WHAT IS EDRIOCRINUS?

Edriocrinus Hall, 1859, is an enigmatic stemless crinoid genus currently classified in the crinoid subclass Flexibilia. The unusual features of Edriocrinus include stemlessness and a cup made of fused plates, reinforced with unusually thick laminae of calcite secreted distally during life. Cups may be conical, hemispherical, sacculate, pyriform, or attached. Forms assigned to the genus are known from high-energy, Lower and Middle Devonian, carbonate and clastic rocks in central and eastern United States, south-central Europe, and northern Africa. The genus is assigned to the subclass Flexibilia based on the shape and width of the uniserial, non-pinnulate, incurving arms known from only two of the 13 named species; neither of the arm-bearing species is the type species. The only characteristics that bind all of the species together are stemlessness and a free-living or attached life mode. New scrutiny of type material suggests that Edriocrinus shares more morphologic features with the cladid crinoids than with the flexibles. For example, the large infrabasal and basal plates of some Edriocrinus species are more similar to those of the cladids. Additionally, the few preserved arms of Edriocrinus lack the characteristic tongue-and-groove, patelloid processes typical of flexible arms.

Ongoing work indicates that the only true Edriocrinus is the type species E. pocilliformis, on which arms are not preserved; it is only preserved as small hemispherical cups of fused infrabasal/basal plates. As stemlessness and fused infrabasal/basal plates are inadequate taxobases to place Edriocrinus in any crinoid group, Edriocrinus should be considered incertae sedis within the Crinoidea. The species with arms (E. sacculus; E. holopoides) have similar arms, may be related, and could be flexibles, but they cannot be linked to the original genus concept of Edriocrinus. Hence, a new genus designation is needed for these forms. The 10 remaining species all lack preserved arms; they cannot currently be associated with flexibles. Only stemlessness and fused plates link them together, and as these are not suitable taxobases, for the time being they must also be considered incertae sedis. Clearly, all “Edriocrinus” species are enigmatic, but the convoluted history of taxonomy has only added to the confusion.