Representatives of basal scleractinian clades (Gardineriidae, Micrabaciidae) vs some Mesozoic and Palaeozoic corals. Overall morphological similarity between Recent Gardineria (A, D), some oldest known Mesozoic scleractinians (B, E; Margarophyllia sp., Triassic, ca. 230 Ma), and Palaeozoic rugosans (C, F; Ptychophyllum sp., Devonian, ca. 380 Ma), and morphological comparison between the skeleton of Recent micrabaciid Letepsammia (G, H), and mould of the Ordovician (ca. 460 Ma) Kilbuchophyllia (I, J). Despite the overall morphological similarity, resulting from occurrence of corrugated, entirely epithecal wall and relatively smooth septa, rugosans exhibit a different pattern of septal insertion than scleractinians (serial vs. cyclic, respectively), which most researchers consider the main argument of their independent origin. Calicular views (A-C); lateral views (D-F). A unique feature of modern micrabaciids is the multiple bifurcation of septa of the third order and straight and nonbifurcate septa of the first order: compare diagrammatic representation of one septal system in Letepsammia (H) and interpretation of the mould of Kilbuchophyllia (J); arrows indicate bifurcations of one branch of third order septa.