A GLOBAL BIOGEOGRAPHICAL ANALYSIS OF BLASTOZOAN ECHINODERMS ACROSS THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT

To date, robust statistical inference of early Paleozoic macroevolutionary patterns of invertebrate taxa have been primarily focused on brachiopods and trilobites, as species-level phylogenies for these taxa are most readily available. However, echinoderms were an important constituent of Paleozoic communities, and their dispersal histories can contribute immense insight into the drivers of Paleozoic evolution. Echinoderm taxa show high disparity and diversity throughout the Ordovician; this disparity and diversity has been linked to changes in paleoecological and paleoclimatic shifts. In this study, we used a previously published phylogenetic hypothesis of early Paleozoic blastozoan echinoderms, those with highly disparate respiratory structures, to reconstruct ancestral biogeographic histories and dispersal pathways.

We time-calibrated the blastozoan phylogenetic hypothesis and used the R package BioGeoBEARS to infer ancestral ranges within the clade. Species occurrences were culled from the primary literature and online diversity databases. Eight geographic areas were defined following those of previous Ordovician paleobiogeographic analyses and include areas in Laurentia, Gondwana, and Baltica.

This study represents one of the first attempts to quantify biogeographic patterns of blastozoan echinoderms within a rigorous phylogenetically-informed statistical framework. As blastozoan echinoderms have highly disparate morphologies, it is hypothesized that changing biogeographic patterns and resulting changes in morphology were likely caused by changing climatic regimes throughout the Paleozoic. This study provides a foundation to begin understanding how major paleoceanographic and tectonic events during the early Paleozoic (e.g., the end-Hirnantian Ordovician glaciation, Taconian Orogeny) affected rates and modes of speciation in a global context. Further, this study provides a framework with which to understand how echinoderm biogeographical patterns compare to other groups of invertebrate taxa, such as brachiopods and trilobites during the same time period.