjacent carbonate platform. Positive carbon isotope excursion commenced in the same level. New data demonstrated the coincidence of the graptolite crisis with other pelagic and benthic faunal changes and fall in sea-level manifested by facies change and carbon isotope excursion. Subsequent gradual recovery and restoration of the conodont diversity enabled further refinement of regional zonal subdivision of the interval that followed after the Lau Event and thus enabled improvement of relative dating of the interval in global scale.  Graptolite recovery was delayed relative to the recovery of benthic fauna and conodonts and closely post-dated the end of the isotopic excursion. Data obtained from the Prague Synform and their interpretation contributed to general understanding of one of the major global biotic crisis in the history of life on the Earth and its correlation with abiotic indicators of the environmental change.