EVIDENCE FOR A GRAPTOLITE EXTINCTION EVENT LINKED TO SEA-LEVEL FALL AT THE ORDOVICIAN-SILURIAN BOUNDARY

The glacially-linked Late Ordovician Mass Extinction (LOME) was a two phase extinction event that devastated marine communities. The extinction interval extended from the latest Katian through Hirnantian stages. Peak extinction in the graptolites occurred near the Katian-Hirnantian boundary, followed by post-extinction diversification in the latest Ordovician, which continued through the Silurian. Diversity levels tracked changes in sea level and water mass properties, with extinction associated with sea level fall and increased oxygenation, and rediverficiation associated with post-glacial flooding and a return to black shale deposition in the late Hirnantian. New data from the Blackstone River, Yukon, shows that graptolite diversity rebounded there to near pre-extinction maximum levels in the latest Ordovician persculptus Zone, before dramatically dropping at the Ordovician-Silurian boundary (from 16 to 6 species). Diversity levels rapidly recovered in the basal Silurian (ascensus Zone), but with almost no carry-over in taxa. The Ordovician-Silurian boundary diversity drop was principally the result of increased per-taxon extinction rates, which correlate closely with geochemical (neodymium) and lithological evidence of sea level fall. This pattern of increased extinction in the very latest Ordovician, followed by a spike in origination rates in the earliest Silurian, was previously documented in China. Preliminary data from Dob's Linn, Scotland, suggests that same pattern also occurs there. It appears that graptolites experienced a global extinction event at the Ordovician-Silurian boundary after the principal events of the LOME had concluded, linked to continued climate instability.