Does pace angulation correlate with limb posture?

Femoral abduction angle, pelvic rotation angle and motion speed of eight species of modern crocodiles and lizards were measured to determine the factors affecting pace angulation and to assess whether limb posture can be estimated from trackways. Film sequences of dorsal views of free walking reptiles were filmed with a digital video camera recorder, then positions of the ankle, knee and hip joints were digitized and x–y coordinates were obtained for each of the half frames. Pace angulation and abduction angle of the supporting femur were calculated from the coordinates, together with motion speed and pelvic rotation. Multiple stepwise regression analysis showed that the femoral abduction angle was the most influential factor, explaining 47% of the variation in pace angulation value. The parameter estimate for the femoral abduction angle (− 0.71) indicates that an animal with a more erect limb posture (i.e., lower femur abduction angle) makes a trackway with a higher pace angulation. Pelvic rotation also contributed significantly to the model, whereas motion speed did not. By inverse prediction, a trackway with an average pace angulation value of 108° or less is unlikely to have been produced by an animal with a fully erect gait. A major implication of our study is that limb posture of extinct tetrapods can be reconstructed from fossilized trackways. Fossil trackways are an important source of information about posture. Analyses of fossilized trackways can complement information obtained from body fossils or, in case where body fossils are rare or lacking, fossilized trackways may be the primary source of information about posture, locomotion and behavior.