Wall structure and evolution in cyclostomate Bryozoa

The Stenolaemata comprise both basically single-walled forms (Cyclostomata) and doublewalled forms (Trepostomata, Cystoporata and Cryptostomata) from their first appearance in the geological record. At the end of the Palaeozoic the main groups of apparently double-walled stenolaemates died out, and only the single-walled cyclostomates survived. During the Mesozoic, evolution within the Stenolaemata was apparently repeated by the further development of double-walled forms such as cerioporids, lichenoporids and cancelloids, from single-walled ancestors. These double-walled groups are all remarkable homeomorphs of the major Palaeozoic groups of Bryozoa. A monophyletic origin of the post-Palaeozoic Cyclostomata from Palaeozoic single-walled forms is thus suggested.