La radiation des échinodermes au Paléozoïque inférieur, l’exemple des blastozoaires

The subphylum Blastozoa is the most abundant and among the most diversified of echinoderm groups during the Early Paleozoic. Reappraisal of their highly diverse anatomies suggests that their superficially incomparable morphologies are actually relatively homogeneous among the major blastozoan clades. Their generic diversity shows two peaks (Drumian, Sandbian), linked by a single origination event during the Cambrian-Lower Ordovician interval. During the lower Middle Ordovician, blastozoans were distributed in distinct provinces, but became progressively more cosmopolitan during the Upper Ordovician. After the Late Ordovician crisis event, blastozoans were restricted to the Laurentian and the Baltic margins.