BRYOZOANS IN TRANSITION: THE DEPAUPERATE AND PATCHY JURASSIC BIOTA

After elimination of probable synonyms, the described Jurassic bryozoan biota comprises fewer than 200 species. Cyclostomes predominate – the only geological period in which they do so – but there are also soft-bodied ctenostomes, preserved as borings and bioimmurations, and the earliest cheilostomes. Over 120 of the Jurassic species are ‘weedy' encrusters, compared with some 40 erect species. Net-like unilaminate (fenestrate) and free-living colony forms are conspicuously absent. Bryozoans are uncommon in the Lower Jurassic, their diversity rises to a Bathonian peak before declining again to low levels in the Upper Jurassic. The reappearance of Lazarus genera in the Cretaceous suggests that at least some of this decline results from failures in the fossil record. Shallow-water carbonates of the northwestern European Middle Jurassic are the source of the great majority of known species. While this apparent centre of diversity might be explained as an artefact for weedy encrusters, which are inconspicuous in the harder limestones predominating elsewhere, this cannot explain the paucity of large arborescent species outside Europe. It seems possible that a significantly greater patchiness in global bryozoan distribution existed in Jurassic times than at the present day. Bioimmured soft-bodied ctenostomes, including stem-group cheilostomes, are relatively diverse and abundant in the Upper Jurassic. The patchy and depauperate character of the Jurassic bryozoan biota may have provided a favourable ecological context for the emergence of cheilostomes which went on to radiate explosively during the Cretaceous and become the dominant order of bryozoans at the present day.