CLADID CRINOID RADIAL FACETS, ARM DEVELOPMENT, AND ARM APPENDAGES--A TERMINOLOGY SOLUTION FOR STUDIES OF LINEAGE, CLASSIFICATION, AND PALEOENVIRONMENT

Three surface-morphology conditions are recognized as primitive, intermediate, and advanced in the radial facets of cladid crinoids. Primitive radial facets have smooth, shallowly concave surfaces generally lacking other morphologic features, except crenulae on the outer margin. They are generally circular or horseshoe shaped and angustary or peneplenary. These forms have relatively few arms that are formed of uniserial rectilinear brachials and rarely bear appendages. They are present in the primitive cladids. Intermediate and advanced radial facets are trifacial. Intermediate radial facets have arcuate transverse ridges and may have culmina along the outer edge or grooves radiating from the base of the intermuscular notch or center of the transverse ridge. They are commonly circular to horseshoe shaped, rarely oval, and angustary or peneplenary, rarely plenary. These forms have few to many arms formed by uniserial rectilinear brachials, rarely slightly cuneate brachials, normally bear ramules or armlets, and rarely are pinnulate. They are present in the primitive cladids. Advanced radial facets have straight transverse ridges with complex surface morphology. They are commonly oval in outline and may be angustary to plenary. They bear pinnulate arms that may be formed of uniserial rectilinear or cuneate brachials or biserial cuneate or wedge-shaped brachials. Advanced radial facets begin with the advanced dendrocrinids and include cladids previously included in the poteriocrinids, excluding the poteriocrinids, which have intermediate radial facets. Terminology of ramules, armlets, and pinnules has not been applied uniformly. Ramules are defined as unbranched minor arm appendages, regardless of length and girth, occurring irregularly along the arm or may become regularly spaced distally such as every fourth or greater brachial. Pinnules are defined as unbranched minor arm appendages, regardless of length and girth, occurring in a regular pattern of one or more per brachial or every second brachial. Armlets are defined as minor secondary appendages of the arm composed of a branchlet (the main stem) and pinnulets (the branches). Future studies must provide detailed information of radial facet surface morphology whenever possible for use in lineage, classification, and paleoenvironmental applications.