SPECIES DIVERSIFICATION DURING MASS EXTINCTION: GRAPTOLITES IN THE LATE ORDOVICIAN

In general, all marine organisms were decimated during the brief (0.5 my), but dramatic, global climatic and oceanographic changes in the latest Ordovician. With generic diversity declining 60%, this was the second greatest of the “Big 5” Phanerozoic mass extinction events. Graptolites - pelagic, colonial, suspension feeding, macro-zooplankton - experienced nearly complete extinction with one clade, the normalograptids surviving. In fact, the normalograptids diversified during the extinction interval! Results from multi-disciplinary study of a stratigraphically complete section in the Vinini Formation in the Roberts Mountains, Nevada, and their comparison to those from correlative sections in the region and elsewhere in the world indicate that, while the habit favored by most graptolites was more-or-less completely lost, the habitat of normalograptids was little affected. Most graptolites inhabited denitrifying waters at the margins of the oxygen minimum zone beneath areas of oceanic upwelling, where they fed on nitrate reducing bacteria that flourished in these waters. This habitat was lost worldwide in the latest Ordovician and, as a consequence, the graptolites adapted to it suffered nearly complete extinction. Normalograptids, however, inhabited the photic zone, fed on phytoplankton, flourished and diversified. Thus, graptolites were affected differentially by Late Ordovician oceanographic changes that devastated one habitat but had minimal effect on the other. 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. Yet, in sections worldwide, the normalograptid record is one of diversification during this same brief time interval. Such a gradual, sporadic, diachronous, differentiatial pattern is expected when extinction results from dramatic, but selective environmental change worldwide.