ABSTRACT
Echinoderms provide some of the best palaeontological material for the study of evolution on a large scale. Their skeletons closely reflect soft anatomy, they are easily recognised because of their complexity, they are well dated and they are now very well studied by modern techniques. Using taxonomic and morphological approaches it can be shown that the major Palaeozoic evolutionary changes in this phylum took place in the Cambrian and Ordovician. During this interval new Baupläne appeared several times. The group shows no correlation between rates of cladogenesis and rates of morphological change. The availability of physical space did not trigger morphological change. The crossing of adaptive thresholds did not produce diversification, at least until long periods of time had elapsed, and many higher taxa probably became extinct by chance before diversification could take place. A model that involves rapid phenotypic expression of accumulated genetic change seems to fit the data best. Such change seems to have been possible only within a limited period after a major new genome was established.
