Neoichnology of the burrowing mayfly Tortopus circumfluus and its bearing on the ichnofossil Glossifungites gingrasi

Larvae of the burrowing polymitarcyid mayfly Tortopus circumfluus are recognized for the first time and positively identified as the excavators of simple U– and pouch–shaped, inclined to vertical, burrows in well–consolidated, firmground clay, silt, and sand cutbanks of the Colorado River in Columbus, Texas, USA. Hydro–stone casts of these burrows reveal architectural details that facilitate comparison to other reported modern mayfly burrows as well as ichnofossils attributed to Ephemeroptera. Polymitarcyid burrows from rivers in Poland and the Czech Republic resemble those excavated by T. circumfluus but are larger with a more regular pattern of bioglyphs. Glossifungites gingrasi from Turonian fluvial deposits of central Utah, USA, is architecturally similar to T. circumfluus burrows, although G. gingrasi are larger and nearly horizontally inclined. The large ephemerid mayfly Hexagenia limbata is sympatric with T. circumfluus in the Colorado River and constructs sinuous U–, J–, and S–shaped burrows in poorly consolidated, softground deposits in deeper sections of the channel thalweg. If encountered in the rock record, H. limbata burrows might be referred to Arenicolites variabilis. Detailed investigations of extant mayfly burrows while thus far rare, can inform not only interpretations of the rock record, but also conservation assessments for imperiled taxa.