CHAPTER NINETEEN Ecologic Radiation of Cambro-Ordovician Echinoderms Echinoderms represent a modest component of the initial metazoan radiation during the Cambrian but responded to global environmental changes across the CambroOrdovician boundary with rapid and prolific diversification to more varied lifestyles. | 19-C1099 8 10 00 2 19 PM Page 428 CHAPTER NINETEEN Thomas E. Guensburg and James Sprinkle Ecologie Radiation of Cambro-Ordovician Echinoderms Echinoderms represent a modest component of the initial metazoan radiation during the Cambrian but responded to global environmental changes across the Cambro-Ordovician boundary with rapid and prolific diversification to more varied lifestyles in expanded habitats. Many attached echinoderms were preadapted to exploit carbonate hardgrounds and other stable substrates that became abundant on shallow carbonate platforms at that time whereas other attached and many new free-living echinoderms evolved structures to cope with soft substrates. Early to Middle Cambrian echinoderms are primarily known from soft substrate environments where attached suspension-feeding eocrinoids crinoids and edrioas-teroids clung to skeletal debris by suctorial attachment disks or were skeletally cemented by a holdfast helicoplacoids perhaps employed other means. Vagile surface deposit-feeding echinoderms included stylophorans homosteleans homoiosteleans and ctenocystoids. Echinoderms reached a diversification bottleneck in the Late Cambrian but stemmed eocrinoids with cemented holdfasts were among the first skeletonized animals to colonize hardgrounds that became common at that time. Stylophorans homoiosteleans and edrioasteroids were also represented. Attached crinoids and free-living rhombiferans led the Early Ordovician radiation among suspension-feeding echinoderms and were accompanied by several other newly evolved groups with generally similar lifestyles. Vagile herbivorous echinoids and carnivorous asteroids greatly expanded echinoderm ways of life by the Middle Ordovician. This overall diversification pattern for echinoderms supports a model of two sequential evolutionary faunas in which shallow-water habitats fostered onshore origination and radiation followed by offshore expansion for many attached forms. However the diversification pattern
