PLATE SHAPE VARIATION AND CONSTRUCTIONAL MORPHOLOGY IN THE ORDOVICIAN CRINOID ANOMALOCRINUS

Early Paleozoic crinoids are sessile, mostly attached, filter-feeders with a large degree of subdivision and specialization within this niche. One such specialized mode of life is that of large-bodied, long-stemmed, long-lived crinoids that live in a given environment over several generations. Commonly this niche is occupied by camerates, which have a large degree of flexibility in their body plan. The Cincinnatian disparid Anomalocrinus also appears to occupy this niche in the Late Ordovician, however, its cup construction is much more rigid. Anomalocrinus is a giant among co-occurring crinoids growing over a meter tall and also displays unusual morphology in its holdfast, stem, calyx, tegmen, and arms. Many of these anomalous features can be traced to adaptations to this particular niche, particularly the features of the cup. As this lineage evolved a larger body size, the cup changed shape from a slender cylinder (as seen in Ectenocrinus or Cincinnaticrinus) to the bowl-shape of Anomalocrinus in order to house an increase in the visceral mass. This change in size and shape is easy to accomplish in camerates by adding small plates to the cup, but difficult for Anomalocrinus, whose cup is limited to thirteen plates thus creating constructional difficulties. If the aforementioned scenario is accurate, these constructional limitations should cause an increase in abnormally plated individuals as well as an increase in the variation in the shape of cup plates in Anomalocrinus compared with co-occurring disparid crinoids.

To test this, several specimens of Anomalocrinus and other co-occurring disparid crinoids were examined. Individual plates were traced for each specimen using a camera lucida and these outlines were quantified using ImageJ. The collected points were then scaled and re-orientated for each homologous plate. The first seven elliptical Fourier descriptors of the plate outlines were calculated and then used to construct a morphospace using principal coordinate analysis. Preliminary results indicate that there is higher variation (~60%) in plate shape in Anomalocrinus’ radial circlet than is present in that of Ectenocrinus which is consistent with the constructional morphology model that we proposed for the evolution of Anomalocrinus into its preferred ecological role.