MULTIPLE CROSSOVERS BETWEEN MONOCYCLIC AND DICYCLIC CUP DESIGNS: EXPERIMENTATION AND PLASTICITY IN EARLY CRINOID PHYLOGENY

Crinoids have historically been classified into cup designs that have either two circlets of plates beneath the arm-bearing radials (dicyclic), or only one circlet of plates beneath the arm-bearing radials (monocyclic), a “key character” in crinoid classification. Diplobathrid camerates, cladids, flexibles, and fossil articulates have typically been classified as dicyclic groups, whereas monobathrid camerates, disparids, hybocrinids, perittocrinids, and living articulates have typically been classified as monocyclic (or cryptodicyclic) groups. But there has long been unease among specialists about reliance on this single trait in crinoid classification, especially among early crinoids. The presence of pentameres in the proximal stem immediately below the cup provides reliable criteria for evaluating these cases. In 1975, John Warn recognized two additional categories, pseudodicyclic and pseudomonocyclic, for crinoids that had gained or lost the lowest cup circlet. However, except for articulate crinoids, he had few examples of older crinoids that had these alternate designs. The discovery of new Ordovician crinoids in the past 30 years that have either lost an existing plate circlet or added a new plate circlet at the cup base bring into question the use of monocyclic vs. dicyclic cup designs. Recently we have discovered a new Early Ordovician crinoid closely resembling the Late Ordovician cladid Carabocrinus that has apparently become pseudomonocyclic, a pattern that one of us had previously reported for hybocrinids. The presence of closely related large-calyx cladids and hybocrinids and also primitive diplobathrid and monobathrid camerates indicate that multiple crossovers between these presently used groups have occurred early in crinoid history, rendering several of these major groups polyphyletic. We could downgrade this “key character” and discard several major crinoid groups in the present classification, but a better solution might be to save the major groups but recognize these small crossover groups as monophyletic clades in a revised phylogenetic classification and phylogeny.