EXPLOSIVE EVOLUTIONARY DIFFERENTIATION OF UNIQUE GROUP OF MISSISSIPPIAN-PENNSYLVANIAN

Crinoids classed as constituents of the family Acrocrinidae are distinguished from all other Camerata, as well as representatives of remaining subclasses, in having circlets of supplemental calyx plates between the bipartite basais and ring of radials and single pri-manal at summit of the calyx. The supplemental plates, designated as intercalaries, range in number from six to 600 or more. Analysis of their arrangement, combined with other characteristics, furnishes morphological grounds for differentiation of several genera in-stead of a single one (Acrocrinus), which until now has been considered to be the sole generic component of the family. All of the newly recognized acrocrinid genera and seven new species assigned to three of them are determined on seemingly firm and sig-nificant distinctions. The whole assemblage demonstrates remarkable evolutionary changes along different lines, which are expressed in calyx shape, attitude of the basal plates, num-ber and positions of the arms, features of the tegmen, and especially by an explosively rapid increase in number of intercalary plates to a Late Mississippian peak, great decline in the Pennsylvanian, and by diverse patterns of distalmost intercalaries. Two subfamilies are recognized but their interrelations and phylogenetic lines are very obscure