ANALYSIS OF GRAPTOLITE BIOTOPE AFFINITY THROUGH THE LATE ORDOVICIAN MASS EXTINCTION

The Late Ordovician Mass Extinction was the result of a combination of factors, including changes in sea surface temperature, sea level, and ventilation of the oceans, among others. These factors resulted in restricted habitable areas and changes within the habitats themselves. We can investigate these changes using a Bayesian analysis of graptolite species occurrence data from 42 study sites (mainly paleotropical localities but also mid to high latitude locales, including the North African margin of Gondwana) in an effort to estimate the biotope affinities of 79 graptolite species and the water depths at the study sites. During the Paraorthograptus pacificus Biozone, preceding the HICE, the graptolites examined behaved as expected, with the temperate regions containing mostly epipelagic neograptine species, and the tropical diplograptine species inhabiting both shallow and deep localities. The biotope affinities of the surviving graptolites in the succeeding Metabolograptus extraordinarius Biozone, however, exhibit a profound shift in the biotope affinities of the surviving graptolites. Eighteen percent were reassigned from epipelagic to mesopelagic biotopes, i.e., to a more restricted depth designation. We suggest that the changing oceanic conditions altered the distribution of the resources that these particular graptolites relied upon. As those resources became more restricted and localized to deeper water sites, epipelagic graptolites that previously occurred in on-shore sites moved to more offshore locations, and possibly to greater depths in the water column, thus behaving as mesopelagic taxa.