LIFE IN A SAND SEA: BURROWS EXCAVATED BY MAMMALS OR THERAPSIDS IN THE NAVAJO SANDSTONE AND THEIR ASSOCIATION WITH OTHER ORGANISMS REPRESENTED BY TRACE FOSSILS IN A WET DESERT ECOSYSTEM

Enigmatic cylindrical structures in eolian cross-bedded Navajo Sandstone near Moab, Utah, are interpreted as mammal or therapsid burrows based on architectural and surficial morphologies. Four localities were studied where cylindrical casts, associated trace fossils, and stratigraphic sections were measured and described to interpret depositional environments and postdepositional histories of those units. In three localities cylindrical casts occur below carbonate beds interpreted as interdune lake deposits. The horizons that contain the cylindrical casts destroy the cross bedding of the Navajo Sandstone. Casts occur in mounds measuring 33 m by 22 m, and ~1 m above present-day horizontal weathered surfaces. Burrow casts are preserved in full relief and crosscut bedding. Architectural morphologies include sinuosity, Y- and T- branching, spiral ramps, and chambers. Burrow casts are elliptical in cross section with an average of 9.3 cm in diameter and 4.2 cm high, and are infilled with structureless sand. Smooth walls dominate the cast surfaces. One particular cast preserves important architectural and surficial morphologies as it passes from eolian sandstone into interdune mudstone. The upper portion of the ~6.2-m-long cast is preserved poorly in the sandstone. In the mudstone, the cast exhibits well-preserved bilobate morphology; the diameter is ~35 cm, the height is ~20 cm, and the walls preserve a series of thin, inclined scratch marks from about mid-height of the wall to the base of the wall along the bilobate floor. Multiple individuals living together in social groups best explains the highly complex Navajo burrow systems, rather than one individual making multiple, interconnected tunnels. These burrows are most similar to burrows of extant social mammals (e.g., naked mole rats, prairie dogs) and to Early Triassic therapsid burrows from South Africa based on the complex architectural morphology. Traces associated with the burrow casts are attributed to rhizoliths, dewatering pipes, arthropod backfilled burrows (Naktodemasis), and termite nests (Termitichnus) based on their architectural and surficial morphologies. Environments represented by the Navajo Sandstone were thought to be devoid of life, but these traces indicate the presence of a viable ecosystem, possibly due to pluvial episodes.