HETEROPODY INDEX: A SQUARE PEG IN A ROUND HOLE?

Abstract: Heteropody Index (HI) is a tool used to calculate area differences between the manus and pes of fossil trackways. HI uses a simple length × width calculation to estimate area. However, since most foot impressions are rarely close to square in shape, HI using a different area calculation could potentially more accurately reflect differences in manus and pes foot area. In this study, accuracy of length × width (L×W) as an area estimate for basic shapes and animal footprints, was tested against two other area calculations, the area of a circle: πr2, and length × carpal width (L×CW) (the width at the most proximal point of the foot in contact with the ground). In addition, accuracy of HI calculations using these methods was tested against HI calculations using actual area of the corresponding shape or underfoot area. It was discovered that in general L×W is a better estimate for area than πr2, in most animals except ungulates. However, for those animals where L×W was a better estimate than πr2, L×CW was more accurate. This paper additionally proposes that by combining the findings of these tests with those of Strickson et al. (2019), foot area estimates for dinosaurs can be estimated more accurately using L×CW, to return an area close to estimates for soft tissue. Previous HI measurements may have overestimated extreme heteropody in sauropod dinosaurs.