Evolution of chameleon locomotion, or how to become arboreal as a reptile.

High-speed, biplanar X-ray motion analysis, X-ray reconstruction of moving morphology (XROMM) and morphological studies have led to the identification of those traits which are considered to be crucial for the evolution of arboreal locomotion in chameleons. The loss of the extensive lateral undulation typical of reptiles needs to be compensated by high mobility in the shoulder girdle and a clear functional regionalization of the trunk. Large limb excursion angles provide a compliant gait and are made possible by a functional parasagittalization of fore- and hind limbs, at least temporarily. All these evolutionary novelties parallel very similar modifications in the evolution of the locomotor apparatus in therian mammals. We propose that the convergent "invention" of dynamic stability and a compliant gait seem to be responsible for the locomotor similarities between chameleons and mammals.