BIOTIC IMMIGRATION EVENTS, SPECIATION, AND BIODIVERSITY IN ORDOVICIAN SEAS OF LAURENTIA

Biotic immigration events (BIMEs), the dispersal of taxa from one biogeographic area to another, have impacted biodiversity throughout geologic time. BIMEs associated with biodiversity increases have been linked to ecologic and evolutionary processes such as niche partitioning, species packing, and higher speciation rates. Yet substantial biodiversity decline has also been documented following BIMEs due to elevated extinction and/or reduced speciation rates.

In this contribution, we explore the relationships among BIMEs, biogeographic mode of speciation events, and biodiversity during the Middle through Late Ordovician interval (Dapingian to Katian Stages) in the epicontinental seas of Laurentia. The focal interval begins with the Great Ordovician Biodiversification Event (GOBE) and ends after the Richmondian Invasion. BIMEs during this temporal interval include both long-distance dispersal between Laurentia and surrounding paleocontinents and within-craton dispersal occurring between geographically adjacent depocenters. The distance, magnitude, and timing of BIMEs were constrained for a suite of rhynchonelliform brachiopod and trilobite clades.

Two macroevolutionary regimes alternated during the study interval: (1) vicariance speciation regime characterized by limited BIMEs with active tectonism and increased physical separation between marine basins (2) dispersal regime characterized by frequent BIMEs and high basinal connectivity.

The alternation of the vicariance and dispersal-dominated macroevolutionary regimes established a “taxon-pulse” for generating diversity. In the first phase, the high frequency BIMEs increase α-diversity but γ-diversity remains stable as speciation rate declines. β-diversity decreases due to the sharing of taxa between basins. In the vicariance phase, speciation rate increase dramatically elevates γ-diversity and produces modest increases in α- and β-diversity. The repetition of the two-phase diversification system occurred at least twice during the Middle to Late Ordovician study interval and forms a useful hypothesis against which to examine diversification in other Ordovician clades. Thus, generation of substantial diversity requires mechanisms that alternately promote and restrict immigration between basins.