Functional controls on monticule height and spacing in Permian stenolaemate bryozoans

Abstract One of the challenges for bryozoans is to avoid refiltering 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 the height and density of spacing of monticules are inversely proportional to curvature of the colony surface. Larger, flatter colonies should have higher and more closely spaced monticules. We compare two Permian stenolaemate bryozoans whose colonies form branches with elliptical cross sections: the smaller and more elliptical cystoporate Evactinostella crucialis (Hudleston, 1883) 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 half 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.