![](https://sciencepress.mnhn.fr/sites/default/files/periodiques/visuels/grand/bandeau-superieur-palevol-modele-2023_0.jpg)
Echinoids (sea urchins) originated in the Early Paleozoic and, after a first period of diversification during that era, participated in an intense evolutionary radiation during the Mesozoic and Cenozoic. In the context of a reconsideration of homologies for echinoderm body regions, we present several major aspects of echinoid evolution: the origins of irregularity and bilateral symmetry; a model of radiation exemplified by the spatangoids; the diversification of modes of echinoid reproduction. These examples allow us to understand the extent of and capacity for innovation demonstrated by sea urchins during their post-Paleozoic radiation.
Echinoids, Evolution, Radiation