First Report on Reptile Tracks from the Moenkopi Formation (lower-?middle Triassic) in Dinosaur National Monument, Utah

The Lower-Middle Triassic Moenkopi and Red Peak formations, which crop out throughout western North America, contain ubiquitous occurrences of tetrapod tracks attributed to reptiles (Peabody, 1948; Klein and Lucas, 2010; Lovelace and Lovelace, 2012). Although reptile skeletal remains from these formations are rare to non-existent, and largely restricted to the Holbrook Member in Arizona (e.g., Heckert et al., 2005; Nesbitt, 2005) and the Anton Chico Member in New Mexico (Schoch et al., 2010), this track assemblage indicates that a medium-high diversity reptile fauna must have existed in this region during Early-Middle Triassic times. Tracks thus provide valuable insights into the composition, geographic distribution, and paleoecology of Early-Middle Triassic reptiles in North America that cannot be obtained from the body fossil record alone. Tetrapod tracks from the Moenkopi Formation were first discovered in the Red Fleet Reservoir region west of Dinosaur National Monument (DNM) and north of Vernal by a field crew from the University of California Museum of Paleontology in the 1940s. These consisted of an unidentified trackway showing an undulating tail mark and several extensive surfaces bearing numerous marks interpreted as swim tracks (Peabody, 1956). Additional specimens from these track sites are now housed at the Utah Field House of Natural History in Vernal as well as at the University of Colorado Museum in Boulder. Here we report on three new track localities from the Moenkopi Formation located within DNM (Fig. 1). DNM 481 was discovered in 2010 by Madeline Weigner and Jacob Grosskopf (Geologist-in-Parks GeoCorps interns) in the “racetrack” exposure along the nose of Split Mountain in the Green River District of DNM. DNM 485 and 486 were discovered later by one of us (TJT) in 2011 along this same exposure. Two tracksites occur near the Sound of Silence trail (DNM 481, DNM 485) and another near the Desert Voices trail (DNM 486). The only other record of vertebrate fossils from the Moenkopi Formation at DNM is the putative occurrence of “amphibian bone” near Vivas Cake Hill by Untermann and Untermann (1954), but more specific locality data are lacking and no repository was indicated by those authors. Institutional abbreviations. DINO: Acronym for museum catalog numbers, Dinosaur National Monument; DNM: Acronym for fossil locality numbers, Dinosaur National Monument, Vernal, Utah.; UGS: Utah Geological Survey, Salt Lake City, Utah.