THE LATE ORDOVICIAN MASS EXTINCTION OF GRAPTOLITES: ONE HUMP OR TWO?

Comparison of species occurrences in 26 paleotropical sites around the globe that span shelf to deep oceanic settings permits reassessment of zooplankton turnover during the Late Ordovician Mass Extinction (LOME). We compiled taxonomically standardized occurrences and the associated paleoenvironmental proxies into a high-resolution composite via the automated sequencing tool Horizon Annealing. This composite overcomes the limitations of habitat change and facies displacement that created the incomplete local diversity trajectories that seemed to show two pulses of extinction. Instead, we find an ongoing selective turnover that commenced in latest Katian time as rates of extinction reached ~0.8 species/my near the end of the P. pacificus Zone interval (~ 445.25 ma). This onset of rising extinction rates is associated with the rising limb of the δ¹³C (HICE) and δ¹⁵N isotopic excursions, as well as related change in the phytoplankton communities. Turnover accelerated in the early Hirnantian along with sea level and oceanographic changes and the accompanying invasion of the paleotropics by the Neograptina (e.g., M. extraordinarius and related species), which formerly were confined to temperate latitudes. Contemporaneously, diplograptines disappeared or became rare at most of the sites they formerly occupied, indicating biogeographic range contraction and population size decline. Extinction rates rose ~eight-fold through the Hirnantian to a peak of ~6.5 s/my in the latter half of the M. persculptus Zone (~444.2 ma) in association with the falling limb of the HICE and coincident with widespread transgression and expansion of water column anoxia. Few diplograptine species are known from this interval and all are rare. Meanwhile, extinctions among neograptines were few and their diversity increased sharply over the course of the Hirnantian Age. Biased origination reinforced extinction as neograptines entirely replaced the Diplograptina by the end of the LOME. Late Ordovician climate change drove graptolite turnover via an interplay of habitat loss and ecosystem disruption, which lead to range contraction and subsequent extinction.