A COMPUTATIONAL FLUID DYNAMIC APPROACH TO POSTURE MODELING IN GLYPTOCYSTITOID RHOMBIFERANS (ECHINODERMATA)

As glyptocystitoid rhombiferans are an extinct group of Paleozoic echinoderms without an extant morphologic analogue, it is necessary to create models to examine possible posture stability. The bauplan can be divided into brachioles attached to a theca and a stem holding the theca above the sediment interface for suspension feeding. In a few derived taxa, the stem is cemented to the substrate in a sessile life mode. Most of these animals have a stem strongly specialized for motility that changes in size, shape, and flexibility along its length. The proximal portion is highly flexible and bears alternating inner and outer heteromorphic columnals. Spiraling pivot points of the proximal portion confer rotational movement along the stem, allowing for increased flexibility and a greater range of physiologically possible postures. The stiffer distal stem has barrel shaped homeomorphic columnals and likely functions as support.

There is much controversy in the literature over how these organisms lived and moved. Three hypothesized positions for the distal stem are found: 1) the linear posture, where the distal stem extends parallel to the substrate, 2) the bent posture, where an s-bend is introduced in the distal portion, and 3) the snake posture, where the distal stem curves around in a C shape. In scaled clay model experiments, the snake model was found to be the most stable posture. Fluid dynamic forces increase in aquatic environs, so clay and air are non-analogous to water and skeletal materials. Herein, a computational method was used to assess posture models in a more controlled and comparable simulated environment.

Three posture and one control model were built in the ANSYS suite using Cheirocystis fultonensis for scaling. The theca, brachioles, and proximal stem were kept consistent. Models were tested oriented 0, 45, 90, 135 and 180 degrees to a unidirectional flow. Inlet flows of u= 0.05 to 0.5 m/s were used for each model and orientation. Model evaluation utilized drag and lift forces as a metric of overturning verses stability. From these results, a suite of possible postures was created by ranking postures based on the number of model failures in different flow speeds and directions.