Biotic recovery after mass extinction: The role of climate and ocean-state in the post-glacial (Late Ordovician Early Silurian) recovery of the conodonts

The pattern of recovery in conodents 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 proposed in which progenitor 'survivor' apecies evolved in the bathyal ecozone during Ashgill global cooling. Their appearance in low latitude shallow water environments occurred as the result two phases of migration from the bathyal ecozone during Ashgill global cooling. Their level fall, included species attributed to the Ozarkodina and Oulochus? 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 emeogence 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 extonction, measuted from an Upper Ordovician to Lower Silurian (low latitude) graphical composite referece 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 extinections were non-random, a feature consistent with the dramatic changes in environment at this time.