TRACE FOSSILS OF THE AZTEC SANDSTONE IN SOUTHERN NEVADA INCLUDING AN UNDESCRIBED MAMMALOID ICHNOTAXON

The Aztec Sandstone represents the southwestern margin of the great Jurassic Navajo-Nugget-Aztec sand sea. A diverse ichnofauna has long been known to occur in the Navajo and Nugget formations, and in 2006 multiple ichnotaxa were reported from the Aztec Sandstone in the Mescal Range of eastern California. Anomalously, however, prior to 2011 only one ichnotaxon of trace fossils had been reported in the widely exposed Aztec Sandstone in southern Nevada; that occurrence consisted of multiple trackways of the therapsid track Brasilichnium at a single site in Valley of Fire State Park.

The known track record in the Aztec Sandstone of southern Nevada has increased dramatically in the past five years. In 2011 hikers reported the discovery of poorly-preserved dinosaur tracks in Red Rock Canyon National Conservation Area (RRCNCA). Publicity about this discovery soon led to the discovery of other tracksites. This in turn has led to the systematic search for―and documentation of―fossil tracks and trackways in RRCNCA, Valley of Fire State Park, and the Gold Butte region. During the past four years we have been documenting and studying these tracksites. We provide here an update of the trace fossils that have been documented to occur in the Aztec Sandstone of southern Nevada.

A total of approximately three dozen tracksites are now known to be present in the Aztec Sandstone of southern Nevada, with eight ichnogenera represented. These eight ichnogenera include two dinosaur ichnotaxa [Grallator and an unidentified theropod(?) track], two mammaloid ichnogenera (Brasilichnium and an undescribed form), and four arthropod ichnogenera (Entradaichnus, Octopodichnus, Paleohelcura, and Planolites).

Generally speaking, this ichnofauna is a very familiar one in the Navajo-Nugget-Aztec erg. Among the vertebrate tracks, mammaloid tracks are much more common than dinosaur tracks. We attribute the relative rarity and low diversity of dinosaur tracks to a paucity of interdune deposits in the Aztec Sandstone compared to the Navajo. One of our mammaloid track types is a small one with a track width of approximately 1 cm and a stride of 8 to 10 cm. These tracks are smaller than the range we have documented for Brasilichnium, and we tentatively recognize them to represent an undescribed ichnotaxon.