Skeletal morphology and phylogenetic affinity of Paleozoic scleractiniamorph corals are analyzed by ordinating scleractiniamorphs among solitary genera within subclass Rugosa and order Scleractinia. Scleractiniamorph corals are solitary and phaceloid in form and occur in Ordovician and Permian rocks. As implied by their name, scleractiniamorph morphologies are similar to those of corals in the order Scleractinia. However, corals of the subclass Rugosa were prominent in the fossil record of the Paleozoic until their end-Permian extinction. Although order Scleractinia first appeared in the middle Triassic with an unusually widespread geographic distribution and high morphological diversity, the fossil record belies the molecular data that suggest a late Ordovician, soft-bodied origin for scleractinians. In essence, these rare, skeletonized scleractiniamorph fossils with disputed relationships to Scleractinia occur in Paleozoic strata.

We test the hypothesis that scleractiniamorphs fall within a zone of overlap between the hulls of Rugosa and Scleractinia in PCoA analyses of skeletal morphologies. A morphologic database of solitary coral skeletal characters from 74 rugosan families and 483 genera, 50 scleractinian families and 236 genera, and three scleractiniamorph families and three genera was curated from descriptions and images within the World Register of Marine Species, Corallosphere, the Treatise on Invertebrate Paleontology, museum specimens, and publications on systematic paleontology. Data for 85 generic character states were analyzed using PCoA. Eigenvalues for the first two axes total 8.22%, suggesting high independence of traits. Both scleractiniamorph genera fall in the overlap of hulls between Rugosa and Scleractinia but importantly, at an extreme of Permian rugosan morphologies and near the central-most cluster of scleractinian morphologies including Triassic, Modern, and molecularly identified basal clade scleractinian genera. This work reveals the morphological convergence of scleractiniamorphs to specific scleractinian genera, and the breath of scleractinian and rugosan morphologies through time. The PCoA plot also reveals positioning of the ‘typical’ horn coral often associated with rugosan morphology.

Results generated by this research bring scleractiniamorph, scleractinian and rugosan morphologies into clearer focus and reveal morphology of the three coral groups with time. With the realization that convergence of form in the marine environment results in morphologic similarities in corals that may not be indicative of evolutionary relationships, this database advances the potential taxonomic placement of scleractiniamorphs within coral systematics.