PATTERNS AND CHANGES OF GRAPTOLITE BIOTOPE AFFINITY DURING THE LATE ORDOVICIAN MASS EXTINCTION

The Late Ordovician Mass Extinction (LOME) was the result of a combination of environmental changes that resulted in restricted habitable areas and altered habitat properties. We investigated the habitat changes experienced by graptolites through Bayesian analysis of a dataset of species occurrences at 93 paleotropical to high latitude sites that span the late Katian to early Rhuddanian. We employ these data to estimate the biotope affinities of 267 graptolite species and the water depths at the study sites. During the interval of the Paraorthograptus pacificus Biozone, the temperate regions contained mostly neograptine species, which were all epipelagic, whereas tropical regions were dominated by diplograptine species, which inhabited both epipelagic and mesopelagic biotopes. In the succeeding Metabolograptus extraordinarius Biozone interval, however, tropical graptolites exhibited a profound shift in their biotope affinities. Eighteen percent of the surviving diplograptine species shifted from epipelagic to mesopelagic biotopes, i.e., to a more restricted depth designation, whereas all of the newly dominant neograptines exhibit epipelagic biotope affinities. This trend continued into the Metabolograptus persculptus Biozone interval, wherein all the remaining diplograptine species with clear biotope assignments became mesopelagic. Neograptines diversified during the latter part of this interval with nearly a third of the species attributed to mesopelagic biotopes. Finally, within the early Silurian Akidograptus ascensus/Parakidograptus acuminatus Biozone interval, neograptines continued to diversify and came to inhabit the mesopelagic and epipelagic biotopes nearly equally. We suggest that the changing oceanic conditions during the LOME altered the distribution of the resources upon which these graptolites relied. As those resources became more restricted and localized to deeper water sites, epipelagic diplograptine graptolites that previously occupied on-shore sites moved to more offshore locations, and possibly to greater depths in the water column, thus behaving as mesopelagic taxa prior to going extinct. During the post-glacial interval, Neograptines reoccupied ecospace vacated by the extirpated Diplograptina.