Abstract
AbstractEchinoderms make up a substantial component of Ordovician marine invertebrates, yet their speciation and dispersal history as inferred within a rigorous phylogenetic and statistical framework is lacking. We use biogeographic stochastic mapping (BSM; implemented in the R package BioGeoBEARS) to infer ancestral area relationships and the number and type of dispersal events through the Ordovician for diploporan blastozoans and related species. The BSM analysis was divided into three time slices to analyze how dispersal paths changed before and during the great Ordovician biodiversification event (GOBE) and within the Late Ordovician mass extinction intervals. The best-fit biogeographic model incorporated jump dispersal, indicating this was an important speciation strategy. Reconstructed areas within the phylogeny indicate the first diploporan blastozoans likely originated within Baltica or Gondwana. Dispersal, jump dispersal, and sympatry dominated the BSM inference through the Ordovician, while dispersal paths varied in time. Long-distance dispersal events in the Early Ordovician indicate distance was not a significant predictor of dispersal, whereas increased dispersal events between Baltica and Laurentia are apparent during the GOBE, indicating these areas were important to blastozoan speciation. During the Late Ordovician, there is an increase in dispersal events among all paleocontinents. The drivers of dispersal are attributed to oceanic and epicontinental currents. Speciation events plotted against geochemical data indicate that blastozoans may not have responded to climate cooling events and other geochemical perturbations, but additional data will continue to shed light on the drivers of early Paleozoic blastozoan speciation and dispersal patterns.
Highlights
The basins in this study follow those defined in Lam et al (2018) so a direct comparison among dispersal paths could be made for the blastozoan echinoderms in this study and brachiopods and trilobites studied in Lam et al (2018)
The best-fit model for the blastozoan tree presented here was DIVALIKE + j, with the DEC + j model having a slightly weaker fit. The results from these analyses are very similar, with only slight differences
The most probable ranges estimated from the BioGeoBEARS analysis indicate the earliest likely ancestors of the species represented, based on their respiratory structures (G. spiralis; Fig. 2), originated in Gondwana and western North American, with the earliest diploporan blastozoans occurring within the Ordovician
Summary
Echinoderms show complex patterns of morphological disparity and taxonomic diversity throughout their evolutionary history (Sumrall 1997; Deline 2015; Deline and Thomka 2017; Deline et al 2020) and have been suggested to respond to long-term environmental and climatic patterns (Paul 1968; Clausen 2004; Dickson 2004; Clausen and Smith 2005, 2008; Zamora and Smith 2008; Rahman and Zamora2009). The Ordovician Earth System.—The Ordovician Period (485.4–443.8 Ma; Ogg et al 2016) was a time of increased tectonic activity, widely dispersed continents, and major climatic shifts. The Early Ordovician was characterized by relatively high sea level, with the Iapetus Ocean at its widest. Continents were at their most widely distributed due to increased rates of seafloor spreading and high continental drift rates for Baltica, Siberia, Gondwana, and Avalonia (Scotese and McKerrow 1991; van de Pluijm et al 1995; Harper et al 1996; Cocks and Torsvik 2011). Throughout the Ordovician, the Iapetus Ocean between Laurentia and Baltica constricted, leading to the eventual collision of Laurentia, Baltica, and Avalonia in the latest Ordovician to early Silurian (Hutton and Murphy 1987; Soper et al 1992; Cocks and Torsvik 2011; Zagorevski and Van Staal 2011)
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