DELTA DIVERSITY, MIGRATION AND GLOBAL WARMING DURING THE LATE ORDOVICIAN

Migration is an important evolutionary process that is often neglected in paleontological studies conducted at the regional or continental scale. Migration has a profound impact on delta diversity, which in recent years has become a feature of fossil assemblages that is of increasing interest to paleontologists. Delta diversity is similar to beta diversity, but rather than measuring the diversity gained when environmentally homogenous, local samples are pooled, delta diversity measures the diversity gained when samples from environmentally heterogeneous landscapes or continents are pooled. A null model is presented that evaluates the role of migration, in concert with origination and extinction, in shaping patterns of delta diversity. Delta diversity is computed as one minus the Jaccard coefficient of similarity. The model results show that, over time, delta diversity reaches a dynamic equilibrium and that migration is a process that reduces delta diversity while origination and extinction are processes that increase delta diversity.

Delta diversity patterns from an Ordovician data set generated from the Paleobiology Database and comprised of the Trilobita, Brachiopoda, Monoplacophora, Gastropoda and Bivalvia is compared to the model results. The Ordovician data, which are for the paleocontinents of Laurentia and South China, show statistically significant variation in delta diversity, especially during the Late Ordovician. Delta diversity drops from the Caradoc to the Ashgill, and this drop corresponds to a two-fold increase in the percent faunal similarity between the two paleocontinents. This drop in delta diversity is interpreted as the result of increased migration between Laurentia and South China. The Boda event (Fortey and Cocks 2005), a mid-Ashgill episode of global warming, has recently been implicated in enhancing faunal exchange between high and low latitude paleocontinents, but it was concluded that migration did not increase among the low latitude paleocontinents of Laurentia and South China. The delta diversity results presented here suggest that there may have indeed been increased migration between these two paleocontinents during the Ashgill.