The Secretion and Structure of the Skeleton of Living and Fossil Bryozoa

Eighty species of living and extinct bryozoans have been studied to ascertain the mode of secretion of the skeleton and its structural diversity throughout geological time. In the growing tip of the cheilostomes Membranipora and Electra and the ctenostome Bowerbankia , periostracum secreted by a cap of palisade cells expands forwards by intussusception. Older periostracum, left behind by the advancing cell cap in cheilostomes and cyclostomes, becomes the seeding sheet of a calcitic layer (primary layer) of vertically disposed crystallites with minor banding, nodules and lenses, deposited by epithelium generated immediately proximal of the cell cap. The periostracum is composed of mucopolysaccharide and some chitin but its outer surface varies, being a fibrillar triple-unit membrane in cheilostomes and the cyclostomes Berenicea and Lichenopora , and a homogenous granular layer in Bowerbankia and the cyclostome Crisidia . The mineral skeleton of most cheilostomes is secreted on periostracum by succeeding epithelium. But coelocystic structures, deposited within folds of epithelium, also occur and vary from frontal walls and cryptocysts to almost complete skeletons of Cupuladria and Iodictyum . In some living cheilostomes and in most fossil species examined, only a primary shell, more commonly calcitic than aragonitic, is found, but in the majority of living species there occurs an organic-carbonate secondary layer usually composed of discrete laminar or lenticular aggregates of vertically disposed crystallites enclosed in proteinous sheets. In Celleporella , however, the laminae are more strongly defined because they consist of spirally growing calcitic plates separated from one another by simultaneously secreted proteinous sheets. More rarely, a tertiary layer is found in some species. It is almost entirely calcitic or aragonitic and may be banded, granular or composed of acicular crystallites. Puncta and basal kenozooids, as in Schizoporella and Cupuladria, penetrate the shell of some species and accommodate papillae with storage cells. The mineral skeleton of most cyclostomes, including the earliest known genera like the Ordovician Corynotrypa, has always been subperiostracal, and may also be pierced by puncta accommodating storage papillae and distally closed by periostracal plugs (e.g. Crisidia, Berenicea ). The skeleton of Heteropora , Hornera, Lichenopora and related genera which first appeared in early Mesozoic times, is coelocystic. It may also be ornamented by inwardly pointing cones of secondary shell (pseudopuncta) with or without axial rods of granular calcite (e.g. Hornera ) , or include vesicular tissue between radiating rows of zooecia as in Lichenopora . All cyclostomes, however, have always had a double-layered calcitic skeleton consisting of a primary layer of vertically disposed acicular crystallites, granules or tablets altered to a granular texture in the fossil state, and a laminar secondary layer with spirally growing plates and/or overlapping fibres coated by protein sheets. The mineral skeleton of the remaining stenolaemates represented by extinct trepostomes, cystoporates and cyclostomes, is exclusively double-layered and coelocystic. The primary layer is granular, the secondary composed of lenticular fibres. Pseudopuncta were almost invariably developed and interzooecial cavities (mesopores) commonly occurred and were filled with vesicular tissue as in Lichenopora . Phylogenetic considerations suggest that the prototypic bryozoans possessed a pseudopunctate coelocystic skeleton, indented by mesopores and composed of a chitinous or proteinous periostracum, a primary layer of acicular crystallites and a secondary layer of lenticular fibres. This structure was inherited by trepostomes, cystoporates and cryptostomes. It reappeared in Mesozoic cyclostomes subsequent to their divergence from a cystoporate-like ancestor by the development of a variably punctate subperiostracal skeleton which, however, was composed of the same shell types. The subperipheral skeleton of early cheilostomes may have evolved by extension of the ctenostome secretory regime. In any event a new type of secondary shell, composed of acicular crystallites arranged in proteinbound laminae and lentides, became characteristic mainly of Tertiary and Recent species. Nevertheless, a coelocystic skeleton and even a stenolaemate-like secondary shell have already appeared within the Order.