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Palaeozoic |
Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
The pattern of recovery in conodonts following the Late Ordovician mass extinction does not conform to the classical adaptive radiation model of transgression, shelf area expansion and cladogenesis. An alternative hypothesis is proposed in which progenitor ‘survivor’ species evolved in the bathyal ecozone during Ashgill global cooling. Their appearance in low latitude shallow water environments occurred as the result of two phases of migration from the bathyal ecozone. The first, during the extraordinarius Biozone sea-level fall, included species attributed to the Ozarkodina and Oulodus? assemblages. This migration is considered to have been a response to the upward movement of the permanent thermocline, as high latitude climate and ocean conditions developed at low latitudes. The second phase of emergence occurred during the persculptus Biozone transgression when species of the Dapsilodus-Distomodus assemblage, appeared ahead of advancing anoxia.
Mean rates of per taxon origination and extinction, measured from an Upper Ordovician to Lower Silurian (low latitude) graphical composite reference section, were unequal at 0.09 and 0.19 taxa per standard time unit. The mean value for net rate of change in diversity was –0.66. Species originations and extinctions were non-random, a feature consistent with the dramatic changes in environment at this time.
