USING SDMS TO ASSESS PALEOBIOGEOGRAPHIC RESPONSE OF GRAPTOLITE TO THE LATE ORDOVICIAN MASS EXTINCTION IN SOUTH CHINA

The Late Ordovician mass extinction, the second largest mass extinction in geologic history, results in not only the great reduction in large number of taxa, but also the evolutionary differentiation, ecological reconstruction and biogeographic reorganization after the mass extinction. Graptolites, as the primary marine planktons, suffered from the mass extinction and showed a significant change in paleogeographic distribution patterns during the extinction event. A quantitative study on the graptolite paleobiogeography will be helpful for the understanding of the macroevolution dynamics of the graptolite fauna.

Species distribution models (SDMs), a numerical tool used in modern ecological research, were employed to study the paleobiogeographic changes of graptolite species through the Late Ordovician mass extinction. Based on the habitat requirements of species, we can use SDMs to establish the link between the observed distributions of species and the environmental data in their known distribution area, and then predict the potential geographic distribution in the study area. The studying time interval, late Katian to Hirnantian, was divided into four time slices through the extinction event. Twenty-seven long-ranging species of graptolites were chosen as the study objects, among which, twenty-one belong to the Diplograptina and six to the Neograptina. Fifty-nine geographic distribution maps of graptolites species were generated using Maxent, a powerful program for SDMs analysis, and the ArcGIS software. By investigating the dynamics of the predicted distributions of each graptolite species during the successive time slices, we found that (1) Different clades of graptolites showed different evolutionary strategy while facing the same extinction event: the Ordovician Diplograptina species decreased their geographical ranges, while those Neograptina species increased their geographical ranges and became the major clade in Silurian. (2) Environmental factor analysis indicates that the hydrodynamic condition was the most important among the factors affecting the geographic distribution of graptolites.