TRACKWAYS OF MODERN BASAL INSECTS (JUMPING BRISTLETAILS AND SILVERFISH) AND THE ORIGIN OF PENNSYLVANIAN ARTHROPOD TRACE FOSSILS FROM EASTERN MASSACHUSETTS

A quarry in Plainville, MA is the source of arthropod traces from the Pennsylvanian-aged Rhode Island Formation, including the body impression Tonganoxichnus buildexensis, which is attributed to a basal, wingless insect (a monuran or jumping bristletail), as well as trackways tentatively referred to Stiaria isp. The trackways consist of alternating sets of two to three foot impressions oriented at an angle to the trackway midline, and a median impression consisting of single, double, or triple drag marks. The alternating sets of three foot impressions and the co-occurrence of T. buildexensis suggest a monuran or related insect lineage might have made the trackways. To test this hypothesis, experimental trackways were produced by wingless insects of the orders Archaeognatha (jumping bristletails; Pedetontus saltator) and Thysanura (silverfish; Thermobia domestica) on smoked glass and in mud. Both P. saltator and T. domestica produced trackways with alternating sets of two to three footprints. However, T. domestica typically produced only a single median impression as a consequence of dragging the medial terminal filament along the substrate; rarely, double or triple median grooves were produced, but they were intermittent and did not resemble the fossils. P. saltator often produced multiple median impressions as the terminal filament, lateral cerci (paired posterior appendages), and/or styli (paired abdominal appendages) dragged along the substrate. For example, a double median impression was produced by the styli, and triple median impressions were produced when the terminal filament and lateral cerci dragged along the substrate. The experimental trackways indicate that bristletails can account for all of the trackway morphologies seen in the fossils. The extinct monurans, which were anatomically similar to jumping bristletails in many respects, remain candidates for having made the fossil tracks. Silverfish similar to T. domestica could have produced trackways with single median drag marks, but the full suite of Pennsylvanian traces cannot be attributed to these derived silverfish. However, less derived silverfish (e.g., Tricholepidion gertschi, the relict silverfish) are also anatomically similar to bristletails and thus cannot be ruled out as trackmakers without further experimentation.