EVENNESS AND SPECIES ABUNDANCE IN GRAPTOLITE COMMUNITIES: A NEW PROXY FOR CLIMATE CHANGE DURING THE END ORDOVICIAN MASS EXTINCTION

The Hirnantian glaciation generated what may be the second largest mass extinction of the Phanerozoic. Graptolites were particularly affected, experiencing an extinction episode that greatly reduced species diversity. Previous analyses involving other taxonomic groups have demonstrated that species abundance distributions can reflect community responses to environmental perturbations that cannot be detected by studies of taxonomic rates alone. We examined change in the distribution of abundance among species in late Katian and Hirnantian graptolite communities at two important late Ordovician sections located at Vinini Creek in Nevada and Blackstone River in the Yukon. We quantified change in the distribution of abundance among species at these two sections using multinomial models, evenness indices and traditional species abundance distributions. Graptolite communities at both sections showed two drops in evenness in the late Paraorthograptus pacificus Zone separated by a brief return to high levels. At both sections the first drop in evenness preceded falling species richness while the second occurred synchronously with the interval of accelerated extinction in the latest Katian. Following the second drop evenness values remain low through the Katian-Hirnantian boundary and the transition from Diplograptina to normalograptid dominance. Evenness reached its nadir in the early part of the Normalograptus extraordinarius Zone where normalograptids increased from zero to more than 90% of specimens present. The trend in evenness at Vinini Creek mirrors the trend in δ¹⁵N, which previous studies suggest reflected changes in algal productivity driven by contraction of the Oxygen Minimum Zones (OMZ) induced by glacial cooling. The high productivity edges of the OMZ were the main habitat of the diverse offshore graptolite communities. We therefore hypothesize that changes in graptolite community evenness during the late Katian and early Hirnantian tracked the expansion and contraction of the OMZ and thus responded directly to changes in the global oceans during this interval.