Neoichnology of Thyonella gemmata: A Case Study for Understanding Holothurian Ichnofossils

Abstract

While the fossil record of holothurians extends from the Cambrian to the Holocene, adequately evaluating their temporal and spatial distribution is difficult due to their poor preservation potential. Several extant holothurians, however, produce abundant shallow burrows that a have a high preservation potential. Neoichnological experiments allow for the direct observation of burrowing behaviors and the resulting biogenic structures. Data obtained from these experiments are invaluable to the interpretation of ichnofossils lacking associated body fossils. The burrowing behaviors of the holothurian Thyonella gemmata were studied in a laboratory setting under varying environmental conditions. Specimens were exposed to variations in grain size, salinity, and water temperature to assess their behavioral response to environmental change and any resulting biogenic structures. Thyonella gemmata burrows by intruding itself into the sediment using muscular contractions and limited use of tube feet. Thyonella gemmata contracts its body into a U-shape and maintains contact with the sediment surface. The resulting burrow is a wide, U-shaped concentration of disrupted sediment with or without spreite. Sediment size had the greatest effect on burrowing activity and morphology; individuals were able to easily burrow into fine- and medium-grained sand, but experienced difficulty in coarse-grained sand. Altering water temperature yielded no significant results. While increasing salinity had no effect, lowering salinity had an adverse physiological effect on the specimens and inhibited burrowing. Studying the varying burrow morphologies produced in these experiments will aid in the interpretation of potential holothurian ichnofossils and the interpretation of paleoenvironmental conditions.