86 Grallator Tracks as a Window to Growth Strategies of Small-Bodied Theropods

Mentor: Randall Irmis (Geology and Geophysics, University of Utah)

Because the soft tissue of non-avian dinosaurs rarely preserves, studies of their ontogenies have been limited largely to skeletal remains. Fossil tracks are far more abundant than skeletal remains and record both locomotory information and impressions of the soft tissue that covered the feet. Thus, they have large sample sizes and can act as proxies for determining growth regimes among living dinosaur feet. The St. George Dinosaur Discovery Site at Johnson Farm (SGDS) in St. George, Utah, USA, has a wealth of tracks preserved on closely-spaced bedding planes. Thus, the site is as close to population sampling as possible. In particular, the SGDS contains a wealth of tridactyl Grallator tracks, which most likely were made by early, small theropods. Tracks were measured using eleven different linear metrics, which were then used to determine whether these early theropods experienced allometric or isometric growth of their feet, and evaluate the degree of individual variation. Analysis of track metrics using bivariate plots and regressions suggests strong, negative allometric linear relationships between the width of the track and the length of the track digits, as well as between anterior width and digit length. Linear relationships between R (distance from the middle of the fourth pad on digit IV to the second pad on digit III) and both total track width and the width between the claws on digits II and IV display more scatter, though they also indicate negative allometry. Independent of size, there appears to be significant variation in the angle between digit II and IV (divarication), suggesting that divarication angle is significantly influenced by substrate and/or locomotory behavior. All measurements in the dataset suggest a single continuous sample, suggesting that the tracks were likely made by a single species or closely related species having similar foot morphologies. These data suggest that as early small theropods increased in size, growth in toe length was slower than growth in foot width. This could indicate that growth progressed to convey larger theropods greater stability to support their increased mass.