Fig. 6: Evolution of skeleton regularity in black corals.

a Bayesian analyzes concatenating morphology and molecular data for selected fossil and extant black corals (see detailed topology in Supplementary Fig. 6c); fossil families are shown in bold. b schematic construction of typical regular network types observed in eight extant black coral families. c–e Construction of irregular networks of three fossil species of the family Sterictopathidae fam. nov.: Sterictopathes sp. (c), S. seira (d) and S. radicatus (e). f Conceptual ancestor of the Sterictopathidae fam. nov. with fully-developed irregular networks. g–j Hypothetical evolutionary trend of black coral skeletal networks and ridges. Erected colonial black coral with (h) or without (g) ridges; i encrustation of ancestral colonial black corals by asexual reproduction; j ancestral solitary polypoid periderm of hexacorallian with a basal periderm. k–l Artist reconstructions of the black coral Sterictopathes seira sp. nov. Abbreviations: CO coenenchyme, BP basal plate, CC central canal, CR complete ridge, IR incomplete ridge, PO polyp, RI ridge, SP spine, TH theca.