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Free-lying colonies of bryozoans that grew as thin sheets over fine-grained marine sediments were common during the Paleozoic but apparently have been uncommon for the past 245 million years. Decline in this free-lying growth habit corresponded in general with rise in intensity of marine bioturbation, which has been implied to be a cause of the decline. The occurrence of two short-lived populations of free-lying, sheet-like colonies of the anascanchcilostome bryozoan Caplersia nobilis is consistent with the hypothesized relationship. C. nobilis normally grows as uni- to multilaminate sheets on shells and rock, including common circumrotatry growth. The species recently appeared at two occasions as dense hut ephemeral populations of free-lying unilaminate colonies up to 20 cm diameter on muddy sand substrata, 45–50 m deep, in the Northern Adriatic Sea offshore from the southern Istrian Peninsula, Croatia. These population blooms of free-lying C. nobilis occurred adjacent to or within areas where the benthic communites, including bioturbators, recently have been reduced and destabilized by severe anoxic events. □Bryozoa, Calpensia nobilis, bioturbation, marine anoxia, population blooms, Adriatic Sea.