Fenestrate theoretical morphology: geometric constraints on lophophore shape and arrangement in extinct Bryozoa

A geometric analysis of fenestrate bryozoan lophophore shape and arrangemen t is conducted by creating a theoretical morphospace of apertural positioni ng within the colonial meshwork. Working from the assumption that fenestrat e bryozoans needed to form a continuous filtering surface with contact betw een adjacent lophophores, we show that within the morphospace three regions exist for optimum close-packing of lophophores with circular projections; all other close-packing configurations in the morphospace require the exist ence of noncircular lophophores. Examination of the actual distribution of 251 fenestrate colonies within th e morphospace reveals that the morphospace regions occupied by fenestellids and polyporids are displaced and have little overlap, but that they are ve ry similar in size and shape, and that the colonies scale similarly. With i ncreasing size, fenestrate meshworks expand laterally faster than the branc hes widen and the proximodistal spacing of the apertures increases, apparen tly because the larger zooids require disproportionately more room for thei r lophophores. Two of the optimum close-packing regions of the morphospace are occupied by fenestrates. The positioning of the fenestellid region within the morphosp ace suggests that these biserial bryozoans followed a proximodistal-row pla cement of the lophophores, and that the lophophores were generally equitent acular, with circular projections. The positioning of the polyporid region within the morphospace suggests that these polyserial bryozoans followed a diagonal-row placement of the lophophores, and that the lophophores were he teromorphic, with medial lophophores on the branch being more equitentacula r whereas the laterally placed lophophores were obliquely truncate. The thi rd optimum close-packing region in the morphospace, corresponding to a hypo thetical lateral-row placement of the lophophores within the colony, is uno ccupied. We suggest that hypothetical fenestrate morphologies in the vacant region of morphospace have branches that would be too narrow to support no rmally shaped zooids, and that the lateral-row placement of the lophophores would have required the branches of the colony to have been perfectly alig ned throughout growth.