Phylum Bryozoa, also called phylum Ectoprocta, sorts itself into three classes: Stenolaemata, Gymnolaemata, and Phy-lactolaemata. Stenolaemata are exclusively marine bryozoans, while Gymnolaemate bryozoans are mostly marine, plus a few freshwater types, and Phylactolaemates bryozoans are entirely reshwater.
The fossil record of class Stenolaemata extends back as far as 400 million years, into the early Ordovician Era. The Steno-laemata have passed their peak of diversification and are now in decline. Stenolaemata comprises one extant order, Cy-clostomata (also called Tubuliporida), which first appeared in the Ordovician and scraped by with small colonies and little diversification through the Paleozoic, then exploded into extraordinary diversity during the Cretaceous. Its diversity declined toward the end of the Cretaceous, the number of genera was whittled down from about 175 to about 50, and the order has maintained that level of diversity into the present. There are about 500 extant species.
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