Quantifying change in community structure and extinction selectivity in late Katian graptolite communities
Hawkins, Andrew Donald
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The end Ordovician glaciation greatly impacted Ordovician biodiversity, producing one of the largest mass extinctions of the Phanerozoic. Planktonic graptolites in particular suffered a severe decline. Changing climatic and oceanographic conditions drastically altered the environment to which late Katian graptolite species were adapted, including eliminating the primary graptolite habitat at the edges of well developed oxygen minimum zones. Previous analyses of this interval have focused on quantifying extinction rates. Species abundance data from the Vinini Creek section in Nevada as well as preliminary species abundance data from the Blackstone River section in the Yukon provide an opportunity to test hypotheses about changing community structure in late Katian and Hirnantian graptolite communities. High resolution data on species biostratigraphic ranges also provides the opportunity to test hypotheses about the ecological processes responsible for species extinction. A set of hypotheses related to late Katian graptolite community change and species extinction are tested: (1) That a shift in community structure observed in the late Katian at Vinini Creek occurs at the Blackstone river section as well; (2) That the composition of graptolite communities shifted in the late Katian independent of species loss as the relative abundances of remaining species converged on a consistent composition; (3) That late Katian graptolite extinction was non-random with respect to species depth preferences; (4) That late Katian graptolite extinction was non-random with respect to species morphological complexity. Analysis of late Katian graptolite community structure revealed a consistent pattern of evenness change across the two studied sections. The trend in graptolite community evenness closely mirrors the trend in δ 15 N, a proxy for the contraction and expansion of the oxygen minimum zone. I therefore hypothesize that the trend in evenness in graptolite communities tracked the contraction and expansion of their primary habitat. Graptolites that inhabited the deep water biotope are shown to have been more likely to go extinct in the late Katian. This is likely also due to the impact of the initial phase of climate change on the primary deep water graptolite habitat.