CLADID CRINOID (ECHINODERMATA) ANAL CONDITIONS: A TERMINOLOGY PROBLEM AND PROPOSED SOLUTION

Abstract

Abstract: Analysis of anal plates of type species of most cladid crinoid genera (excluding codiacrinid and some gasterocomacids) recognizes four catagories of anal plating conditions designated: Parioplax, Menoplax, Mesoplax and Opioplax. These four plating conditions are subdivided into 23 anal subconditions. These various anal conditions reflect recognized general evolutionary trends within all orders of the cladids. This corrects, describes more precisely, and replaces the widely used terms ‘primitive’ and ‘advanced’ to describe anal conditions, either of which can occur in older or younger cladid crinoids. The morphological terms primanal, secundanal and tertanal should be used in place of the terms radianal, anal X and right tube plate, respectively, in most cladid and flexible crinoids. Anal X should be used only for those genera for which it has been demonstrated that the primanal was resorbed or migrated out of the cup. In most genera having cups with a single anal this rectifies the misnomer of referring to the single anal as anal X when it is actually the primanal that has migrated into the position previously occupied by the secundanal, pushing the secundanal out of the cup. The origin of cladid anals is not considered homologous to camerate anals, but the use of similar terms for descriptive purposes is given precedence over use of a misnomer. Recognition of the anal‐plate positions and clarification of anal‐plate homologies are important in cladid phylogenetic analysis. Results of this study strongly support punctuated and mosaic evolution within the anal‐plate series of cladid crinoids. Application of the new terminology further implies that the classification of some polyphyletic genera and families may be the result of convergence, wherein greater emphasis has been placed on the anal condition for genus or family assignment than has been placed on other equally significant morphological features.