The significance of corallite spacing in heliolitid corals

Heliolitids (Heliolitina, Tabulata) are one of the few groups of Paleozoic corals exhibiting a plocoid form of the corallum, indicative of a relatively high level of colony integration. The difference in the spacing of individual corallites within their plocoid colonies (measured in corallites per cm2) has been broadly used as one of the main characteristics for distinguishing between heliolitid taxa. In the present study, corallite spacing was analysed in three coral species: Propora tubulata and Heliolites spongodes from the Silurian of Gotland, and H. porosus from the Devonian of Morocco. This study aims to offer a new perspective on the usefulness of corallite spacing in heliolitids as a characteristic for the purpose of taxonomic, palaeoecological, and evolutionary analyses. Additionally, a new measure of this trait, corallites per (10d)2, is proposed, in order to facilitate more direct comparisons of corallite spacing despite intra- and interspecific differences. Observations on P. tubulata and H. porosus show that their corallite spacing was adaptable. The adaptability of offset insertion in heliolitids was likely advantageous and could be the reason behind the evolutionary convergence between them and the modern octocoral Heliopora coerulea, which develops a very similar colony structure.