Early Paleozoic diversification of echinoderms: The example of blastozoans

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.Original Abstract: Le sous-phylum Blastozoa est un des groupes d'echinodermes les plus diversifies (dix classes) au debut du Paleozoique. Apres revision critique de leur squelette, leurs morphologies en apparence tres variees sont en fait homogenes. Leur diversite montre deux pics (Drumien, Sandbien) lies par un evenement de fortes apparitions generiques au Cambrien superieur-Ordovicien inferieur. Les blastozoaires montrent un fort endemisme au Cambrien et un important provincialisme a l'Ordovicien inferieur et moyen. Ils deviennent cosmopolites a l'Ordovicien superieur, par plusieurs evenements migratoires. Ils sont restreints a la Laurentia et a Baltica au Silurien inferieur.