Historical record of erect bryozoan growth forms

Information compiled from literature on marine bryozoans suggests that erect taxa have had a limited range of repeatedly evolved growth forms, each with characteristic branch widths or surface morphology, through the nearly 500 Ma history of the phylum. The range of erect growth forms among the Bryozoa has been limited to six (with only rare exceptions) that have co-occurred constantly throughout the history of the phylum. Ciliated tentacles of bryozoan lophophores are located just above the surface of colonies and draw a flow of food-bearing water. Outflow of the filtered water is confined to the space between the tentacles and the colony surface, until some point of escape is reached. In almost all erect bryozoans, lophophores are closely spaced and, in extant species, feed simultaneously. Therefore, for broad surface areas to develop, colony margins must be supplemented by other points of outflow (chimneys), commonly situated on surface disruptions, to keep local volume and velocity within manageable bounds. Presence of chimneys apparently permits branch widths to increase beyond certain size limits into less tightly constrained morphospace, and branch widths have been narrowly limited in the various growth forms except where regularly spaced chimneys can be demonstrated or reasonably inferred. It is hypothesized that the observed limits on form, size and surface morphology are derived primarily from the phylogenetic constraint of byrozoan lophophore size and attendant limitations on flow of filtered water across colony surfaces.