MASS EXTINCTION AS AN ARTIFACT OF STRATIGRAPHIC RESOLUTION - AN EXAMPLE FROM THE UPPER ORDOVICIAN

The second greatest of the “Big 5” Phanerozoic mass extinction events occurred in the latest Ordovician. Driven by global climate change, two rapid, successive glacio-eustatic sea-level falls within an interval of 0.5 m.y, resulted in two stages of extinction. The general opinion is that the “first strike" resulted in the sudden global extinction of graptolites and other planktic organisms. Recent integrated multi-disciplinary study of a stratigraphically complete, expanded section in the uppermost Vinini Formation in the Roberts Mountains, Nevada, and its comparison to correlative sections in the region and elsewhere in the world suggest otherwise. Although the graptolite fauna inhabiting denitrifying waters at the margins of the oxygen minimum zone beneath areas of oceanic upwelling suffered nearly complete extinction with habitat loss, the graptolite fauna inhabiting the photic zone flourished and diversified. Thus, graptolites were affected differentially by Late Ordovician oceanographic changes that devastated one habitat but had minimal effect on the other. In addition, the extinction of denitrification-zone graptolites was 1) gradual, occurring over several meters in the Vinini Creek and other sections, 2) sporadic, with the denitrification-zone habitat and associated fauna being restored briefly in some areas before total habitat loss and complete extinction, and 3) diachronous, with complete habitat loss and extinction occurring earlier in some regions (e.g. Asiatic Russia and Scotland) and progressively later in other regions (e.g. South China) over a period of approximately 0.3 m.y. Such a gradual, sporadic, diachronous pattern is expected when extinction results from habitat loss worldwide. Development of this new perspective on the Late Ordovician “mass extinction” is possible only because key sections are stratigraphically complete and graptolite species were abundant and cosmopolitan. It would not be possible if, as is so often the case, stratigraphic sections are incomplete and most species are rare and endemic.