Abstract

Trace fossils of basal, apterygote (wingless) insects from the Pennsylvanian-aged Rhode Island Formation of southeastern New England include the body imprint Tonganoxichnus buildexensis and the trackways Mitchellichnus cf. ferrydenensis, Siskemia elegans, Stiallia pilosa, and Stiaria intermedia. Trackways with double and triple medial impressions (S. elegans and M. ferrydenensis) are rare among trace fossils attributed to such insects. To determine how these unusual trackway morphologies were produced, and why they are so rare, experiments were conducted over a range of media ( = sedimentary substrates) with modern archaeognathans (jumping bristletails) and thysanurans (silverfish). Our experimental results suggest that archaeognathans produced the fossil body imprints, as well as trackways exhibiting opposite symmetry, but cannot rule out thysanurans for trackways exhibiting alternate symmetry. The results also show that a variety of appendages leave medial impressions. The rarity of fossil trackways with more than one medial impression results from the low mass of the animal and the delicate nature of the appendages making such structures, coupled with behavior (jumping and landing) and media properties (saturation and cohesiveness). The production of different numbers of medial impressions by a single archaeognathan species suggests that fossils should show similar variety, albeit in rare cases, and that the named ichnotaxa are likely derived from a single biological taxon. Finally, the experimental work provides new insights into the production and preservation in Stiallia and Tonganoxichnus.

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