Skeletal Ultrastructure in Some Articulate Cyclostome Bryozoans

Abstract

AbstractThe ultrastructure of the calcareous skeleton is described in 11 species of articulate cyclostome bryozoans with elastic joints. Ten species have interior walls comprising semi‐nacreous and pseudofoliated fabrics without a precursory granular layer. Exterior walls consist of outer, finely granular and planar spherulitic layers, succeeded by semi‐nacreous and pseudofoliated fabrics like those of interior walls. Outer fabrics are calcified as longitudinal strips, each corresponding to a planar sphcrulitic unit. Articulation surfaces comprise ring diaphragms of very fine granular fabric with concentric laminations. The semi‐nacre of walls adjacent to ring diaphragms contains minute holes. Crisulipora occidentalis is unique in having interior walls of transverse fibres succeeded by pseudofoliated fabric, articulation surfaces festooned with deep pits but lacking well‐differentiated ring diaphragms, and pseudopores containing sieve‐like closure plates. The ultrastructure of most articulates resembles tubuliporine cyclostomes with dominantly semi‐nacreous walls, although the lack of precursory granular fabric in the interior walls and the presence of subcircular tablets of semi‐nacre (without six‐fold sectoring) may be peculiar to articulates. In contrast, Crisulipora is more similar to other tubuliporines with transverse fibres. evidence which, together with other skeletal characters, suggests that Crisulipora evolved jointing independently of the rest of the articulate cyclostomes.