Lower Jurassic corals from Morocco with skeletal structures convergent with those of Paleozoic rugosan corals

Abstract

Neorylstonia nom. nov. pseudocolumellata, a replacement name for Mesophyllum pseudocolumellatum Beauvais, 1986, is only known from uppermost Sinemurian to uppermost Pliensbachian (Lower Jurassic) strata of Morocco. This solitary coral species went extinct during the Pliensbachian-Toarcian transition (~183.5 My), which is currently considered to be a second order biodiversity crisis linked to a period of rapid and global paleoenvironmental disturbances. Neorylstonia pseudocolumellata has a distinctive skeletal organization. The axial structure of the corallite, which has no equivalent in Mesozoic taxa, is reminiscent of the skeletal organization of some Paleozoic rugose corals such as Amygdalophyllum Dun and Benson, 1920, or Rylstonia Hudson and Platt, 1927. This similarity is based on the occurrence of a calicular boss also named pseudocolumella. As Rugosa and Scleractinia do not appear closely related, we consider that this feature cannot be due to evolutionary inheritance from a Rugosan ancestor. The morphological aspects of the skeleton have been assessed in order to understand the function and growth of this axial “calicular boss” as well as to more precisely describe the genus and species characters. The organization of the septal apparatus points to a typical Scleractinian pattern, which is also supported by the original aragonitic mineralogy of the skeleton. This implies a convergence phenomenon, leaving open the question of the functional significance of such a calicular organization. We posit that this feature was used for sexual reproduction and so did not always develop in a population.