Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution

Bioturbation in neritic siliciclastic settings during the Proterozoic-Phane rozoic transition increased in depth and intensity, causing a change in sub strates from the matgrounds characteristic of the Proterozoic to the mixgro unds characteristic of the Phanerozoic. This change in bioturbation increas ed the water content of surficial layers of sediment and blurred the sedime nt-water interface, leading to the first appearance of a mixed layer. Devel opment of a mixed layer throughout neritic environments would have had a st rong impact on any benthic metazoans, particularly sessile suspension feede rs, that were well adapted for survival on relatively unbioturbated Protero zoic substrates. The impact of this substrate transition on benthic metazoa ns has been termed the "Cambrian substrate revolution." The unusual Early C ambrian helicoplacoid echinoderms were well adapted for survival on typical Proterozoic-style substrates. The examination of new helicoplacoid specime ns collected during this study, combined with extensive study of the rocks in which they are preserved, indicate that helicoplacoids lived as sediment stickers on a muddy substrate that underwent only low to moderate levels o f strictly horizontal bioturbation and did not have a mixed layer. The sign ificant increase of bioturbation through the Cambrian in neritic siliciclas tic settings is likely to have led to the extinction of the helicoplacoids. Other similarly adapted sessile suspension-feeding echinoderms may have al so been driven to extinction by the effects of the Cambrian substrate revol ution. The co-existence during the Cambrian of organisms adapted to the var iety of substrates characteristic of this transitional period may also have contributed to the high degree of perceived morphological disparity during the Cambrian "explosion".