LARGE ARACHNID TRACKS FROM THE EOLIAN TRIASSIC/JURASSIC NUGGET SANDSTONE, NORTHEASTERN UTAH

The Nugget Sandstone of northern Utah, along with the Navajo and Aztec sandstones to the south, was deposited as part of a vast sand sea. Although these Triassic-Jurassic erg deposits generally lack body fossils, vertebrate and invertebrate trace fossils are more common and can be locally abundant. In northern Utah the Nugget preserves a diverse vertebrate and invertebrate ichnoassemblage. The diversity and associations of these trace fossils provide unique insights for understanding this arid terrestrial ecosystem. A slab preserving five large arthropod trackways and was recently recovered from a flagstone quarry in the Nugget Sandstone near Heber, Utah and donated to the Natural History Museum of Utah by Marshall McFarland. Two trackways belong to the ichnogenus Paleohelcura sp., two to Octopodichnus tridactylus, and one is cf. Octopodichnus raymondi. Both ichnogenera are attributed to arachnids, either spiders (Octopodichnus) or scorpions (Paleohelcura). The largest trackway on the slab is a well-defined Paleohelcura sp. with a maximum external width of 12 cm and an average stride length of 6.2 cm. The smallest is a faint Octopodichnus tridactylus with a maximum external width of 5 cm and an average stride length of 6.5 cm. All the trackways run subparallel, with trackway orientations within 25 degrees of one another. Differences in the degree of preservation of detail between trackways are likely due to factors such as tracemaker size, differences in the moisture content of the substrate, or undertrack fallout. The two best preserved trackways, a large Paleohelcura and a large Octopodichnus, were probably made on the same depositional surface, and show well developed sediment displacement rims and tic traces which indicate the tracemakers were traveling in the same direction either on flat ground or upslope. Although this particular slab only contains invertebrate surface traces, Octopodichnus and Paleohelcura are associated elsewhere in the Nugget with the small tetrapod surface trace Brasilichnium (often all traveling in the same direction) and the burrowing arthropod trace Entradichnus. These associations are typical of the Chelichnus vertebrate ichnofacies, which correlates with the Octopodichnus invertebrate ichnofacies, and represents an eolian dune lower slipface environment.