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Palaeozoic |
Palaeontological Institute, Russian Academy of Sciences, Profsouyznaya 123, Moscow 117647, Russia
Revised and new stratigraphic data indicate a complex extinction event in the late Early Cambrian which consisted of two, temporally separate, but related phases. The earliest phase (mid-Botomian, Sinsk Event) may be related to widespread anoxia due to eutrophication and phytoplankton blooms. The later event (early Toyonian, Hawke Bay Event), is connected to a world wide regression. This double extinction event severely injured the reefal biota which has undergone a further rejuvenation during the remainder of the Cambrian. As a result of the lowering of grazing pressure and unhealthy metazoan-calcimicrobial interactions, the remaining metazoan reef-builders were eliminated by the end of the Early Cambrian. Consequent reduction of space heterogenity led to the decline of calcified microbes (Renalcis, etc.) in both diversity and abundance which gave way to the thrombolite-stromatolite community. The recovery of the reefal biota occurred during the very end of the Late Cambrian-Early Ordovician and may be attributed to Elvis-taxa, with the exception of the spicular demosponges. This recovering biota intruded into the thrombolite-stromatolite community and created a space for the more successful encrusting reef dwellers of Middle-Late Ordovician time.
