SPECIES TURNOVER DYNAMICS DURING THE HIRNANTIAN (LATE ORDOVICIAN) MASS EXTINCTION: NEW GRAPTOLITE AND CARBON ISOTOPIC DATA FROM VININI CREEK, NEVADA (USA)

The fossil record of graptolites provides an ecologically unique and unusually high-resolution picture of species turnover during the great Hirnantian Mass Extinction (HME).  Previous work in the Yangtze Platform region, China, suggested that the species turnover there was dramatically more gradual than seen in any other part of the world and may have been the result of locally preferential survival and origination.  New graptolite collections from the Vinini Creek section (Roberts Mountains, Nevada) enable us to compare species turnover in these two regions.  Faunas obtained from atypically large samples include a substantial number of rare taxa not previously reported from this region.  These taxa are most numerous in the pre-extinction P. pacificus Biozone and post-extinction N. persculptus Biozone intervals.  In the latter case, several of the newly recorded species are Lazarus taxa of the Orthograptidae.   Also newly recorded in the Hirnantian here are several Normalograptus lineages that arose during the extinction interval and proliferated in the Early Silurian.  These revisions make the pattern of species turnover much more like that seen in the intensively studied Yangtze data set and suggest that at least some of the apparent geographic variation in turnover may be an artifact of differences in specimen abundance and sampling.  We also obtained organic carbon isotopic data from this region.  These data match patterns seen in China and the Canadian Arctic: strong positive excursions in the lower N. extraordinarius and lower N. persculptus biozones separated by a more negative set of values in the upper N. extraordinarius Biozone.   The more shallow water Monitor Range section exhibits a similar pattern but a 4‰ excursion compared to 2‰ changes in δ¹³C-org values at Vinini Creek.  This supports suggestions that the source of variations in the oceanic carbon isotopic reservoir may have been driven by carbonate erosion and terrestrial runoff during the Hirnantian glacial low-stand.  As in China, the Lazarus Orthograptidae reappear during the return of C-org values to pre-Hirnantian levels as post-glacial sea levels rose.  Based on these new data it appears that graptolite species turnover during the HME was high but took place gradually throughout the low latitude realm without an evolutionary lag and at pace with glacially-drive oceanic changes.