Aberrations in the infrabasal circlet of the cladid crinoid genus Cupulocrinus (Echinodermata) and implications for the origin of flexible crinoids

摘要

Flexible crinoids (subclass Flexibilia) likely originated from the cladid crinoid genus Cupulocrinus or one of its immediate ancestors in the Middle to Late Ordovician. A remarkably constant and Glade-defining character of flexible crinoids is the possession of three plates in the infrabasal circlet of the calyx. This character was a significant morphological modification for the origin of the flexible Glade, resulting from the reduction of the number of infrabasal plates from five plates in the ancestral cupulocrinid.