CHANGES IN LATE ORDOVICIAN (PRE-HIRNANTIAN) ECHINODERM ASSEMBLAGES:PALAEOENVIRONMENTAL AND PALAEOBIOGEOGRAPHICAL IMPLICATIONS

摘要

1 INTRODUCTIONThe phylum Echinoderrnata underwent two successive,major diversifications in Early Palaeozoic times.The first one was probably triggered by the dramatic palaeoenvironmental shift from Proterozoic-type to Phanerozoic-type sea-floors (“Cambrian substrate revolution'’),which took place in the Early to Middle Cambrian time interval(Lefebvre and Fatka,2003；Dornbos,2006；Nardin et al.,2006).