Ecological Reorganization of Graptolite Communities during the Hirnantian Mass Extinction

Graptolite faunas in paleotropical shelf to slope habitats during the Late Ordovician (Katian and Hirnantian ages) consisted of a diverse suite of diplograptid, dicranograptid, and climacograptid (DDC) species. These clades were wiped out during the Hirnantian mass extinction and subsequently replaced by the Normalograptidae and its descendents. New data from Laurentia (Nevada, Yukon, and Scotland), coupled with a reassessment of data from China and other paleotropical sites, reveal that normalograptids were rare to absent at Katian paleotropical sites but were common in higher latitude locales such as Baltica and Perunica. DDC species diversity began to decline in the latest Katian. Normalograptids invaded the paleotropics at the onset of the Hirnantian carbon isotopic anomaly and rapidly expanded in abundance and species diversity during the extinction interval. By the early part of the N. extraordinarius Zone interval, normalograptids constituted >95% of specimens in preserved graptolite assemblages. The DDC species appear to have become extinct in latest Katian and earliest Hirnantian, however, our large collections reveal that many DDC species survived into the post-glacial, latest Hirnantian, where they are represented by extremely rare, primarily juvenile rhabdosomes.

The apparently abrupt graptolite species extinction is an artifact of a truly abrupt change in species abundance patterns (an inverse Signor-Lipps effect). The Hirnantian graptolite species turnover occupied an extended interval (> 2MY), however, radical ecological reorganization, expressed as major changes in taxonomic and morphological diversity, occurred in the early Hirnantian with the onset of Gondwanan glaciation. Taken together with sedimentological and geochemical data, these observations suggest that the primary drivers of graptolite mass extinction were most likely oceanographic changes leading to loss of habitat and disruption of the bacterial and phytoplankton communities that provided their food source. Recovery took place as these conditions returned to normal following deglaciation and was rooted entirely among the immigrant normalograptids.