THE RECOGNITION AND IMPLICATIONS OF THE WOOD-BORING TRACE FOSSIL ASTHENOPODICHNIUM XYLOBIOTUM IN UPPER CRETACEOUS STRATA OF GRAND STAIRCASE ESCALANTE NATIONAL MONUMENT, UTAH

A prolific, macroscopic, wood-boring trace fossil is present in the Late Cretaceous upper and capping sandstone members of the Wahweap Formation and the Kaiparowits Formation in Grand Staircase-Escalante National Monument, Utah. Detailed examination of these borings warrants assignment to Asthenopodichnium xylobiotum (Thenius, 1979). The morphology and paleoenvironmental occurrence of these Asthenopodichnium are discussed and contrasted with the similar wood-boring ichnogenus Teredolites, which is widely considered a marine/estuarine trace fossil.

The scoop-shaped, pouch-like traces occur as ferruginous casts of tree bark and are found throughout both braided and meandering fluvial deposits in the Wahweap and Kaiparowits formations. The long axis of the trace is typically parallel with or sub parallel to the preserved wood grain. The length ranges from 5.0 to 29.0 mm, width from 1.6 to 9.0 mm, and depths from 0.8 to 6.7 mm. Although found as isolated individual traces, they characteristically occur in dense clusters often with superimposed pouches.

Asthenopodichnium differs from Teredolites by: 1) parallel versus perpendicular orientation of the trace elongation direction relative to the wood surface, 2) scoop-like form versus clavate shaped tube, and 3) semi-circle versus circular cross-section. Asthenopodichnium has been attributed to the freshwater boring of mayfly larvae (Ephemeroptera), although amphipods, isopods and dragonflies are also possible producers. Teredolites, a clam boring, is indicative of marine or estuarine deposits, whereas Asthenopodichnium has only been reported in freshwater fluvial deposits. Caution must be exercised in identifying these two distinct borings because of their specific environmental implications.