MIDDLE JURASSIC DINOSAUR COMMUNITY DYNAMICS IN NORTHERN WYOMING: THEROPOD FAMILY VALUES

Although evidence of gregarious theropods is relatively rare, extensive, state-of-the-art vertebrate ichnology studies at the Red Gulch Dinosaur Tracksite (RGDT) provide exciting insights into the behavioral complexities of a unique Middle Jurassic dinosaur community. In the eastern Bighorn Basin of Wyoming, thousands of tridactyl pes impressions are preserved in a ripple-bedded, oolitic, limestone of the Canyon Springs Member of the Lower Sundance Formation. At the RGDT (UW V-98066), the activity patterns of over 100 small- medium-sized carnivorous dinosaurs (ranging in hip height from approximately 32-120 cm) are preserved. Irregular step lengths, variable straddle widths, and swerving, parallel trackway paths may relate to variations in substrate microenvironments, tidal cycles, and intracommunity dynamics. In addition, dramatic differences in track morphology both within and between scores of distinct trackways (ranging from 2 to 45 steps) may reflect lateral and vertical substrate variations, differential preservation and weathering, variable track generation episodes, ontogenetic variability, and individual trackmaker characteristics. Multivariate analysis of the ichnology data supports interpretations about the family structure and community dynamics of gregarious dinosaurs walking (and perhaps foraging) in the water-saturated sediments close to the shore of the Sundance Sea. The evidence of family groups of these primitive tetanurine theropods (possibly ranging in age from yearling to adult) implies proximity to a nesting area and the altricial nature of young dinosaurs. Interpretations of the intricate "dance" of these organisms on an ancient tidal flat is fascinating as a "live-action" glimpse of the past becomes clearer through continual, intensive research. The exhaustive documentation of the RGDT (one of the most thoroughly documented dinosaur tracksites in the world) has led to our understanding of the behaviors and family structures of carnivorous dinosaurs on the tidally influenced shores of the Western Interior of North America approximately 165 million years ago.