ETHOLOGICAL REINTERPRETATION OF STIALIA PILOSA BASED ON NEOICHNOLOGICAL EXPERIMENTS WITH JUMPING BRISTLETAILS (ARCHAEOGNATHA)

Stialia pilosa Smith 1909 was erected for Devonian traces consisting of two parallel rows of scratch marks and was interpreted as an arthropod feeding trace. Subsequent workers (e.g., Mangano et al. 1998; Minter and Braddy, 2009) have identified these traces in Carboniferous and Permian deposits, where they are associated with archaeognathan resting/jumping traces called Tonganoxichnus and trackways called Stiaria (not to be confused with Stialia). The similar dimensions of the known archaeognathan traces and associated S. pilosa suggested to these workers that these younger occurrences were also produced by bristletails, and they also have been interpreted as feeding traces. Here, we report on neoichnological experiments with extant jumping bristletails (Trigoniophthalamus alternatus) that bear on the interpretation of Stialia. When placed in wet mud (silt and clay mixed with ~40% water by weight), modern jumping bristletails occasionally produced traces indistinguishable from S. pilosa, but these traces did not result from feeding behavior. Rather, they were produced when the animals walked through the mud, dragging their legs to produce the parallel rows of scratches. In one case, an animal became mired and freed itself by clawing at the substrate with its legs, producing three successive traces resembling Stialia pilosa. These modern traces can be thought of as combined locomotion and locomotion/escape traces. The similarity of experimentally produced traces to the fossils suggest that at least some fossil examples may be the result of locomotion and escape, rather than feeding, behavior.