Élise Nardin PhD thesis

The blastozoans radiation during the Early Palaeozoic: phylogenetic, palaeoecologic and palaeobiogeographic aspects



The thesis deals with the characterisation of the blastozoan radiation during the Early Palaeozoic. The subphylum Blastozoa represents an important component of the Palaeozoic invertebrate marine fauna. The appearance of all classes before the Upper Ordovician, suggests a rapid diversification. Blastozoans are characterised by the possession of brachioles, and a skeleton subdivided in three parts. These parts show specific topological sequences. The understanding of the blastozoan skeleton and the application of EAT on the morphologies are the base of the definition of robust homologies and lead to propose a new phylogenetic hypothesis. Main results show the monophyly of blastozoans, and their subdivision in four grades, and several well-defined orders. The diversity and evolution analyses suggest that 1) two crises (Middle Cambrian and Upper Ordovician) highly influence the diversity pattern, 2) the Ordovician radiation starts during the Upper Cambrian, 3) the Cambrian diversity is controlled by the generic turnover process, whereas the Ordovician one is controlled by the additive persistence of the genera, and 4) the morphologic innovations spread along an onshore-offshore gradient. During the Early-Middle Cambrian, blastozoans developed a similar mode of life, while they highly diversified it during the Ordovician. Blastozoan genera have worldwide distribution since the Middle Cambrian. They are cosmopolitan during the Middle Cambrian and the Upper Ordovician. Their geographical distribution suggests a high provincialism between the Lower and the Middle Ordovician.



Echinodermata, Blastozoa, Early Palaeozoic, EAT, macroevolution, palaeocology, palaeogeography, phylogeny