Structural aberrations in the cup in cladid crinoids from the carboniferous of the Moscow region

Aberrations of the discrete characters of dorsal cups and arms in the Carboniferous crinoids of the Moscow region, mainly represented in the collections of the Paleontological Institute, are studied. Aberrations are unevenly distributed across different genera, from the total absence in a sufficiently large sample to nearly 10%. Aberrations in infrabasals, basals, radials, and anal plates as well as in branching of arms are examined separately. It is shown that the aberrations involving one of the circlets usually have almost no influence on neighboring circlets, with the exception of border parts. This demonstrates the significant independence of their formation from each other. Interaction is usually limited by fitting during the ontogeny of border parts of neighboring circlets for the complete closure of the dorsal cup. Nevertheless, in some cases one of the distal part of infrabasal plate or one of the proximal part of radial plate is introduced into the basal circlet, breaking it, making it unlinked. This may indicate that during the ontogeny the basal circlet was formed after infrabasal and even radial circlets, not only in these aberrant forms, but also, perhaps, in the forms with normal course of ontogeny. Aberrations in the anal area are associated mainly with the change of the number of plates, their position within the cup, and the location and shape of the radianal plate. Bilateral symmetry with respect to the crinoid plane of symmetry passing through the A ray and CD interray is detected in branching of arms.