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