NEOICHNOLOGIC TRACKWAY EXPERIMENTS USING THE FRESHWATER CRAYFISH ORCONECTES

This study records the different types of trackways produced by freshwater crayfish in a controlled environment. Different trackway morphologies were created through the variation of media (i.e., sediment) grain size, moisture level, and surface inclination in terrestrial and subaqueous settings. Sediment was sieved into clay and silt, very fine-, fine-, and medium-grained sand. A rectangular container 26 cm wide, 52 cm long, filled with 4-cm-deep sediment was used to run the trackway trials. Media-moisture content for the terrestrial trials was determined by removing a sample of moistened sediment and weighing it before and after heating in an oven for 24 hours. Moisture was recorded in percent water weight. The container was inclined 10° to determine if and how trackways varied with respect to those created on flat surfaces. All subaqueous tracks were made in media in a 20 cm by 35 cm container on a flat surface.

Media-moisture content had the greatest affect on trackway characteristics. Trackway preservation of individual appendage marks decreased as grain size increased. Trackway preservation of individual appendage marks decreased as moisture content increased. The tail of the crayfish was always in contact with the media during the terrestrial trials. Dry clay and silt preserved the most detailed trackways. Intermediate moisture levels created a firm surface which decreased trackway preservation. Crayfish moving through soupy sediment produced continuous furrows to a depth of 1 cm made by the drag of the tail; appendages produced zigzagged furrows. Trackways of crayfish moving down the inclined surface had the deepest impressed appendage marks, including impressions of the chelae, compared to moving up the incline. Subaqueous trackways are distinctively different from trackways produced in terrestrial trials. Individual appendage marks overall are more clearly preserved in subaqueous trackways in fine and very fine-grained sand. Tail drag marks are rare in subaqueous trackways owing to the buoyancy of the crayfish. Results from this study can be used to identify trackways of ancient crayfish. The condition of the trackway may be used to infer the original amount of moisture present at the time the trackway was produced.