Growth form in Silurian heliolitid corals: the influence of genetics and environment

Colony growth form in some Silurian heliolitid corals is analyzed by the measurement of their shape in profile. Data are presented for seven species, Stelliporella parvistella, Heliolites interstinctus, H. megastoma, H. daintreei, H. spongodes, Propora tubulata, and Plasmopora scita from three localities in Gotland, Sweden, and three localities in England. Intraspecific growth-form variation is presented on triangle diagrams. These plots allow variation to be compared between species present at each locality and between localities for each species. Results indicate that the overall potential for growth-form variation is genetically controlled and that levels of response to environmental stimuli may differ markedly between species. Stelliporella parvistella is a very plastic species, the only one developing branching growth in addition to other growth forms. Heliolites interstinctus is much less variable, dominantly tabular, domal, and low bulbous in form, but demonstrates a similar response. Propora tubulata has a tightly constrained bulbous growth form that shows little variation between localities. The other species are represented by few specimens, most of which parallel H. interstinctus. The likely moderating influences of light levels, substrate type, sedimentation rate, energy levels, and other variables on growth-form variation and species range are considered. The main environmental factor including ecophenotypic response is concluded to be sedimentation rate. A close correlation between this factor and growth form in S. parvistella indicates that form in this species is a particularly sensitive indicator of sedimentation rate and substrate conditions. No simple equations can be made between specific environments and one particular growth form in these corals.