| Description |
Fossil tracks are far more abundant than skeletal remains and record both locomotory information and impressions of the soft tissues that covered the feet. Thus, they have large sample sizes and can act as proxies for determining growth regimes among extinct non-avian dinosaurs. The St. George Dinosaur Discovery Site at Johnson Farm (SGDS) is from the earliest Jurassic and is located in the Moenave Formation in St. George, Utah, USA. SGDS contains a wealth of tridactyl Grallator tracks, likely made by early small-bodied theropod dinosaurs, preserved on closely-spaced bedding planes. Therefore, the site is as close to population sampling as possible for the paleontological record. I studied 223 tracks to investigate if early small-bodied theropod feet grew allometrically or isometrically and whether divarication angle was related to growth. Tracks were measured using eleven linear metrics, which were used to determine whether these early theropods experienced allometric or isometric growth of their feet. Analysis of track metrics using bivariate regressions suggests weak, negative allometric linear relationships between the width of the track and the length of digits III and IV and total track length. Analysis of track metrics using bivariate plots and regressions that analyzed only trackways suggest that the variation observed within trackways is similar to the variation observed within different trackways. Photographs of tracks were utilized in twodimensional (2D) geometric morphometrics, which removes the effect of size by establishing common landmarks on specimen photographs and normalizing all specimens to a single size. 2D geometric morphometrics suggest that the majority of shape variation is due to aspects of divarication angle, depth of the division between digits, and the width of the tips of digits. However, no relationship was found between shape and size variation in the geometric morphometric dataset. Both linear measurements and geometric morphometrics suggest a single continuous sample, suggesting the tracks were likely made by a single species or closely-related species having similar foot morphologies. These data suggest that as early small-bodied theropods increased in size, growth in toe length was slower than growth in foot width. This could indicate that these early theropods grew relatively broader feet to provide greater stability to support their increased mass. |
