FUNCTIONAL MORPHOLOGY OF COILED ANAL SACS IN LATE ORDOVICIAN CRINOIDS

Studying the functional morphology of fossil organisms is vital for deciphering their structure and evolution. Anal structures of crinoids are often a focus of study because of the wide variety of forms from simple cylindrical anal sacs to massive spiked tubes. The Late Ordovician disparid Ohiocrinus has a distinctive anal sac, which tightly coils. Surveys of museum collections have revealed that three additional Late Ordovician crinoids also have coiled anal sacs, including two disparids (Anomalocrinus and Daedelocrinus) and a cladid (Merocrinus). These features vary in the tightness, number, and thickness of the coils. In addition, recent phylogenetic analyses indicate that these coiling anal sacs evolved independently. The convergent evolution of these structures strongly suggests an adaptive origin. Three hypotheses relating to the adaptive advantage of a coiled anal sac are evaluated. First, that the coiled form of the anal sac reduced instances of parasitism. Parasitism by snails has been suggested as the impetus for the evolution of several anal structures in crinoids. This hypothesis is unlikely because the crinoids in the current study are concomitant with the first reported association of snails with crinoids. Second, that anal sac coiling lengthened the gut, which resulted in increased nutrient extraction. The ratio of the surface area of the anal sacs compared to the volume of the calyx was calculated for each species and compared to a co-occurring crinoid with a cylindrical anal sac (Iocrinus). Preliminary results indicate a higher ratio in Anomalocrinus, Daedelocrinus, and Merocrinus than Iocrinus, although the ratio of these three crinoids is within the range seen in Iocrinus. The anal sac of Ohiocrinus has a tighter coil with more whorls, which results in a significantly higher ratio than any of the other animals examined. This hypothesis may explain the anal sac of Ohiocrinus, but not the other three crinoids. However, examinations of more specimens are required to confirm this result. Third, that the coiled anal sac affects flow patterns surrounding the filtration fan. To test this hypothesis, 3-D digital models will be constructed with both cylindrical and coiled anal sacs and differences in flow patterns will be explored using computational fluid dynamics.