HYDRODYNAMICS AND ADAPTIVE SIGNIFICANCE OF COILING ANAL SACS IN ORDOVICIAN CRINOIDS
To explore the hydrodynamic function of these structures, we constructed 3-D digital models of crinoids in the open-source computer graphics software Blender. We built a standard model and varied the anal structures such that any differences could be attributed to the morphology of the anal sac. Models approximating the morphology of Ohiocrinus, Anomalocrinus, and Merocrinus were contrasted with a model with a cylindrical anal sac as well as one lacking an anal sac using computational fluid dynamics. The coiling anal structures reduced water flow near the filtration fan compared to the null model. In low nutrient environments in which these animals lived, the coiling anal sacs may have allowed more efficient particle capture, which would have resulted in a selective advantage. Cylindrical anal sacs produce comparable hydrodynamic patterns that would provide a similar selective advantage, which may explain the predominance of large cylindrical anal sacs of co-occurring crinoids (e.g.Iocrinus). As crinoids developed greater flexibility and curvature in their arms through the Paleozoic, the hydrodynamic effects of the anal sac on filtration diminished and this adaptation became obsolete.