NEOICHNOLOGY OF FIRMGROUD AND SOFTGROUND BURROWING MAYFLIES (EPHEMEROPTERA) WITH COMPARISON TO GLOSSIFUNGITES GINGRASI AND OTHER ICHNOTAXA

Plaster and resin burrows casts of modern firmground (Tortopus circumfluus) and softground (Hexagenia limbata) mayfly larvae offer new insights into the paleoenvironmental significance of S-, J-, U-, and pouch-shaped ichnofossils. T. circumfluus in the Colorado River, southeastern TX, USA excavates U- and pouch-shaped burrows up to 35 mm long with a total combined width of 10-16 mm. Individual tubes are 4-5 mm wide and adorned with clusters of short, transverse, linear to cuspate bioglyphs. A poorly developed septum is present in some burrows, separating the descending limbs of the U; however, in most examples, they are connected via a convex excavation without spreiten. Using mandibular tusks, T. circumfluus larvae chisel burrows out of 7-12 ky Holocene firmground clay, silt, and sand along the riverbank at the low water line. Passive fill of fine- to medium-grained sand is common in abandoned burrows, and unoccupied burrows are intersected by those of subsequent generations. H. limbata in the Colorado River and Lake Huron, MI excavate simple, sinuous to U-shaped burrows up to 6.35 mm wide, 114 mm long, and with few bioglyphs in very fine, poorly consolidated silt and organic-rich clay. Both mayfly species’ burrows occur in dense colonies with variable orientation, although the burrow apertures of T. circumfluus are generally aligned parallel to flow direction. H. limbata burrows resemble Arenicolites and possibly Psilonichnus, while those of T. circumfluus resemble Glossifungites gingrasi. In contrast to published examples of G. gingrasi, burrows excavated by T. circumfluus are preserved along channel banks instead of the base of the thalweg and are vertical to inclined, not sub-horizontal. Otherwise, the burrows are very similar such that mayflies cannot be excluded as the maker of G. gingrasi. Burrows of T. circumfluus and H. limbata do not resemble the classic, U-shaped dwellings commonly illustrated as representative mayfly burrows, exemplifying a more diverse and variable range of architectures than is typically assumed. The potential overlap in burrow morphology between mayflies and some marine invertebrates highlights the need for caution when assigning significance to depositional surfaces and inferred paleosalinities based on interpretations of S-, J-, U-, and pouch-shaped burrows.