PALEOBIOGEOGRAPHY OF THE EARLY PALEOZOIC ECHINODERMS ACROSS THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT

Robust statistical inference of early Paleozoic macroevolutionary patterns of invertebrate taxa have been primarily focused on brachiopods and trilobites. These taxa, along with other clades, have been used to infer the drivers and impacts of Ordovician diversification events, namely the Great Ordovician Biodiversification Event (GOBE). To date, echinoderm paleobiogeographic patterns have been excluded from these statistical analyses. In this study, we use a phylogenetic hypothesis of early Paleozoic diplopore-bearing blastozoan echinoderms to reconstruct ancestral biogeographic histories and dispersal pathways before, during, and after the GOBE to infer their drivers of evolution and dispersal. We used the R package BioGeoBEARS to estimate ancestral ranges within the blastozoan phylogeny. The number and type of dispersal events for three Ordovician time slices that encompass the time before, during, and after the GOBE was estimated using Biogeographic Stochastic Mapping (BSM) within BioGeoBEARS. The best-fit model incorporated jump dispersal to explain the biogeographic history of the blastozoans. There are several source regions for this clade, with deep nodes within the tree, indicating a Gondwana or Baltic origin. From the BSM analysis, the most dispersal occurs in the Early to Middle Ordovician, with several events occurring between Gondwana and Laurentia, and Laurentia and Baltica. There are reduced dispersal events within the Middle Ordovician GOBE interval, but it is clear that Baltica and Laurentia exchanged taxa during this time. During the Late Ordovician, there is again an increase in dispersal events among Baltica, Laurentia, and Gondwana. These reconstructed dispersal events indicate that oceanic gyre systems, currents, and upwelling regions were likely important factors to facilitate species movement on a global scale. Speciation events plotted against δ18O and other geochemical data indicate the blastozoan speciation events are not clustered during times of global cooling, unlike speciation events for brachiopods and trilobites. Additional phylogenetic hypotheses of this group will further reveal drivers of echinoderm speciation through the early Paleozoic.