HYDRODYNAMICS AND ADAPTIVE SIGNIFICANCE OF COILING ANAL SACS IN ORDOVICIAN CRINOIDS

The adaptive and ecological significance of features of fossil organisms are most clearly explored through the study of functional morphology. Computational methods, such as biomechanical analyses, have proven promising for investigating the functional morphology of extinct species. The disparate anal structures of fossil crinoids have long been the focus of study and hypotheses regarding function. These structures have been interpreted as aiding in reproduction, defense, feeding, hydrodynamic stability, and the reduction of fouling. Particularly puzzling are the anal structures of the Late Ordovician disparid Ohiocrinus, which has a tightly coiling anal sac. Surveys of museum collections revealed three additional Late Ordovician crinoids with coiling anal sacs. Although, all four of these crinoids share this feature, their anal sacs vary in height, diameter, tightness, and number of coils. These morphological differences together with recent phylogenetic analyses that show the taxa are not closely related, which strongly indicates that the coiling anal sacs evolved independently and in all likelihood had an adaptive origin.

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.