DEVELOPMENTAL BREAKDOWN DURING THE EARLY EVOLUTION OF THE CODIACRINIDAE

Recognition that Parazophocrinus Strimple is not a disparid crinoid but a cladid belonging to the Codiacrinidae indicates that the a breakdown in the strictures of development occurred during the early evolution of this family. The entire family is characterized by paedomorphic crinoids, however Parazophocrinus is neotenic, whereas the other members of this family are progenetic.

The Codiacrinidae ranges from the Silurian through the Permian and is characterized with cladid microcrinoids regarded as paedomorphic. These typical codiacrinids are very small (commonly less than nine mm), may lack arms, may have the anal opening from the side of the aboral cup. The small size and arrested development is considered progenetic. In contrast, Parazophocrinus is large in size (maximum diameter of 29 mm), but it also has arrested development of the aboral cup. Parazophocrinus is composed only only infrabasal plates, basal plates, radial plates, and orals; arms are absent; and the anal opening is along the side of the aboral cup. The radial plates are hypertrophied laterally to produce a disc-shaped aboral cup. This very unusual crinoid is considered to be neotenic.

Parazophocrinus is the second oldest genus in the Codiacrinidae. Apparently, when "developmental rules" broke down early during the evolution of this family, both neotenic and progenetic crinoids evolved. Even though Parazophocrinus evolved, this morphology a large crinoid without arms was ultimately not successful through time. Presumably feeding in Parazophocrinus only occurred with tube feet projected from between opened oral plates or from epidermal nutrient uptake.

Evolution of both progenetic and neotenic organisms in the same lineage is not common. Is it possible that when "developmental rules" ease that any development is possible but that functional and ecological constraints typically do not permit various developmental deviations within a lineage?