Bilateral symmetry in ontogeny and regeneration of solitary Rugosa (Cnidaria; Paleozoic)

The widespread lateral attachment in solitary Rugosa (tetracorals) is evidence that their planula settled down and became attached by its ventral side. The symmetry plane of Rugosa corallites, which is marked by the cardinal and counter septa, coincides with the larval plane of symmetry, which is marked by the position of the cardinal septum and attachment scar on the same side of the corallite. This strongly suggests that rugose coral larvae had paired mesenteries at least on the ventral side. Direct or remote soft-bodied rugose coral ancestors had a planula-like body shape and paired ventral and, probably, dorsal mesenteries. They were benthic, crawling on the ventral side, and fed on bottom semi-decomposed organic matter. The study of regeneration in rugose corals has shown that all of three ways of regeneration occur in their skeletons (epimorphosis, morphallaxis, and compensatory regeneration). The morphallaxis is manifested in small buds on dead parent corallites. The study of the development of these buds has shown that their septa were formed under the influence of the surface relief of the parent corallite bearing the bud. At the early stages of bud development, mesenteries were probably absent.