ARE CONTINENTAL TREPTICHNUS FEEDING BURROWS?

Based on its three-dimensional geometry, the ichnogenus Treptichnus is often interpreted as a feeding trace, or fodinichnion (e.g., Vallon et al., 2015). Various animals make Treptichnus-like traces in modern environments, so hypotheses about the behavior of the tracemaker can be tested with direct observation and experimentation. For example, Treptichnus occurs in marine settings, and Vannier et al. (2010) presented neoichnological evidence that these forms could have been made by priapulids, perhaps while searching for prey. Treptichnus is also found in continental settings from the Carboniferous onward, and modern Treptichnus-like burrows in similar settings are made by the larvae of multiple families of flies (e.g., Muñiz Guinea et al., 2014). These flies have a distinctive lifestyle marked by feeding, locomotion, and pupation (Denlinger and Ždárek, 1994). In many of these flies, burrowing does not occur during the feeding phase, but rather as the animal is in search of a suitable location to pupate. Once a suitable site is found, larvae of some species spiral inward toward the site, thus offering a possible search criterion for identifying fossilized pupation traces. We have observed spiraling Treptichnus from the Jurassic of the Hartford Basin in Massachusetts, and these fossils may represent pupation sites. The burrowing behavior of the modern flies suggests that at least some continental occurrences of Treptichnus are locomotion, rather than feeding, traces that were produced by insect larvae as they searched for a pupation site. The locomotion traces produced by these insect larvae also indicate that caution should be used when inferring ethology from burrow geometry alone.