LARGE, COMPLEX BURROWS ARE FOR TERRESTRIAL INVERTEBRATES TOO: NEOICHNOLOGY OF PANDINUS IMPERATOR AND HETEROMETRUS SPINIFER (SCORPIONES: SCORPIONIDAE)

While predatory arthropods constitute a large portion of the burrowing fauna in most terrestrial environments, their subsurface biogenic structures are relatively unknown in the fossil record. In many cases the size and complexity of the biogenic structures produced by these animals are greater than expected. Scorpions have comprised a significant portion of the diversity of predatory arthropods since the Late Paleozoic. Many of these animals are active burrowers in the modern and likely have a substantial, if unrecognized, trace fossil record. This project involved the study of the burrowing behavior and trace morphologies of the scorpions Pandinus imperator and Heterometrus spinifer (Scorpiones: Scorpionidae). Groups of five animals of each species were placed into sediment-filled terrariums under stable temperature and moisture conditions for six-week periods after which the open burrows were cast with plaster, excavated, and described. Descriptions of the subsurface structures included basic architecture, dimensions, bioglyphs, complexity, and tortuosity. Additional experiments were conducted with differing soil composition, compactness, and moisture levels to evaluate the animal’s behavioral response to altering environmental conditions. Both species burrowed by excavation using the first three pairs of walking legs. Burrow openings were consistently triangular in appearance. The subsurface biogenic structures produced consisted of subvertical ramps, helical burrows, and interconnected burrow networks. All burrow types contained a laterally expanded chamber. Tunnels averaged 10 cm in width while chambers were up to 15 cm wide. Both tunnels and chambers were 4 cm high on average. The burrow elements were elliptical in cross section with concave floors and ceilings. Increasing sediment density and moisture increased the complexity and tortuosity of the burrows. Decreasing these variables reduced the complexity of the subsurface structures and reduced the likelihood of their preservation as a result of gravitational collapse before the end of the experimental period. Data collected from these and similar experimental studies can be applied to ichnofossil assemblages found in continental paleoenvironments in order to better interpret the paleoecology of ancient soil ecosystems.