NEW TRIASSIC VERTEBRATE TRACKS FROM THE MOENKOPI FORMATION OF LAKE MEAD NATIONAL RECREATION AREA: A RECORD OF THE OLDEST MESOZOIC TRACKS FROM THE STATE OF NEVADA

Trace fossils are important proxies for biodiversity, paleoenvironmental interpretation, biogeography, and animal behaviors. Terrestrial Mesozoic trace fossils are known from Nevada in the Lower Jurassic Aztec Sandstone and the Cretaceous Baseline Sandstone. As yet there have been no known animal traces of Triassic age from Nevada, and here we present the first known occurrence from the state. The locality initially caught the attention of University of Nevada-Las Vegas geosciences undergraduate, Alex Purcell, who notified Lake Mead National Recreation Area Geoscientist-in-the-Park who confirmed and collected several tracks as float. The tracks are found stratigraphically high in the Moenkopi Formation as exposed in southern Nevada. Many formations from the Lower Mesozoic of Nevada are considered by some to be stratigraphically distinct from their equivalents in surrounding southwestern states; likely an artifact of geographic sedimentary variability across the region.

The traces are found along bedding planes in epirelief. The host sediment is dark brown, very fine-grained sandstone to siltstone with symmetrical ripple marks. Traces are shallow in relief with a least one track-maker being quadradactyl. Some shallow claw impressions are reminiscent of swimming tracks from Triassic units in neighboring Utah.

By measuring a detailed stratigraphic section of the Moenkopi Formation throughout the area, combined with sedimentological data, we conclude that the tracks were produced in a tidal flat depositional paleoenvironment. Interbedded with the ripples, trace fossil-bearing beds are finely-laminated mudrocks more indicative of subtidal deposition. Photogrammetry will be used to digitally model the tracks in three dimensions. The models can reveal details of the fossils not apparent to the naked eye. Due to the partial preservation of tracks from this site, ichnotaxonomic identifications cannot be made, nor potential track-makers, at this time. However, these tracks do represent the oldest Mesozoic vertebrate traces from Nevada, and some of the western-most terrestrial Triassic traces in North America expanding the record of Moenkopi ecosystems further afield. Further work in the region will likely reveal additional tracks making identifications possible.