301 The boring ctenostomate Bryozoa: taxonomy and paleobiology based on cavities in calcareous substrata

The boring ctenostomate Bryozoa: taxonomy and paleobiology based on cavities in calcareous substrata - BAP #301

The boring Bryozoa (Upper Ordovician-Recent) live immersed in calcareous substrata that the organism penetrates by purely chemical means. All known species are here assigned to the order Ctenostomata Busk, 1852, [Including Penetrantia Silén, 1946]. Until now, the group has been known in anatomical detail almost exclusively from studies on the internal soft parts of Holocene species. Fossil remains of these bryozoans have been confused with various borings and shallow pits of diverse origin.

            The classification of previously recognized species is revised, and 13 genera including 48 species are recognized. There are six new genera: Casterogenera, Cookobryozoon, Fischerella, Marcusopora, Orbignyopora, and Voigtella. Casteropora and Fischerella are assigned to the Bryozoa with slight reservation. The following 20 new species are erected: Cookobryozoon lagaaiji, Immergentia atypica, I. boydekina, I. lanceolata, I. losangelina, I. patagoniana, I. subangulata, Marcusopora ripleyensis, Orbignyopora? Cornbrashica. O.? tridelta, Penetrantia soulei, Ropalonaria? bugei, Spathipora ambidextra, S. brevicauda, S. cheethami, S. magnivorticellopsis, S. occidentalis, Terebripora Miniatura, Voigtella regalis, and V. secunda.

            New and figured material includes the ancestrulae and early astogenetic stages of several fossil species. In addition, various heterozooids, reversed zooids and teratological abnormalities are recognized in fossils of several genera. Some groups are characterized by the occurrence of enantiomorphic autozooids. There is substantial evidence that at least the initial portions of entire colonies occur in dextral and sinistral forms in some species.

            The confinement of zooids beneath an inflexible calcareous shield has sometimes been accompanied by the development of features resembling those which are generally associate with the cheilostomes. The occurrence of ovicells in Spathipora cheethami, a boring species from the Middle Jurassic (Cornbrash) of England, is an example.

            In Penetrantia densa Silén (Recent), a probable ctenostome, the presence of ovicells, an operculum, and a calcareous body wall suggest indirectly that the Cheilostomata may be polyphyletic. Ovicells (and opercula?) may be more likely to develop in lineages wherein the zooids are protected by a solid external skeleton (of bryozoan or molluscan origin). The possibility of independent evolution of these features in at least two groups of “shielded” ctenostomes (i.e., in Penetrantia Silén and in one or more independent lines of calcified, encrusting ctenostomes) is worthy of thorough consideration.