GROWTH IN THE DINOSAUR COELOPHYSIS: IMPLICATIONS FOR DINOSAUR SPEED ESTIMATES FROM TRACKWAYS

In order to estimate dinosaur speeds from their trackways, it is assumed that hip height can be calculated from footprint length; speed estimates are based on footprint and stride lengths. However, our initial data shows that there is much greater variation in foot length/hip height ratios than previously reported, both within and between clades. Combined with the inability to identify a trackmaker to the species level (or sometimes even to clade), it becomes difficult to ascertain the hip height from the footprint length. In this study we investigated ontogenetic growth and intraspecific variation within Coelophysis bauri and Dromaius novaehollandiae (emu), in order to determine the degree of variability in limb element ratios within a single species. Ideally, the ventral (trackmaking) surface of the foot should be measured; and due to variation in length of the ungual sheath, claws should be excluded from footprints in order to best estimate foot length from the footprint. However, most specimens of Coelophysis are preserved and prepared with only the dorsal surfaces of the foot accessible for study. Within Coelophysis, the length of the femur strongly correlates to the tibia length, and likewise the metatarsus is proportional to the digits. However, there is great variability in the relationship between the long bone (tibia and/or femur) and pedal (metatarsus and/or digits) elements. Therefore, it is not possible to reliably estimate hind limb length from foot length. Emus exhibit even greater variability, with less strong correlations both between femur and tibia, and metatarsus and digits; the relationship between long bone and pedal elements is weaker than in Coelophysis. In summary, it is not possible to accurately predict hip height from footprint length within either of these species. Add to this the uncertainty in trackmaker identity – whether a small footprint made by an adult of a small species or a juvenile of a large species – and any estimates of dinosaur speed based on trackway measurements will be highly inaccurate.