There are five orders of stenolaemate bryozoans, and four of them evolved to their greatest diversity during the Palaeozoic. These include the well-known fan-shaped Fenestrata and the stony, usually stick-like, Trepostomata. These orders were severely affected by the end-Permian mass extinction event and all four were extinct by the end of the Triassic. The remaining order of stenolaemates, the Cyclostomata, were a minor component of Palaeozoic faunas but survived into the Mesozoic and radiated to a great abundance, especially during the Cretaceous. Many genera of cyclostomes became extinct at the end of the Cretaceous, but a few survive to the present day.
Gymnolaemate bryozoans are known from the Ordovician, but reached their acme in the Cenozoic. They are divided into two orders: the minor Ctenostomata, which have been a small element of bryozoan faunas throughout the Phanerozoic, and the hugely diverse order Cheilostomata. This group first appeared in the Jurassic and has come to dominate bryozoan assemblages (Fig. 5.3). Ctenostomes are entirely soft bodied and are usually preserved as borings on calcareous substrates. Cheilostomes are the most highly evolved bryozoans, with the most diverse and integrated colonies.
It is sometimes argued that cheilostomes outcompeted cyclostome bryozoans around the end of the Cretaceous. However, it appears that although both groups have similar ecological tolerances they have continued to co-occur in assemblages. The relative decline of one group cannot be attributed with certainty to the rise of the other.
Fig. 5.3 Range and abundance of bryozoan classes. The dark shaded classes were the dominant Palaeozoic forms and are all stenolaemates. The dotted forms were the dominant Mesozoic and Cenozoic forms, the stenolaemate cyclostomes, and the gymnolaemate cheilostomes.
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