INTERPRETING THEROPOD COMMUNITY DYNAMICS AND DISPELLING THE MYTHS OF THE SUNDANCE VERTEBRATE ICHNOFAUNAL PROVINCE: COMPARISON OF BATHONIAN DINOSAUR TRACKSITES IN THE BIGHORN BASIN, WYOMING

Jurassic strata exposed along the eastern flank of Wyoming's Bighorn Basin contain a plethora of vertebrate fossil localities, including world-famous dinosaur bonebeds and tracksites. Near the town of Shell, the sporadic exposures of a limestones facies at the top of the Canyon Springs Member of the Lower Sundance Formation (i.e.,"Sundance Footprint Bed") preserve thousands of delicate, gracile tracks of theropod dinosaurs. Extensive study of a number of these Middle Jurassic tracksites reveals a complex interrelationship between track morphologies, track densities, and microenvironments. As a result, numerous track morphotypes, which might appear to represent multiple ichnogenera, actually represent the unique preservational, deformational, and multiple age/gender morphologies of a single and possibly new ichnotaxon of theropod dinosaur. Many of the tracksites contain high concentrations of tracks (up to 18 tracks per square meter). However, most exposures of the track-bearing unit are devoid of footprints. Thus, extrapolations of the number and extent of tracks need to be made with caution. Although not extensive enough to be considered a megatracksite, the moderate to high dinoturbation of the "Sundance Footprint Bed" is referred to as a "High Density Track Assemblage." Due to common ichnofaunal content, similar depositional environment, and correlative stratigraphic position, these tracksites are grouped into the newly defined "Sundance Vertebrate Ichnofaunal Province." This province may represent an endemic fauna of theropods in North America during the Bathonian. As changes in track morphology and density occur in this ichnofaunal province, so also do the apparent trackmaker interactions and activity patterns. Behavioral interpretations about the trackmakers are speculative, but valuable information about the family structure and community dynamics of a large population of gregarious, carnivorous dinosaurs living along the shores of the Sundance Sea has been provided by intensive ichnological documentation. Research continues on numerous individual tracksites in the "Sundance Vertebrate Ichnofaunal Province," as each appears to tell a different story about the complex and fluctuating paleoenvironment of a previously unknown dinosaur population.