EXPERIMENTAL TRACKWAYS OF SCORPIONS, TARANTULAS, AND CRAYFISH UNDER SUBAERIAL AND SUBAQUEOUS CONDITIONS: IMPLICATIONS FOR DETERMINING WATER CONTENT OF FINE SAND AT THE TIME OF TRACKWAY FORMATION

While many invertebrate fossil trackways (ichnites) have been formed by arthropods, there is still a need for more formal, comprehensive, and quantitative neoichnological studies in order to create statistical models that can predict the most probable (paleo)sedimentary environment(s). Here we present part of a new qualitative and quantitative neoichnological analysis using scorpions (Hadrurus arizonensis), tarantulas (Grammostola rosea), and crayfish (Procambarus clarkii) tested on a 60:40% mixture of very-fine and fine-grained sand under a range of sand moisture levels (defining five target conditions: dry, damp, wet-drying, wet-saturated, and subaqueous) and incline angles (0°, 15°, and 25°) when viable to account for variability due to slope. Damp and wet sand moisture content ranged from about 2-5% and 21-23% H₂O, respectively. The sloped wet-drying condition produced a moisture gradient as water percolated down, resulting in stiffening sand from the mid-bottom to the top. The bottom was grouped with the similar wet-saturated condition. But for the sloped wet-saturated condition, to maintain nearly saturated sand on the slope, a ¼” soaker hose was buried at the upslope end, with water output regulated using a peristaltic pump and valves to prevent slumping. The sloped wet-saturated condition gave much more consistent trackway morphology than wet-drying slopes. Scorpions and crayfish made tracks fully submerged (tarantulas could only wade). Trackways were characterized and measurements (standardized by external trackway width) obtained by detailed digital photo and video analysis of 490 symmetric trackway segments. Individual parameter summary diagrams and MDS visualizations show subaqueous walking trackways tend to be morphologically medial to wet-drying and wet-saturated ones, with dry trackways most like wet-saturated ones. In dry and wet-saturated (loose) sand, imprints were broad and tapering. Multifid imprints always occurred in stiff sand (often rare or none in loose sand). Only 8% of scorpion trackways had telson drag marks (never on stiff [and rarely subaqueous] sand). Moving forward, the quantitative data obtained will enable the creation of multinomial logistic regression models, including paleoenvironmental predictions for apparent arthropod ichnites in sandstones.