In addition, tegus are facultative bipeds, while monitors are obligate quadrupeds. There was considerable debate whether pterosaurs ambulated as quadrupeds or as bipeds. The dinosaurs are classified in 6 families&colon; large carnivorous, armored quadrupeds, light bipeds, etc. Like most theropods, abelisaurids were carnivorous bipeds. Humans are obligate bipeds, not facultative bipeds. Few modern species are habitual bipeds whose normal method of locomotion is two-legged. Historically, most indeterminate ornithischian bipeds were lumped in as ornithopods. The aliens are humanoid bipeds, but see in the infrared portion of the spectrum. All early dinosaurs and many later ones were bipeds. The early bipeds eventually evolved into the australopithecines and still later into the genus "Homo". "Paranthropus" were bipeds, and their hips, legs, and feet resemble "A. afarensis" and modern humans. Because humans are bipeds, motor control is needed for keeping balance. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats. As a main means of locomotion, this is usually found in bipeds, or semi-bipeds. All birds are bipeds when on the ground, a feature inherited from their dinosaur ancestors.