| Abstract: | Abstract The ultrastructure of the calcareous skeleton is described in nine species of Recent cyclostome bryozoans belonging to the suborder Cerioporina. Two species of Heteropora have interior zooecial walls comprising a granular precursory layer followed by a thick layer of transverse fibres and a subordinate foliated fabric with, in mature proximal walls, a semi-nacreous layer. The remaining seven species have interior walls with no transverse fibres and instead predominantly comprise a distally-imbricated, regularly foliated fabric overlying a granular precursory layer. Older, proximal surfaces often have abundant screw dislocations, but true semi-nacre is absent. Basal walls comprise an outer finely granular precursory fabric and planar spherulitic layer, succeeded by the same ultrastructural succession seen in the interior zooecial walls of the respective groups. Exterior walled diaphragms, peristomes and gonozooids similarly comprise an external fabric of planar spherulitic calcite, lined internally by the predominant fabric seen in the interior walls. Ultrastructurally, therefore, cerioporines may be split into two groups with different fabric suites, the first resembling cinctiporids and many tubuliporines in having interior walls with fabrics of transverse fibres, foliated crystallites and semi-nacre; and the second resembling the rectangulates Lichenopora and Disporella in having interior walls comprising only the foliated fabric. These findings support the close phylogenetic relationship between cerioporines and other cyclostomes but suggest that the cerioporines may constitute either a diphyletic or a paraphyletic group. |
