LIVING POPULATIONS OF FREE-LYING BRYOZOANS - IMPLICATIONS FOR POSTPALEOZOIC DECLINE OF THE GROWTH HABIT

Free-lying colonies of bryozoans that grew as thin sheets over fine-gr ained marine sediments were common during the Paleozoic but apparently have been uncommon for the past 245 million years. Decline in this fr ee-lying growth habit corresponded in general with rise in intensity o f marine bioturbation, which has been implied to be a cause of the dec line. The occurrence of two short-lived populations of free-lying, she et-like colonies of the anascan cheilostome bryozoan Calpensia nobilis is consistent with the hypothesized relationship. C nobilis normally grows as uni- to multilaminate sheets on shells and rock, including co mmon circumrotatory growth. The species recently appeared at two occas ions as dense but ephemeral populations of free-lying unilaminate colo nies up to 20 cm diameter on muddy sand substrata, 45-50 m deep, in th e Northern Adriatic Sea offshore from the southern Istrian Peninsula, Croatia. These population blooms of free-lying C. nobilis occurred adj acent to or within areas where the benthic communities, including biot urbators, recently have been reduced and destabilized by severe anoxic events.