The specialization of holopodid crinoids to a sessile habit, by directly cementing the cup to a hard substrate and having short arms that interlock on closure to give a watertight seal, led F.A. Bather to speculate in 1928 that they might once have led an intertidal, barnacle‐like existence in shallow water. Although this remains unproven, holopodid‐barnacle associations from the Danian now provide indirect supporting evidence of their similar environmental preferences at that time. It has long been recognized that the holopodid Cyathidium holopus Steenstrup was encrusted by the verrucomorph Verruca prisca ? Bosquet in Danian submarine caves. This crinoid is now known to occur with abundant plates of the brachylepadomorph barnacle Pycnolepas bruennichi Withers in open sea bed settings, suggesting that they lived together on a substrate now lost due to diagenesis. These were most likely to have been azooxanthellate scleractinian corals (benthic; 100+ m estimated water depth) and/or logs (either on sunken benthic islands (100+ m) or floating as pseudoplankton). If the tight, barnacle‐like closure of the holopodid arms was primarily an anti‐predatory adaptation, suggested as an alternative hypothesis, then it was of limited effectiveness and insufficient to prevent the group having to migrate from shallow into deeper water with the stalked crinoids since the Late Cretaceous.
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