Speed from fossil trackways: calculations not validated by extant birds on compliant substrates

Fossil trackways serve as a valuable tool in understanding the behaviour and locomotion of extinct animals. Calculating speeds from trackways has become a standard approach, particularly for dinosaurs. However, the original equation was derived from predominantly mammalian data. There have been few validation studies using modern birds, the descendants of theropod dinosaurs. We used high-speed video recordings of two helmeted guineafowl (Numida meleagris) traversing mud of varying consistency. Associated trackways were digitized using photogrammetry, and stride and track lengths were measured to calculate speed. The speed calculated from trackways was higher than the measured speed for multiple speed formulae. Within-trackway stride lengths were variable, even when speed was relatively constant, but there were cases where two identical stride lengths were produced by birds moving at very different speeds. Prior work of guineafowl locomoting on a treadmill found good correspondence between measured speed and speed calculated from stride length. We attribute the discrepancy seen in our data between measured and calculated speed as resulting from non-steady state locomotion on compliant substrates. Given that tracks require compliant substrates to form, and are made by free-moving individuals, our data indicate that speed estimates from trackways are inaccurate, if not outright misleading.