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hinoderms with mineralized skeletons entered the fossil record in the early Cambrian (540 mya), and during the next 100 million years, the crinoids and blastoids (also stalked filter-feeders) were dominant. At that time, the Echinodermata included twenty taxa of class rank, only five of which survived the mass extinction events that followed. The long and varied geological history of the crinoids demonstrates how well the echinoderms had adapted to filter-feeding. The crinoids underwent two periods of abrupt adaptive radiation, the first during the Ordovician (485 to 444 mya), and the other during the early Triassic (around 230 mya). This Triassic radiation resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent than sessility. This radiation occurred somewhat earlier than the Mesozoic marine revolution, possibly because it was mainly prompted by increases in benthic predation, specifically of echinoids. There then followed a selective mass extinction at the end of the Permian period, during which all blastoids and most crinoids became extinct. After the end-Permian extinction, crinoids never regained the morphological diversity and dominant position they enjoyed in the Paleozoic; they employed a different suite of ecological strategies open to them from those that had proven so succes