PALEOECOLOGY AND TAPHONOMY OF TRACE FOSSILS IN THE EOLIAN UPPER TRIASSIC/LOWER JURASSIC NUGGET SANDSTONE, NORTHEASTERN UTAH

The Upper Triassic/Lower Jurassic Nugget Sandstone represents a portion of a vast inland erg of eolian sand dunes that were populated by diverse vertebrates and invertebrates, represented primarily by their trace fossils. Body fossils are rare, making the trace fossil record essential for deciphering the paleoecology of the ancient dune system. Trace fossils in the Nugget Sandstone near Vernal, Utah, include invertebrate burrows (Entradichnus meniscus, Entradichnus isp., Planolites beverleyensis, Taenidium isp. "A", Taenidium isp. "B", Skolithos, and Planolites isp., burrow clusters, large oblique burrows, flared burrows) and trackways (Paleohelcura and Octopodichnus). Arthropods (insects and arachnids) probably were the tracemakers of most, if not all, the trace fossils. Sediment moisture must have played a key role in the production and preservation of the trace fossils, indicating that moisture was important for supporting such complex ecosystems. Extended wet climatic intervals must have persisted intermittently between arid intervals. New fossil evidence for plants (sphenophytes, cycads, and algal buildups), ichnologic evidence of herbivorous insects and carnivorous arachnids, as well as indirect evidence for environmental moisture content during deposition of the Nugget Sandstone, provide a picture of the paleoecology of this ancient sea of sand.