Small Theropod Track Assemblages from Middle Jurassic Eolianites of Eastern Utah: Paleoecological Insights from Dune Ichnofacies in a Transgressive Sequence

Abstract

New discoveries show that very small theropod tracks (cf. Wildeichnus) are abundant in the upper part of the Moab Member, recently assigned to the Curtis Formation (formerly considered part of the Entrada Formation) in the Mid-?Late Jurassic of eastern Utah. The tracks represent a distinct small-theropod ichnofacies associated with eolian dune deposits that is easily differentiated from the water-lain beds of the overlying Megalosauripus-Therangospodus ichnofacies, which comprises the single-surface Moab megatracksite. Pterosaur track assemblages, representing the Pteriachnus ichnofacies, are found a few meters above the megatracksite surface in the upper tongue of the Summerville Formation.

The small theropod ichnofacies is reminiscent of other early Mesozoic dune facies ichnofaunas from the Wingate and Navajo formations (Late Triassic and Early Jurassic) where small theropod tracks occur in association with other small tetrapod footprints. All such examples evidently represent a recurrent dune facies ecosystem dominated by diminutive vertebrates. Because the small theropod ichnofacies is one of three ichnofacies found in a thin stratigraphic sequence (<20 m) that contains no body fossils, it is clear that vertebrate tracks play an important role in providing insight into the paleoecology of units previously considered devoid of any useful fossil evidence. The three successive ichnofacies represent a transgressive transition from sand dunes, through sandy shoreline to shallow marine environments, each with its quite distinct vertebrate fauna.

Keywords:

• small dinosaur tracks
• Middle Jurassic
• Moab Member
• eolian facies

ACKNOWLEDGEMENTS

We particularly thank Fran and Terby Barnes for taking us to the sites he discovered and generously sharing locality information. We also thank Donna Turnipseed of the Bureau of Land Management (Moab Office) for support in conducting our tracksite survey, which brought several new sites to light. Work was completed under BLM permit number UT-S-04-005. We thank Spencer Lucas and Josh Smith (New Mexico Museum of Nature and Science) for their reviews of this manuscript.