Swimming through the substrate: the neoichnology of Chalcides ocellatus and biogenic structures of sand-swimming vertebrates

Neoichnological experiments involving a species of sand-swimming skink, Chalcides ocellatus, demonstrate the diversity of biogenic structures produced by desertdwelling lizards. The skinks were placed into terrariums with ten, 1-cm-thick layers of fine-, medium-, or coarse-grained sand for periods of 7 to 14 days. Sediment moisture content was held constant at either 0% or 20%. The sand skinks exhibited locomotion, escape, and resting behaviors. They burrowed to a maximum depth of 4.2 cm with most activity restricted to the upper 2 cm of the sediment. The biogenic structures produced included surface mounds and depressions; layer truncations; surficial and subsurficial sinuous, bilobate trails; isolated-to-connected, U-to V-shaped divots; flame structures; and open burrows. The size and diversity of biogenic structures produced by C. ocellatus changed with variations in sediment grain size and moisture content. Biogenic structures tended to be better defined in fine-grained sand. The width of the isolated U-to V-shaped divots was greater in the medium-and coarse-grained sand. Connected U-to V-shaped divots only occurred in the dry, fine-and medium-grained sand whereas open burrows were only produced in moist sand. While engaged in sand swimming the skinks altered grain sorting, loosened the sediment fabric, increased porosity and permeability, and added organic matter in the form of food remnants, fecal material, and shed skin. Data collected in this study can be used to better interpret terrestrial trace fossil assemblages in arid, eolian paleoenvironments to better assess the paleoecology of these ancient settings.