ORIGIN OF STANDARDIZED CUP AND POSTERIOR PLATING IN EARLY CRINOIDS

Recently discovered stem-group crinoids provide evidence for the origination of all crown-group crinoids with their standardized cup plate patterns. These earliest crinoids have rudimentary cup organization with a cup-base primary circlet (CBPC) below and fixed ray plates extending into the cup from above. Mid cup and interray plating is unorganized. In crown-group crinoids, the lowest cup plates have traditionally been designated as basals if the cup is monocyclic (one plate circlet below the radials), or as infrabasals if dicyclic (two plate circlets below the radials). However, these lowest plate circlets appear homologous, both derived from the CBPC, based on the presence of 1-2 distinctive posterior gap plates and alternation of these lowest cup plates with the stem meres. Thus, the CBPC has a perradial orientation in stem-group and dicyclic crinoids, but must have rotated 36 degrees to an interradial orientation in monocyclic crinoids. Correct plate homologies can be re-established in dicyclic crinoids by renaming infrabasals as basals and the previous overlying "basals" as epibasals, following Jaekel, 1918. Two derived posterior plate arrangements involving the C-ray plating arose from within the wider CD interray plating of stem-group crinoids, a pattern continued in later camerates where it originates at or above the radials. Posterior plating of cladids originates beneath the C-ray, but continues to exit the cup in the CD interray, whereas that of disparids is eliminated from the lower CD interray and branches from the C-ray plating at or above the radial. Posterior plating is more consistent than plate circlet number, and therefore of greater phylogenetic significance. The new plate homologies help explain seemingly anomalous early crinoids. Aethocrinusis a stem-group cladid apparently having two epibasal circlets and stem meres aligned with the basals. Perittocrinids are monocyclic cladids with radials, tiny isolated intercolates, and basals, whereas hybocrinids are (pseudo)monocyclic cladids that have lost the basal circlet.