One of the challenges for bryozoans is avoiding refiltering of water that has already had its plankton removed. Larger colonies develop colony-wide maculae-centered feeding currents to avoid refiltering water, and generally have elevated maculae (monticules). We hypothesize that height and spacing of monticules is inversely proportional to curvature of the colony surface. Larger flatter colonies should have higher and more closely spaced monticules. We compare two Permian palaeostomate bryozoans whose colonies form branches with elliptical cross-sections: the smaller and more elliptical cystoporate Evactinostella crucialis from Western Australia (n = 17) and the larger and flatter trepostome Tabulipora sp. from eastern North Greenland (n = 15). Using calipers and digital elevation models we measured curvature, monticule height, and number of monticules per area. Results indicate that Evactinostella branches are at least twice as curved as those of Tabulipora, their monticules are 1/2 the height of Tabulipora, and their monticules are 22% less densely spaced than those of Tabulipora. In Evactinostella colonies, surface curvature is inversely proportional to monticule height and spatial density which is not true for Tabulipora. Therefore, we conclude that the smaller and more curved the colony surface, the less the colony needs robust colony-wide feeding currents created by tall closely spaced monticules.

We measured branch width, branch thickness, and monticule height with digital calipers to the nearest 0.01 mm. Branch curvature in mm^-1 was calculated as ½ branch thickness/(½ branch width)². Monticule spatial density in number per mm² was calculated as number of monticules per branch side/area of branch side. Using the Structure from Motion (SfM) method, we built digital elevation models (DEMs) of two branch surfaces. The DEMs were built in Agisoft’s Metashape Pro version 1.64 with 0.1 mm resolution on the x and y coordinate axes and 0.01 mm resolution on the z (vertical axis). Topographic profiles through the colony surface were made with QGIS version 3.30 from monticule crest to monticule crest.