Inplenary and explenary radial facets in cladid crinoids

Abstract

Three types of radial facets have been recognized in the cladid crinoids. These are: • Angustary—the facet occupies less than 70% of the width of the distal end of the radial. • Peneplenary—the facet occupies between 70% and 99% of the width of the distal end of the radial. • Plenary—the facet occupies the full width of the radial facet. Both the angustary and peneplenary radial facets have interradial notches between adjacent radial facets along the sutures on the distal ends of the radials. The radial facets are here referred to as inplenary if an interradial notch (here referred to as exinterradial notch) is between the distal ends of the transverse ridges and the aboral parts of the muscle fields are abutting at the suture. If the interradial notch is between the muscle fields and the distal ends of the transverse ridges are abutting, then the radial facet is referred to as explenary and the notch as ininterradial. The origin of these conditions is uncertain. However, they may be an evolutionary transition between the peneplenary and plenary conditions or they may be a secondary response to evolutionary changes in the cup structure. The explenary radial facets may have resulted in minimal contact of the primibrachials when opening, closing, or manipulating the arms. It is here proposed that the interradial notch be restricted to the condition wherein the nonarticular surface extends along the suture entirely across the distal end of the radial completely separating adjacent radial facets. Recognition of the inplenary and explenary radial facets should be useful in consistency of description, evolutionary studies, paleoecologic implications, and classification of the cladid crinoids.