CONTRASTING PATTERNS OF ORIGINATION AND EXTINCTION AMONG GRAPTOLITE SPECIES DURING THE END-ORDOVICIAN MASS EXTINCTION

Graptolites experienced near total extinction during the Hirnantian Mass Extinction (HME) in the latest Ordovician. Near extirpation followed by less than 3 MY a mid Ashgill peak diversity (in species/my) that was the greatest since the early Darriwilian, some 20 MY prior. Using graphic correlation, we have compiled species ranges from the extraordinary Yangtze Platform faunas (Dicellograptus complexus to Parakidograptus acuminatus zones). These data allow us to investigate the manner in which the dramatic history of graptolite diversity change differs significantly from random or pseudo-random patterns. Using Monte Carlo methods, we tested whether peak species extinction or origination rates (based on several metrics: number of events per interval, per-taxon rate, and estimated per-capita rate) differ from those that might arise at random in two alternate models: a simple form with diversity independently determined in each interval and a model with logistic coupling of diversity among intervals. In both models, the observed peak of extinction during the N. extraordinarius-N. ojsuensis Zone differs significantly from random expectations. Furthermore, chi-square contingency tests of clade survivorship suggest that the strong observed taxonomic bias (all extinction survivors are normalograptids despite their rarity prior to the extinction) is also non-random: the dominant Ordovician clades (climacograptids, orthograptids, and dicellograptids) were over represented among species that went extinct. Application of Capture-Mark-Recapture methods (including implementation of a unique bootstrapping method to obtain confidence limits on rates) facilitates testing for homogeneity of extinction and origination rates through the HME. Our results suggest that the Ashgill diversity peak was a product of very low extinction rate and slightly higher, but also low, origination rate. In contrast to the Late Devonian depressed origination event, the HME truly was an interval of accelerated extinction. In addition, both extinction and origination remain comparatively high in the early Llandovery following the HME. Thus, the Hirnantian mass extinction may have fundamentally altered graptolite species dynamics as well as clade dominance patterns.