Bipedal locomotion has been observed mostly in a group of lizards known as Iguania in small species with slender bodies and long hind limbs. However there have been a few observations of this behavior in larger species as well. The phylogeny above (edited from shows some of the taxa with species known to use bipedal locomotion. The widespread use of this behavior in lizards leads to many debates about its origin. Some theories are that it is a left-over trait from dinosaur ancestors or that it is simply a result of lizard limb structure. Most believe howeever, that it has arisen several different times in lizards.