Bryozoa: Fossil Record

The Bryozoa are the only animal phylum with an extensive fossil record that does not appear in Cambrian or late Precambrian rocks. The oldest known fossil bryozoans, including representatives of both major marine groups, the Stenolaemata (tubula r bryozoans) and Gymnolaemata (boxlike bryozoans), appear in the Early Ordovician. It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some other reason; perhaps they evolved from a phoronid -like ancestor at about this time.

Stenolaemate Stories
The stenolaemate bryozoans quickly radiated in the early Paleozoic and are very characteristic fossils of Paleozoic rocks, sometimes making substantial contributions to the formation of reefs, calcareous shales, and limestones. They included forms with robust skeletons, such as the trepostome Hallopora pictured above; such forms were common in shallow-water habitats that today are dominated by corals. There were also forms with delicate, branching fanlike skeletons such as the fenestrates pictured below (from the Mississippian of Domodedovo, near Moscow, Russia). With the exception of one order of stenolaemates, the Tubuliporata or Cyclostomata, all of these Paleozoic bryozoan lineages were severely impacted in the Permian extinction: cryptostomates disappeared at the end of the Permian (245 million years ago), while a few other lineages lingered until the end of the Triassic, about 210 million years ago. Tubuliporate bryozoans have survived to this day, and in fact underwent a remarkable radiation in the Cretaceous, but are no longer dominant today.

Gymnolaemate Grandeur
Uncalcified gymnolaemates are known as fossils from the Late Ordovician on, almost exclusively as distinctive borings in carbonate substrates such as shells. Non-boring, non-calcified gymnolaemate bryozoans are extremely rare as fossils and known from the Jurassic and Cretaceous only. Calcareous gymnolaemates did not appear in the oceans until the Cretaceous, during which time they diversified rapidly from a very few species in the early Cretaceous. By the end of the Cretaceous, there were over 100 genera of gymnolaemates. They continued to diversify in the Cenozoic: today there are over 1000 genera, comprising the bulk of bryozoan diversity in today's seas.