Brooding in the Cretaceous bryozoan Stichomicropora and the origin of ovicells in cheilostomes

The coilostegan Stichomicropora has unusual ovicells normally preserved only as a transverse row of 3-10 small pores located along the proximal mural rim of the distal zooid. Intact ovicells in S haccata show the pores to be bases of flattened, hollow spines overarching proximally and tightly juxtaposed to form a costate roof. Spine bases in some other species of Stichomicropora may have been articulated, which would explain their non-fossilization. In all species, however, the ovicell is produced entirely by the distal zooid, the roof from modified mural spines and die floor from proximal gymnocyst. Stichomicropora may form a clade (Monoporellidae) with some other coilostegans, including Monoporella spp. and ‘Macropora’ multilamellosa . Comparison of ovicells in Stichomicropora and Wilbertopora , the oldest known ovicellatc cheilostome, permits the primitive cheilostome ovicell to be reconstructed as consisting of several articulated mural spines belonging to the distal zooid and having no direct structural association with any pore chambers.