CALCITIC NACREOUS ULTRASTRUCTURES IN BRYOZOANS - IMPLICATIONS FOR COMPARATIVE BIOMINERALIZATION OF LOPHOPHORATES AND MOLLUSKS

The occurrence of nacreous-type fabrics in bryozoans has broad implica tions for our understanding of similarities between lophophorate and m olluscan biomineralization. Calcitic semi-nacreous ultrastructures in the skeletons of several species of cyclostome bryozoans belonging to the suborders Tubuliporina and Articulata are described. In these taxa semi-nacre is the dominant ultrastructure of the walls, and often suc ceeds a precursory fabric of tiny wedge-shaped crystallites. The semi- nacre comprises laminar sheets of irregularly stacked, typically six-s ided tablets with frequent screw dislocations. Tablets usually have in clined edges and an upper surface with a central depression. The surfa ces of etched six-sided tablets have six subtriangular sectors: three soluble sectors alternating with three less-soluble sectors. The less- soluble sectors have outward-sloping edges and sometimes have slightly longer sides than the more soluble sectors, which have inward-sloping edges. Newly seeded crystallites comprise only the three less-soluble sectors separated by a 'trilete' suture. They are not nucleated on a particular sector type. Similar fabrics have previously been recorded in the cyclostome suborder Cancellata. The substructure of the calciti c semi-nacreous tablets of cyclostomes resembles molluscan aragonitic nacre, particularly that of bivalves, but differs in the precise arran gement of the sectors. Morphologically, the bryozoan semi-nacre is at least as similar to bivalve nacre as bivalve nacre is to gastropod and cephalopod nacre. The existence of a distinct substructure in the com ponent tablets of molluscan nacre has been used as evidence that a gre ater evolutionary potential characterizes molluscan rather than lophop horate (bryozoans and brachiopods) biomineralization. This claim is co ntradicted by the recognition of substructured semi-nacre in cyclostom e bryozoans.