THE STRUCTURAL AND FUNCTIONAL MORPHOLOGY OF ANOMALOCRINUS; ATTEMPTING TO OVERCOME ANCESTRAL LIMITATIONS

Paleozoic crinoids were largely benthic, sessile, filtration feeders. Within this broad ecological niche, crinoids diversified in terms of their filter morphology, elevation above the substrate, attachment structures, and life history strategy in order to better colonize different environments, utilize different food sources, and presumably reduce competition. One prominent and extensive ecological strategy during the Paleozoic is an attached, long-lived, large-bodied, and long-stemmed crinoid that occupies a given habitat over several generations. This particular niche is often filled with camerate crinoids that have multi-plated calyces that can add circlets of plates to grow to a larger size, such as the Ordovician Cleiocrinus and Silurian Eucalyptocrinites.

Disparid crinoids, which normally have small, cylindrical calyces composed of only 11-20 plates, rarely fill this ecological role with the exception of the Anomalocrinidae. The Anomalocrinidae are a species-poor and relatively rare family of Late Ordovician crinoids. This family is typified by Anomalocrinus (Late Ordovician of the Cincinnatian region), which has a broad bowl-shaped cup, a relatively dense filtration fan, a plated oral surface (tegmen), and stem lengths exceeding one meter. In addition, Anomalocrinus holdfasts are routinely preserved attached to hardground surfaces with varying degrees of taphonomic degradation, indicating persistence in an area over multiple generations.

The anomalous features of the Anomalocrinidae appear to be consequences of a shift in ecological position toward a larger body size. The large increase in body and widening of the cup without an increase in the number of plates appeared to have placed a strain on the constructional morphology of the genus based on an increase in abnormally plated individuals (i.e., the addition of extra plates) compared with co-occurring heterocrinid crinoids. In addition, the change from a cylindrical- to a bowl-shaped calyx required the re-evolution of a calcified tegmen that was previously lost in the disparid clade.