Uncalcified gymnolaemates are known as fossils from the late Ordovician on, almost exclusively as distinctive borings in carbonate substrates such as shells. Nonboring, noncalci-fied gymnolaemate bryozoans are extremely rare as fossils and are known from the Jurassic and Cretaceous only. Calcareous gymnolaemates did not appear in the oceans until the Cretaceous, during which they diversified rapidly. There were very few species of gymnolaemates in the early Cretaceous, but by the end of the period, there were more than 100 genera. Gymnolaemates continued to diversify in the Cenozoic era. There are two gymnolaemate orders—Ctenostomata and Cheilosto-matida—and more than 1,000 genera. The Ctenostomata are stoloniferous or compact colonies in which the uncalcified ex-oskeleton is membranous, chitinous, or gelatinous and the usually terminal orifices lack an operculum. Order Cheilo-stomatida contains colonies composed of boxlike zooids that are adjacent but have separate calcareous walls. The orifice of cheilostomes is covered with an operculum.
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