The Ecology of the Cambrian Radiation

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 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 freeliving—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 edrioasteroids 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 is not as clear among free-living echinoderm groups, and the expansion direction for several of these could have been from offshore to onshore. Bathymetry is a simplification of what must have been a complex list of controls. Most Ordovician echinoderms had regular and sturdy construction; these advanced

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