Fossil record of echinoderm regeneration with special regard to crinoids

Regeneration of skeletal parts of fossil echinoderms is reviewed. Among regenerations of various fossil echinoderms, those of crinoids are most common and many examples have been reported. Mesozoic and Cenozoic crinoids, the subclass Articulata, are characterized by the dominance of muscular articulations in the arms and they have only a small number of ligamentary articulations in localized positions that are specialized for autotomy. The dominance of muscular articulations is most conspicuous in the family Isocrinidae and comatulid (stalkless) crinoids. Most of the regenerated arms of articulate crinoids start on this ligamentary articulation. In contrast, most Paleozoic crinoids have many more ligamentary articulations and fewer muscular articulations in their arms than the Mesozoic to Recent crinoids. The ligamentary articulations of Paleozoic crinoids were probably not capable of autotomy. Fossil and recent stalked crinoids provide data on the degree and style of regeneration after loss of body parts. If the aboral nerve center in the basal part of crown is retained, the entire calyx and arms can be regenerated. In contrast, if the aboral nerve center is lost and only the stalk is preserved, some skeletal parts can only be generated imperfectly and irregularly. This type of imperfect regeneration is better termed as overgrowth.