WALKING WITH EMUS: INSIGHTS INTO DINOSAUR TRACKING IN THE 21ST CENTURY

Understanding the behavioral complexities of extinct animals can be highly speculative. Dinosaur trace fossils pose myriad mysteries, as the trackmaker and its interaction with the substrate are typically unknown. However, large, extant, flightless birds can act as proxies for theropod dinosaurs; and the footprints they make show strong similarities to the pes impressions of carnivorous dinosaurs. Various researchers have created models to see the dynamic interactions and subsequent 3-D subsurface signatures of tridactyl feet upon malleable surfaces. Complementary to these studies, are the field observations of the formation of tracks and trackways by modern emus (Dromaius novaehollandiae) and rheas (Rhea americana), providing direct comparison of the actions and activities of trackmakers with the traces they leave. Information gained from these lab and field studies has been utilized in the interpretations of thousands of tridactyl pes impressions (i.e., Carmelopodus) from the Middle Jurassic Sundance Vertebrate Ichnofaunal Province (SVIP) of the Bighorn Basin, Wyoming. Within the SVIP, a 3-4 cm thick, oolitic limestone package of the Lower Sundance Formation preserves a unique multilayered preservational continuum of tracks. The majority of footprints represent undertracks preserved in the lower level of this package. However, some sites also preserve the more fluid, thixotropic, upper levels, which illustrate the collapse and flow of the sediment upon pes removal. In addition to the understanding of track formation, observations of modern ground birds provide insight into the complexities associated with walking gaits. It is now clear that trackways interpreted as belonging to walking dinosaurs may not always represent continuous movement, and thus calculations of walking speeds may be misleading. In reality, many trackways probably represent animals stopping in midstride, which although common in emus will not be observable from trackway evidence. Also, puzzling trackways, such as those with pace angulations greater than 180 degrees (e.g., crossovers) and those with random orientations in restricted areas, are easy to understand when ratites are observed. Finally, observations of modern birds help us understand the spacing, movement patterns, age distributions, and community dynamics which may be represented by vertebrate trace fossils.