Developmental breakdown during the early evolution of the Codiacrinidae:Parazophocrinus callosusStrimple, 1963 (class Crinoidea)

Abstract

Parazophocrinus callosusStrimple, 1963, is redescribed as a cladid crinoid belonging to the Codiacrinidae.Parazophocrinusis a reasonably large cladid crinoid that is composed primarily of basal plates, radial plates, and oral plates; arms are absent; and the anal opening is along the side of the aboral cup. The radial plates are hypertrophied laterally, producing a disc-shaped aboral cup, and this very unusual crinoid is considered to be neotenic. Presumably, feeding inParazophocrinusoccurred only with tube feet projected through opened oral plates or from epidermal nutrient uptake. Typical codiacrinids may lack arms, may have the anal opening from the side of the aboral cup, and commonly have prominent oral plates; but in contrast toParazophocrinus, they are small to very small in size. This small size and arrested development is considered progenetic.Parazophocrinusis the second oldest genus in the Codiacrinidae. Apparently, when developmental controls broke down during the early evolution of this family, both neotenic and progenetic crinoids evolved. However, ultimately, theParazophocrinusmorphology, a large crinoid lacking arms, was not successful through time. Thus, although more than one heterochronic pathway was possible, functional and ecologic constraints determined which developmental pathway would have long-term success.