From a survey of population density and size frequency in 1423 Trypanites boreholes from symmetrical, high domical Silurian stromatoporoids, two factors are shown to have controlled borer distribution. Progressive burial left areas of high “absolute” elevation available for continuing growth and further colonization over long periods. As well as serving to aid borehole concentration on topographic “highs”, this process gave rise to populations which included both old and young individuals. In addition to this environmental control there operated a larval settling strategy which favoured the selection of prominences, presumably because they intercepted stronger or more persistent currents. Such prominences did not necessarily coincide with the areas of greatest “absolute” elevation, so permitting the two controls to be distinguished.