SWIMMING THROUGH THE SUBSTRATE: NEOICHNOLOGY OF THE SAND SKINK CHALCIDES OCELLATUS

Skinks are the most diverse group of lizards, and are identified from body fossils as early as the Cretaceous. Although most skinks are ground dwellers or burrowers, little is known about the biogenic structures they produce. The biogenic structures and burrowing behaviors of an extant species of sand-swimming skink, Chalcides ocellatus (Squamata: Scincidae), was studied in a laboratory setting in order to determine the morphology of the traces produced, and to apply these modern traces to the study of ichnofossils. Chalcides ocellatus is a generalist sand swimmer that inhabits arid and semi-arid environments in northern Africa, southern Europe and western Asia. The objectives of this research were to determine the morphology of the biogenic structures produced by sand-swimming skinks and to evaluate the effect of environmental factors on their burrowing behaviors and the resulting biogenic structures. Specimens of C. ocellatus were placed in 10 and 30 gallon sediment-filled terrariums. The specimens were exposed to variations in sediment composition and moisture within the normal ranges of the species. The sediment consisted of 1 cm layers of fine-, medium-, or coarse-grained sand. Sediment moisture was kept constant at either 0% or 20%. The skinks used two methods of burrowing, sand-swimming and excavation. Fast and slow sand-swimming were observed in fine- and coarse-grained sand, respectively, while excavation occurred in the presence of obstacles such as rocks or terrarium walls. Biogenic structures were produced by C. ocellatus 1-3 cm below the sediment surface, and included general sediment mixing and homogenization as well as changes in grain sorting. Soft sediment deformation structures consisting of isolated and connected concave divots, layer truncation, vertical tunnels, and internal mounds. Surficial and subsurface sinuous bilobate traces were also produced. The laboratory study of the burrowing behaviors of C. ocellatus can be used to aid in the identification of ichnofossils produced by other sand-swimming vertebrates, to aid in the interpretation of the paleoenvironmental conditions in which the biogenic structures were produced, and to determine the role of small reptiles in pedogenesis in arid and semi-arid environments.