ICHNOLOGICAL EVIDENCE FOR FAMILY GROUPS OF MIDDLE JURASSIC DINOSAURS IN WYOMING: EVIDENCE FROM FOOTPRINTS AND GROWTH RATES OF EMUS AND THEROPODS

In Wyoming's Bighorn Basin, the Sundance Vertebrate Ichnofaunal Province (SVIP) preserves the activities of a monotaxonomic community of hundreds of Middle Jurassic (Bathonian), carnivorous dinosaurs on the shores of the ancient Sundance Sea. Within the Canyon Springs Member of the Lower Sundance Formation, thousands of tridactyl pes impressions (assigned to the ichnotaxon Carmelopodus) are arranged into hundreds of trackways, providing evidence for the family structure and community dynamics of a population of dinosaurs. As the result of extensive documentation and analysis at the Red Gulch Dinosaur Tracksite and other localities within the SVIP, these tracks are interpreted as having been made by primitive, tetanuran theropod dinosaurs. These dinosaurs (ranging in age from yearling to adult) appear to have been traveling together and possibly exhibiting parental care. The evidence for gregarious, family groups of theropods implies a precocial nature for the young dinosaurs, as well as proximity to a nesting area. To better understand the meaning of the ontogenetic and behavioral implications of the fossil footprints, the tracks and activities of modern emus (Dromaius novaehollandiae) have been studied. As emus make theropod-like, tridactyl footprints, are gregarious, have precocial young, exhibit parental care, and nest on the ground, they are excellent modern analogues for studying growth and community dynamics of small- to medium-sized carnivorous dinosaurs, such as those that left their tracks in the SVIP. As part of the UW Geological Museum's Undergraduate Research Program in Paleontology, ichnological data were collected from emus of various ages and footprint growth curves developed. These data were compared to previously established growth rate curves for theropod dinosaurs. By integrating these data, the approximate ages of carnivorous dinosaurs represented only by their footprints can be determined for the first time. In addition, not only are emus useful in establishing the ages of extinct trackmakers, observations of their activities and the associated tracks and traces provide valuable information in the understanding of the behaviors preserved in the ichnological record of theropod dinosaurs.