Cheilostomes first appeared in the Late
Jurassic (
Pyriporopsis) but diversified very slowly during the Early
Cretaceous, with only 1 family known up to the
Albian. During the Late Cretaceous, cheilostomes diversified rapidly to reach a level of more than 20 families in the
Maastrichtian, replacing
cyclostomes as the dominant group of bryozoans. At the same time new forms evolved which partly or fully used
aragonite instead of
calcite in their exoskeleton. This diversification is thought to be a consequence of the evolution of a new larval type. Though the
Cretaceous–Paleogene extinction event had some impact on genetic diversity, the rapid diversification continued into the Eocene, then apparently reaching a plateau of about 50 families up to the
Holocene. Most species incubate their offspring in brood chambers which has evolved independently at least 10 times in the order. ==References==